path: root/converter/other/xwdtopnm.c

/* xwdtopnm.c - read an X11 or X10 window dump file and write a PNM image.
**
** Copyright (C) 1989, 1991 by Jef Poskanzer.
**
** Permission to use, copy, modify, and distribute this software and its
** documentation for any purpose and without fee is hereby granted, provided
** that the above copyright notice appear in all copies and that both that
** copyright notice and this permission notice appear in supporting
** documentation.  This software is provided "as is" without express or
** implied warranty.
*/

/* IMPLEMENTATION NOTES:

   The file X11/XWDFile.h from the X Window System is an authority for the
   format of an XWD file.  Netpbm uses its own declaration, though.

   It has been a real challenge trying to reverse engineer the XWD
   format.  This program is almost always broken as people find XWD images
   with which it does not work and we update the program in response.

   We consider an XWD file correct if Xwud displays it properly.
*/


#define _BSD_SOURCE 1      /* Make sure strdup() is in string.h */
#define _XOPEN_SOURCE 500  /* Make sure strdup() is in string.h */

#include <stdio.h>
#include <string.h>
#include <assert.h>

#include "pm_c_util.h"
#include "shhopt.h"
#include "mallocvar.h"
#include "nstring.h"
#include "pnm.h"
#include "x10wd.h"
#include "x11wd.h"

struct compMask {
    unsigned long red;
    unsigned long grn;
    unsigned long blu;
};


struct cmdlineInfo {
    /* All the information the user supplied in the command line,
       in a form easy for the program to use.
    */
    const char * inputFilename;
    unsigned int verbose;
    unsigned int debug;
    unsigned int headerdump;
};


static bool debug;
static bool verbose;

#ifdef DEBUG_PIXEL
static unsigned int pixel_count = 0;
#endif

/* Byte-swapping junk. */

static int
zero_bits(const unsigned long mask) {
/*----------------------------------------------------------------------------
   Return the number of consective zero bits at the least significant end
   of the binary representation of 'mask'.  E.g. if mask == 0x00fff800,
   we would return 11.
-----------------------------------------------------------------------------*/
    int i;
    unsigned long shifted_mask;

    for (i=0, shifted_mask = mask; 
         i < sizeof(mask)*8 && (shifted_mask & 0x00000001) == 0;
         i++, shifted_mask >>= 1 );
    return(i);
}



static int
one_bits(const unsigned long input) {
/*----------------------------------------------------------------------------
   Return the number of one bits in the binary representation of 'input'.
-----------------------------------------------------------------------------*/
    int one_bits;
    unsigned long mask;

    one_bits = 0;   /* initial value */
    for (mask = 0x00000001; mask != 0x00000000; mask <<= 1)
        if (input & mask) one_bits++;

    return(one_bits);
}



static void
parseCommandLine(int argc, char ** argv,
                 struct cmdlineInfo * const cmdlineP) {
/*----------------------------------------------------------------------------
   Note that many of the strings that this function returns in the
   *cmdline_p structure are actually in the supplied argv array.  And
   sometimes, one of these strings is actually just a suffix of an entry
   in argv!
-----------------------------------------------------------------------------*/
    optEntry * option_def;
        /* Instructions to OptParseOptions3 on how to parse our options.
         */
    optStruct3 opt;

    unsigned int option_def_index;

    MALLOCARRAY(option_def, 100);

    option_def_index = 0;   /* incremented by OPTENTRY */

    OPTENT3(0,   "verbose",    OPT_FLAG,   NULL, &cmdlineP->verbose,       0);
    OPTENT3(0,   "debug",      OPT_FLAG,   NULL, &cmdlineP->debug,         0);
    OPTENT3(0,   "headerdump", OPT_FLAG,   NULL, &cmdlineP->headerdump,    0);

    opt.opt_table = option_def;
    opt.short_allowed = FALSE;  /* We have no short (old-fashioned) options */
    opt.allowNegNum = FALSE;  /* We may have parms that are negative numbers */

    optParseOptions3(&argc, argv, opt, sizeof(opt), 0);
        /* Uses and sets argc, argv, and some of *cmdlineP and others. */

    if (argc - 1 == 0)
        cmdlineP->inputFilename = NULL;  /* he wants stdin */
    else if (argc - 1 == 1) {
        if (streq(argv[1], "-"))
            cmdlineP->inputFilename = NULL;  /* he wants stdin */
        else 
            cmdlineP->inputFilename = strdup(argv[1]);
    } else 
        pm_error("Too many arguments.  The only argument accepted\n"
                 "is the input file specification");
}



static void
processX10Header(X10WDFileHeader *  const h10P, 
                 FILE *             const file,
                 int *              const colsP, 
                 int *              const rowsP, 
                 unsigned int *     const padrightP, 
                 xelval *           const maxvalP, 
                 enum visualclass * const visualclassP, 
                 int *              const formatP, 
                 xel **             const colorsP, 
                 int *              const bits_per_pixelP, 
                 int *              const bits_per_itemP, 
                 struct compMask *  const compMaskP,
                 enum byteorder *   const byte_orderP,
                 enum byteorder *   const bit_orderP) {

    int i;
    X10Color* x10colors;
    bool grayscale;
    bool byte_swap;

    *maxvalP = 65535;   /* Initial assumption */

    if ( h10P->file_version != X10WD_FILE_VERSION ) {
        byte_swap = TRUE;
        h10P->header_size     = pm_bs_long(h10P->header_size);
        h10P->file_version    = pm_bs_long(h10P->file_version);
        h10P->display_type    = pm_bs_long(h10P->display_type);
        h10P->display_planes  = pm_bs_long(h10P->display_planes);
        h10P->pixmap_format   = pm_bs_long(h10P->pixmap_format);
        h10P->pixmap_width    = pm_bs_long(h10P->pixmap_width);
        h10P->pixmap_height   = pm_bs_long(h10P->pixmap_height);
        h10P->window_width    = pm_bs_short(h10P->window_width);
        h10P->window_height   = pm_bs_short(h10P->window_height);
        h10P->window_x        = pm_bs_short(h10P->window_x);
        h10P->window_y        = pm_bs_short(h10P->window_y);
        h10P->window_bdrwidth = pm_bs_short(h10P->window_bdrwidth);
        h10P->window_ncolors  = pm_bs_short(h10P->window_ncolors);
    } else
        byte_swap = FALSE;

    for ( i = 0; i < h10P->header_size - sizeof(*h10P); ++i )
        if ( getc( file ) == EOF )
            pm_error( "couldn't read rest of X10 XWD file header" );

    /* Check whether we can handle this dump. */
    if ( h10P->window_ncolors > 256 )
        pm_error( "can't handle X10 window_ncolors > %d", 256 );
    if ( h10P->pixmap_format != ZFormat && h10P->display_planes != 1 )
        pm_error(
            "can't handle X10 pixmap_format %d with planes != 1",
            h10P->pixmap_format );

    grayscale = TRUE;  /* initial assumption */
    if ( h10P->window_ncolors != 0 ) {
        /* Read X10 colormap. */
        MALLOCARRAY( x10colors, h10P->window_ncolors );
        if ( x10colors == NULL )
            pm_error( "out of memory" );
        for ( i = 0; i < h10P->window_ncolors; ++i ) {
            if ( fread( &x10colors[i], sizeof(X10Color), 1, file ) != 1 )
                pm_error( "couldn't read X10 XWD colormap" );
            if ( byte_swap ) {
                x10colors[i].red   = pm_bs_short(x10colors[i].red);
                x10colors[i].green = pm_bs_short(x10colors[i].green);
                x10colors[i].blue  = pm_bs_short(x10colors[i].blue);
            }
            if ( x10colors[i].red != x10colors[i].green ||
                 x10colors[i].green != x10colors[i].blue )
                grayscale = FALSE;
        }
    }

    if ( h10P->display_planes == 1 ) {
        *formatP = PBM_TYPE;
        *visualclassP = StaticGray;
        *maxvalP = 1;
        *colorsP = pnm_allocrow( 2 );
        PNM_ASSIGN1( (*colorsP)[0], 0 );
        PNM_ASSIGN1( (*colorsP)[1], *maxvalP );
        *padrightP =
            (((h10P->pixmap_width + 15) / 16) * 16 - h10P->pixmap_width) * 8;
        *bits_per_itemP = 16;
        *bits_per_pixelP = 1;
    } else if ( h10P->window_ncolors == 0 ) { 
        /* Must be grayscale. */
        *formatP = PGM_TYPE;
        *visualclassP = StaticGray;
        if (h10P->display_planes > sizeof(*maxvalP) * 8 - 1)
            pm_error("XWD header says display_planes = %u, which is "
                     "too large for this program to compute",
                     h10P->display_planes);
        *maxvalP = ((xelval)1 << h10P->display_planes) - 1;
        if (*maxvalP > PNM_OVERALLMAXVAL)
            pm_error("XWD header says display_planes = %u, which is too "
                     "large for maximum maxval of %u",
                     h10P->display_planes, PNM_OVERALLMAXVAL);
        *colorsP = pnm_allocrow( *maxvalP + 1 );
        for ( i = 0; i <= *maxvalP; ++i )
            PNM_ASSIGN1( (*colorsP)[i], i );
        *padrightP =
            (((h10P->pixmap_width + 15) / 16) * 16 - h10P->pixmap_width) * 8;
        *bits_per_itemP = 16;
        *bits_per_pixelP = 1;
    } else {
        *colorsP = pnm_allocrow( h10P->window_ncolors );
        *visualclassP = PseudoColor;
        if ( grayscale ) {
            *formatP = PGM_TYPE;
            for ( i = 0; i < h10P->window_ncolors; ++i )
                PNM_ASSIGN1( (*colorsP)[i], x10colors[i].red );
        } else {
            *formatP = PPM_TYPE;
            for ( i = 0; i < h10P->window_ncolors; ++i )
                PPM_ASSIGN(
                    (*colorsP)[i], x10colors[i].red, x10colors[i].green,
                    x10colors[i].blue);
        }

        *padrightP = (h10P->pixmap_width & 1) * 8;
        *bits_per_itemP = 8;
        *bits_per_pixelP = 8;
    }
    *colsP = h10P->pixmap_width;
    *rowsP = h10P->pixmap_height;
    *byte_orderP = MSBFirst;
    *bit_orderP = LSBFirst;
}



static void
fixH11ByteOrder(X11WDFileHeader *  const h11P,
                X11WDFileHeader ** const h11FixedPP) {

    X11WDFileHeader * h11FixedP;

    MALLOCVAR_NOFAIL(h11FixedP);

    if (h11P->file_version == X11WD_FILE_VERSION) {
        memcpy(h11FixedP, h11P, sizeof(*h11FixedP));
    } else {
        h11FixedP->header_size      = pm_bs_long(h11P->header_size);
        h11FixedP->file_version     = pm_bs_long(h11P->file_version);
        h11FixedP->pixmap_format    = pm_bs_long(h11P->pixmap_format);
        h11FixedP->pixmap_depth     = pm_bs_long(h11P->pixmap_depth);
        h11FixedP->pixmap_width     = pm_bs_long(h11P->pixmap_width);
        h11FixedP->pixmap_height    = pm_bs_long(h11P->pixmap_height);
        h11FixedP->xoffset          = pm_bs_long(h11P->xoffset);
        h11FixedP->byte_order       = pm_bs_long(h11P->byte_order);
        h11FixedP->bitmap_unit      = pm_bs_long(h11P->bitmap_unit);
        h11FixedP->bitmap_bit_order = pm_bs_long(h11P->bitmap_bit_order);
        h11FixedP->bitmap_pad       = pm_bs_long(h11P->bitmap_pad);
        h11FixedP->bits_per_pixel   = pm_bs_long(h11P->bits_per_pixel);
        h11FixedP->bytes_per_line   = pm_bs_long(h11P->bytes_per_line);
        h11FixedP->visual_class     = pm_bs_long(h11P->visual_class);
        h11FixedP->red_mask         = pm_bs_long(h11P->red_mask);
        h11FixedP->green_mask       = pm_bs_long(h11P->green_mask);
        h11FixedP->blue_mask        = pm_bs_long(h11P->blue_mask);
        h11FixedP->bits_per_rgb     = pm_bs_long(h11P->bits_per_rgb);
        h11FixedP->colormap_entries = pm_bs_long(h11P->colormap_entries);
        h11FixedP->ncolors          = pm_bs_long(h11P->ncolors);
        h11FixedP->window_width     = pm_bs_long(h11P->window_width);
        h11FixedP->window_height    = pm_bs_long(h11P->window_height);
        h11FixedP->window_x         = pm_bs_long(h11P->window_x);
        h11FixedP->window_y         = pm_bs_long(h11P->window_y);
        h11FixedP->window_bdrwidth  = pm_bs_long(h11P->window_bdrwidth);
    }
    *h11FixedPP = h11FixedP;
}



static void
readX11Colormap(FILE *      const file,
                int         const ncolors, 
                bool        const byteSwap,
                X11XColor** const x11colorsP) {
                
    X11XColor * x11colors;
    int rc;

    /* Read X11 colormap. */
    MALLOCARRAY(x11colors, ncolors);
    if (x11colors == NULL)
        pm_error("out of memory");
    rc = fread(x11colors, sizeof(x11colors[0]), ncolors, file);
    if (rc != ncolors)
        pm_error("couldn't read X11 XWD colormap");
    if (byteSwap) {
        unsigned int i;
        for (i = 0; i < ncolors; ++i) {
            x11colors[i].red   = pm_bs_short(x11colors[i].red);
            x11colors[i].green = pm_bs_short(x11colors[i].green);
            x11colors[i].blue  = pm_bs_short(x11colors[i].blue);
        }
    }
    if (debug) {
        unsigned int i;
        for (i = 0; i < ncolors && i < 8; ++i)
            pm_message("Color %d r/g/b = %d/%d/%d", i, 
                       x11colors[i].red, x11colors[i].green, 
                       x11colors[i].blue);
    }
    *x11colorsP = x11colors;
}



static bool
colormapAllGray(const X11XColor* const x11colors,
                unsigned int     const ncolors) {

    unsigned int i;
    bool grayscale;

    grayscale = TRUE;  /* initial assumption */
    for (i = 0; i < ncolors; ++i) 
        if (x11colors[i].red != x11colors[i].green ||
            x11colors[i].green != x11colors[i].blue )
            grayscale = FALSE;

    return grayscale;
}



static void
dumpX11Header(X11WDFileHeader * const h11P) {

    const char * formatDesc;
    const char * byteOrderDesc;
    const char * bitOrderDesc;
    const char * visualClassDesc;

    X11WDFileHeader * h11FixedP;

    fixH11ByteOrder(h11P, &h11FixedP);
 
    switch(h11FixedP->pixmap_format) {
    case XYBitmap: formatDesc = "XY bit map"; break;
    case XYPixmap: formatDesc = "XY pix map"; break;
    case ZPixmap:  formatDesc = "Z pix map";  break;
    default:       formatDesc = "???";
    }

    switch(h11FixedP->byte_order) {
    case LSBFirst: byteOrderDesc = "LSB first"; break;
    case MSBFirst: byteOrderDesc = "MSB first"; break;
    default:       byteOrderDesc = "???";
    }

    switch (h11FixedP->bitmap_bit_order) {
    case LSBFirst: bitOrderDesc = "LSB first"; break;
    case MSBFirst: bitOrderDesc = "MSB first"; break;
    default:       bitOrderDesc = "???";
    }

    switch (h11FixedP->visual_class) {
    case StaticGray:  visualClassDesc = "StaticGray";  break;
    case GrayScale:   visualClassDesc = "GrayScale";   break;
    case StaticColor: visualClassDesc = "StaticColor"; break;
    case PseudoColor: visualClassDesc = "PseudoColor"; break;
    case TrueColor:   visualClassDesc = "TrueColor"; break;
    case DirectColor: visualClassDesc = "DirectColor"; break;
    default:          visualClassDesc = "???";
    }

    pm_message("File version: %u", h11FixedP->file_version);
    pm_message("Format: %s (%u)", formatDesc, h11FixedP->pixmap_format);
    pm_message("Width:  %u", h11FixedP->pixmap_width);
    pm_message("Height: %u", h11FixedP->pixmap_height);
    pm_message("Depth: %u bits", h11FixedP->pixmap_depth);
    pm_message("X offset: %u", h11FixedP->xoffset);
    pm_message("byte order: %s (%u)", byteOrderDesc, h11FixedP->byte_order);
    pm_message("bitmap unit: %u", h11FixedP->bitmap_unit);
    pm_message("bit order: %s (%u)", 
               bitOrderDesc, h11FixedP->bitmap_bit_order);
    pm_message("bitmap pad: %u", h11FixedP->bitmap_pad);
    pm_message("bits per pixel: %u", h11FixedP->bits_per_pixel);
    pm_message("bytes per line: %u", h11FixedP->bytes_per_line);
    pm_message("visual class: %s (%u)", 
               visualClassDesc, h11FixedP->visual_class);
    pm_message("red mask:   %08x", h11FixedP->red_mask);
    pm_message("green mask: %08x", h11FixedP->green_mask);
    pm_message("blue mask:  %08x", h11FixedP->blue_mask);
    pm_message("bits per rgb: %u", h11FixedP->bits_per_rgb);
    pm_message("number of colormap entries: %u", h11FixedP->colormap_entries);
    pm_message("number of colors in colormap: %u", h11FixedP->ncolors);
    pm_message("window width:  %u", h11FixedP->window_width);
    pm_message("window height: %u", h11FixedP->window_height);
    pm_message("window upper left X coordinate: %u", h11FixedP->window_x);
    pm_message("window upper left Y coordinate: %u", h11FixedP->window_y);
    pm_message("window border width: %u", h11FixedP->window_bdrwidth);

    free(h11FixedP);
}



static unsigned long
reverseBits(unsigned long arg,
            unsigned int nSigBits) {

    unsigned long input;
    unsigned long output;
    unsigned int i;

    for (i = 0, input = arg, output = 0; i < nSigBits; ++i) {
        output <<= 1;

        output |= (input & 0x1);

        input >>= 1;
    }
    return output;
}



static void
computeComponentMasks(X11WDFileHeader * const h11P,
                      struct compMask * const compMaskP) {
/*----------------------------------------------------------------------------
   You'd think the component (red, green, blue) masks in the header
   would just be right.  But we've seen a direct color image which has
   BGR layout even though the masks say RGB.  It also says bit order
   is LSB first, even though the pixels within the items are arranged
   MSB first.  So we're guessing that LSB first bit order in that
   particular case means the bits within each the pixel are backwards.
   So we reverse the masks to compensate.
-----------------------------------------------------------------------------*/
    if (h11P->visual_class == DirectColor &&
        h11P->bits_per_pixel == 24 && h11P->bitmap_bit_order == LSBFirst) {
        compMaskP->red = reverseBits(h11P->red_mask, 24);
        compMaskP->grn = reverseBits(h11P->green_mask, 24);
        compMaskP->blu = reverseBits(h11P->blue_mask, 24);
    } else {
        compMaskP->red = h11P->red_mask;
        compMaskP->grn = h11P->green_mask;
        compMaskP->blu = h11P->blue_mask;
    }
}



static void
processX11Header(X11WDFileHeader *  const h11P, 
                 FILE *             const file,
                 int *              const colsP, 
                 int *              const rowsP, 
                 unsigned int *     const padrightP, 
                 xelval *           const maxvalP, 
                 enum visualclass * const visualclassP, 
                 int *              const formatP, 
                 xel **             const colorsP, 
                 int *              const bits_per_pixelP, 
                 int *              const bits_per_itemP, 
                 struct compMask *  const compMaskP,
                 enum byteorder *   const byte_orderP,
                 enum byteorder *   const bit_orderP) {

    int i;
    X11XColor* x11colors;
    bool grayscale;
    bool const byte_swap = (h11P->file_version != X11WD_FILE_VERSION);
    X11WDFileHeader * h11FixedP;
 
    fixH11ByteOrder(h11P, &h11FixedP);

    if (byte_swap && verbose)
        pm_message("Header is different endianness from this machine.");
    
    for (i = 0; i < h11FixedP->header_size - sizeof(*h11FixedP); ++i)
        if (getc(file) == EOF)
            pm_error("couldn't read rest of X11 XWD file header");

    /* Check whether we can handle this dump. */
    if (h11FixedP->pixmap_depth > 24)
        pm_error( "can't handle X11 pixmap_depth > 24");
    if (h11FixedP->bits_per_rgb > 24)
        pm_error("can't handle X11 bits_per_rgb > 24");
    if (h11FixedP->pixmap_format != ZPixmap && h11FixedP->pixmap_depth != 1)
        pm_error("can't handle X11 pixmap_format %d with depth != 1",
                 h11FixedP->pixmap_format);
    if (h11FixedP->bitmap_unit != 8 && h11FixedP->bitmap_unit != 16 &&
        h11FixedP->bitmap_unit != 32)
        pm_error("X11 bitmap_unit (%d) is non-standard - can't handle",
                 h11FixedP->bitmap_unit);
    /* The following check was added in 10.19 (November 2003) */
    if (h11FixedP->bitmap_pad != 8 && h11FixedP->bitmap_pad != 16 &&
        h11FixedP->bitmap_pad != 32)
        pm_error("X11 bitmap_pad (%d) is non-standard - can't handle",
                 h11FixedP->bitmap_unit);

    if (h11FixedP->ncolors > 0) {
        readX11Colormap(file, h11FixedP->ncolors, byte_swap, &x11colors);
        grayscale = colormapAllGray(x11colors, h11FixedP->ncolors);
    } else
        grayscale = TRUE;

    *visualclassP = (enum visualclass) h11FixedP->visual_class;
    if (*visualclassP == DirectColor) {
        unsigned int i;
        *formatP = PPM_TYPE;
        *maxvalP = 65535;
        /*
          DirectColor is like PseudoColor except that there are essentially
          3 colormaps (shade maps) -- one for each color component.  Each pixel
          is composed of 3 separate indices.
        */

        *colorsP = pnm_allocrow(h11FixedP->ncolors);
        for (i = 0; i < h11FixedP->ncolors; ++i)
            PPM_ASSIGN(
                (*colorsP)[i], x11colors[i].red, x11colors[i].green,
                x11colors[i].blue);
    } else if (*visualclassP == TrueColor) {
        *formatP = PPM_TYPE;

        *maxvalP = pm_lcm(pm_bitstomaxval(one_bits(h11FixedP->red_mask)),
                          pm_bitstomaxval(one_bits(h11FixedP->green_mask)),
                          pm_bitstomaxval(one_bits(h11FixedP->blue_mask)),
                          PPM_OVERALLMAXVAL
            );
    } else if (*visualclassP == StaticGray && h11FixedP->bits_per_pixel == 1) {
        *formatP = PBM_TYPE;
        *maxvalP = 1;
        *colorsP = pnm_allocrow( 2 );
        PNM_ASSIGN1((*colorsP)[0], *maxvalP);
        PNM_ASSIGN1((*colorsP)[1], 0);
    } else if (*visualclassP == StaticGray) {
        unsigned int i;
        *formatP = PGM_TYPE;
        if (h11FixedP->bits_per_pixel > sizeof(*maxvalP) * 8 - 1)
            pm_error("XWD header says bits_per_pixel = %u, which is "
                     "too large for this program to compute",
                     h11FixedP->bits_per_pixel);
        *maxvalP = ((xelval)1 << h11FixedP->bits_per_pixel) - 1;
        if (*maxvalP > PNM_OVERALLMAXVAL)
            pm_error("XWD header says bits_per_pixel = %u, which is too "
                     "large for maximum maxval of %u",
                     h11FixedP->bits_per_pixel, PNM_OVERALLMAXVAL);
        *colorsP = pnm_allocrow(*maxvalP + 1);
        for (i = 0; i <= *maxvalP; ++i)
            PNM_ASSIGN1((*colorsP)[i], i);
    } else {
        *colorsP = pnm_allocrow(h11FixedP->ncolors);
        if (grayscale) {
            unsigned int i;
            *formatP = PGM_TYPE;
            for (i = 0; i < h11FixedP->ncolors; ++i)
                PNM_ASSIGN1((*colorsP)[i], x11colors[i].red);
        } else {
            unsigned int i;
            *formatP = PPM_TYPE;
            for (i = 0; i < h11FixedP->ncolors; ++i)
                PPM_ASSIGN(
                    (*colorsP)[i], x11colors[i].red, x11colors[i].green,
                    x11colors[i].blue);
        }
        *maxvalP = 65535;
    }

    *colsP = h11FixedP->pixmap_width;
    *rowsP = h11FixedP->pixmap_height;
    *padrightP =
        h11FixedP->bytes_per_line * 8 -
        h11FixedP->pixmap_width * h11FixedP->bits_per_pixel;

    /* According to X11/XWDFile.h, the item size is 'bitmap_pad' for some
       images and 'bitmap_unit' for others.  This is strange, so there may
       be some subtlety of their definitions that we're missing.

       See comments in pixelReader_getpix() about what an item is.

       Ben Kelley in January 2002 had a 32 bits-per-pixel xwd file
       from a truecolor 32 bit window on a Hummingbird Exceed X server
       on Win32, and it had bitmap_unit = 8 and bitmap_pad = 32.

       But Björn Eriksson in October 2003 had an xwd file from a 24
       bit-per-pixel direct color window that had bitmap_unit = 32 and
       bitmap_pad = 8.  This was made by Xwd in Red Hat Xfree86 4.3.0-2.

       In March 2007, Darren Frith presented an xwd file like this:
       Header says direct color, bits_per_pixel = 24, bitmap_unit =
       32, bitmap_pad = 8, byte order and bit order LSB first.  The
       bytes in each item are in fact MSB first and the pixels spread
       across the items MSB first.  The raster is consecutive 24 bit
       pixel units, but each row is padded on the right with enough
       bits to make the total line size 32 x width.  Really strange.
       The header says the bits within each pixel are one byte red,
       one byte green, one byte blue.  But they are actually blue,
       green, red.  Xwud, ImageMagick, and Gimp render this image
       correctly, so it's not broken.  Created by Xwd of X.org 7.1.1.

       Before Netpbm 9.23 (January 2002), we used bitmap_unit as the
       item size always.  Then, until 10.19 (November 2003), we used
       bitmap_pad when pixmap_depth > 1 and pixmap_format == ZPixmap.
       We still don't see any logic in these fields at all, but we
       figure whichever one is greater (assuming both are meaningful)
       has to be the item size.  */
    *bits_per_itemP  = MAX(h11FixedP->bitmap_pad, h11FixedP->bitmap_unit);
    *bits_per_pixelP = h11FixedP->bits_per_pixel;

    if (*visualclassP == DirectColor) {
        /* Strange, but we've seen a Direct Color 24/32 image that
           says LSBFirst and it's a lie.  And Xwud renders it correctly.
        */
        *byte_orderP = MSBFirst;
        *bit_orderP = MSBFirst;
    } else {
        *byte_orderP = (enum byteorder) h11FixedP->byte_order;
        *bit_orderP  = (enum byteorder) h11FixedP->bitmap_bit_order;
    }
    computeComponentMasks(h11FixedP, compMaskP);

    free(h11FixedP);
} 



static void
getinit(FILE *             const ifP, 
        int *              const colsP, 
        int *              const rowsP, 
        unsigned int *     const padrightP, 
        xelval *           const maxvalP, 
        enum visualclass * const visualclassP, 
        int *              const formatP, 
        xel **             const colorsP,
        int *              const bits_per_pixelP, 
        int *              const bits_per_itemP, 
        struct compMask *  const compMaskP,
        enum byteorder *   const byte_orderP,
        enum byteorder *   const bit_orderP,
        bool               const headerDump) {
/*----------------------------------------------------------------------------
   Read the header from the XWD image in input stream 'ifP'.  Leave
   the stream positioned to the beginning of the raster.

   Return various fields from the header.

   Return as *padrightP the number of additional bits of padding are
   at the end of each line of input.  This says the input stream
   contains *colsP pixels of image data (at *bits_per_pixelP bits each)
   plus *padrightP bits of padding.
-----------------------------------------------------------------------------*/
    /* Assume X11 headers are larger than X10 ones. */
    unsigned char header[sizeof(X11WDFileHeader)];
    X10WDFileHeader* h10P;
    X11WDFileHeader* h11P;
    int rc;

    h10P = (X10WDFileHeader*) header;
    h11P = (X11WDFileHeader*) header;
    if ( sizeof(*h10P) > sizeof(*h11P) )
        pm_error("ARGH!  On this machine, X10 headers are larger than "
                 "X11 headers!\n    You will have to re-write xwdtopnm." );
    
    /* We read an X10 header's worth of data from the file, then look
       at it to see if it looks like an X10 header.  If so we process
       the X10 header.  If not, but it looks like the beginning of an
       X11 header, we read more bytes so we have an X11 header's worth
       of data, then process the X11 header.  Otherwise, we raise an
       error.  
    */

    rc = fread(&header[0], sizeof(*h10P), 1, ifP);
    if (rc != 1)
        pm_error( "couldn't read XWD file header" );

    if (h10P->file_version == X10WD_FILE_VERSION ||
        pm_bs_long(h10P->file_version) == X10WD_FILE_VERSION) {
        
        if (verbose)
            pm_message("Input is X10");
        processX10Header(h10P, ifP, colsP, rowsP, padrightP, maxvalP, 
                         visualclassP, formatP, 
                         colorsP, bits_per_pixelP, bits_per_itemP, 
                         compMaskP, byte_orderP, bit_orderP);
    } else if (h11P->file_version == X11WD_FILE_VERSION ||
               pm_bs_long(h11P->file_version) == X11WD_FILE_VERSION) {
        
        int rc;

        if (verbose)
            pm_message("Input is X11");

        /* Read the balance of the X11 header */
        rc = fread(&header[sizeof(*h10P)], 
                   sizeof(*h11P) - sizeof(*h10P), 1, ifP);
        if (rc != 1)
            pm_error("couldn't read end of X11 XWD file header");

        if (headerDump)
            dumpX11Header(h11P);

        processX11Header(h11P, ifP, colsP, rowsP, padrightP, maxvalP, 
                         visualclassP, formatP, 
                         colorsP, bits_per_pixelP, bits_per_itemP, 
                         compMaskP, byte_orderP, bit_orderP);
    } else
        pm_error("unknown XWD file version: %u", h11P->file_version);
}



#ifdef DEBUG_PIXEL
#define DEBUG_PIXEL_1 \
   if (pixel_count < 4) \
       pm_message("getting pixel %d", pixel_count);

#define DEBUG_PIXEL_2 \
    if (pixel_count < 4) \
        pm_message("item: %.8lx", row_controlP->item.l); 


#define DEBUG_PIXEL_3 \
    if (pixel_count < 4) \
        pm_message("  bits_taken: %lx(%d), carryover_bits: %lx(%d), " \
                     "pixel: %lx", \
                     bits_taken, bits_to_take, row_controlP->carryover_bits, \
                     row_controlP->bits_carried_over, pixel);

#define DEBUG_PIXEL_4 \
    if (pixel_count < 4) { \
        pm_message("  row_control.bits_carried_over = %d" \
                   "  carryover_bits= %.8lx", \
                   row_controlP->bits_carried_over, \
                   row_controlP->carryover_bits); \
        pm_message("  row_control.bits_used = %d", \
                   row_controlP->bits_used); \
        pm_message("  row_control.bits_left = %d", \
                   row_controlP->bits_left); \
                   } \
    \
    pixel_count++;
#else
#define DEBUG_PIXEL_1 do {} while(0)
#define DEBUG_PIXEL_2 do {} while(0)
#define DEBUG_PIXEL_3 do {} while(0)
#define DEBUG_PIXEL_4 do {} while(0)
#endif



/*----------------------------------------------------------------------------
   The pixel reader.

   The pixel reader is an object that reads an XWD raster and gives you
   one pixel at a time from it.

   It consists of a structure of type 'pixelReader' and the
   pixelReader_*() subroutines.
-----------------------------------------------------------------------------*/

typedef struct {
    FILE * fileP;
    unsigned long itemBuffer;
        /* The item buffer.  This contains what's left of the item
           most recently read from the image file -- an item goes from the
           XWD raster into here and then bits disappear from it as they
           become part of pixels returned by the object.

           'nBitsLeft' tells how many bits are in the buffer now.  It's
           zero when nothing has ever been read from the file.  Only
           the least signficant 'nBitsLeft' bits are meaningful.

           The numeric value of the member is the number whose pure
           binary representation is the bit string in the buffer.

           That bit string starts out as the contents of one item of
           the XWD raster, with the "byte order" value from the XWD
           header applied.  E.g. Assume bits per item is 16 and the
           and the "byte order" value is "MSB First".  Assume the
           raster contains 0x01 at Offset 109 and 0x02 at Offset 110.
           The value of this member just after that item is read is
           two hundred fifty-eight.  If we are running on a
           little-endian machine, it appears in memory as 0x02 at
           Address A and 0x01 at Address A+1.

           Then we pull bits from either the beginning or end of the
           buffer according to the "bit order" value from the XWD
           header.  E.g. in the example above, assume "bit order" is
           lsb first.  and bits per pixel is 4.  After reading the
           first pixel (0010b) from the buffer, 'itemBuffer' is
           sixteen (0x010) and 'nBitsLeft' is 12.
        */
    unsigned int nBitsLeft;
      /* This is the number of bits in the current item that have not yet
         been returned as part of a pixel.
      */
    int bitsPerPixel;
    int bitsPerItem;
    enum byteorder byteOrder;
    enum byteorder bitOrder;
} pixelReader;



static void
pixelReader_init(pixelReader *  const pixelReaderP,
                 FILE *         const fileP,
                 int            const bitsPerPixel,
                 int            const bitsPerItem, 
                 enum byteorder const byteOrder,
                 enum byteorder const bitOrder) {
    
    pixelReaderP->fileP           = fileP;
    pixelReaderP->bitsPerPixel    = bitsPerPixel;
    pixelReaderP->bitsPerItem     = bitsPerItem;
    pixelReaderP->byteOrder       = byteOrder;
    pixelReaderP->bitOrder        = bitOrder;

    pixelReaderP->nBitsLeft       = 0;
}    



static void
pixelReader_term(pixelReader * const pixelReaderP) {

    unsigned int remainingByteCount;

    if (pixelReaderP->nBitsLeft > 0)
        pm_message("Warning: %u unused bits left in the pixel reader "
                   "buffer after full image converted.  XWD file may be "
                   "corrupted or Xwdtopnm may have misinterpreted it",
                   pixelReaderP->nBitsLeft);


    pm_drain(pixelReaderP->fileP, 4096, &remainingByteCount);

    if (remainingByteCount >= 4096)
        pm_message("Warning: at least 4K additional bytes in XWD input stream "
                   "after full image converted.  XWD file may be corrupted "
                   "or Xwdtopnm may have misinterpreted it.");
    else if (remainingByteCount > 0)
        pm_message("Warning: %u additional bytes in XWD input stream "
                   "after full image converted.  XWD file may be corrupted "
                   "or Xwdtopnm may have misinterpreted it.",
                   remainingByteCount);
}



static void
readItem(pixelReader * const rdrP) {
/*----------------------------------------------------------------------------
   Read one item from the XWD raster associated with pixel reader *rdrP.

   Put the item into the item buffer of the pixel reader object *rdrP.
-----------------------------------------------------------------------------*/
    assert(rdrP->nBitsLeft == 0);

    switch (rdrP->bitsPerItem) {
    case 8: {
        unsigned char const item8 = getc(rdrP->fileP);
        rdrP->itemBuffer = item8;
        rdrP->nBitsLeft = 8;
    }
        break;

    case 16: {
        short item16;

        switch (rdrP->byteOrder) {
        case MSBFirst:
            pm_readbigshort(rdrP->fileP, &item16);
            break;
        case LSBFirst:
            pm_readlittleshort(rdrP->fileP, &item16);
            break;
        }
        rdrP->itemBuffer = (unsigned short)item16;
        rdrP->nBitsLeft = 16;
    }
        break;
        
    case 32: {
        long item32;
        
        switch (rdrP->byteOrder) {
        case MSBFirst:
            pm_readbiglong(rdrP->fileP, &item32);
            break;
        case LSBFirst:
            pm_readlittlelong(rdrP->fileP, &item32);
            break;
        }
        rdrP->itemBuffer = item32;
        rdrP->nBitsLeft = 32;
    }
        break;
    default:
        pm_error("INTERNAL ERROR: impossible bits_per_item");
    }
}



static unsigned long const lsbmask[] = {
/*----------------------------------------------------------------------------
   lsbmask[i] is the mask you use to select the i least signficant bits
   of a bit string.
-----------------------------------------------------------------------------*/
    0x00000000,
    0x00000001, 0x00000003, 0x00000007, 0x0000000f, 
    0x0000001f, 0x0000003f, 0x0000007f, 0x000000ff,
    0x000001ff, 0x000003ff, 0x000007ff, 0x00000fff,
    0x00001fff, 0x00003fff, 0x00007fff, 0x0000ffff,
    0x0001ffff, 0x0003ffff, 0x0007ffff, 0x000fffff,
    0x001fffff, 0x003fffff, 0x007fffff, 0x00ffffff,
    0x01ffffff, 0x03ffffff, 0x07ffffff, 0x0fffffff,
    0x1fffffff, 0x3fffffff, 0x7fffffff, 0xffffffff
};


static unsigned long
pixelReader_getbits(pixelReader * const rdrP,
                    unsigned int  const nBits) {
/*----------------------------------------------------------------------------
  Get the next 'nBits' bits from the stream, and return the last 32
  of them.
-----------------------------------------------------------------------------*/
    unsigned long pixel;
        /* Accumulator for the value we ultimately return.  We shift in
           bits from the right end.  The number of bits presently in the
           accumulator is rdrP->bitsPerPixel - nBitsStillNeeded .
        */
    
    unsigned int nBitsStillNeeded;
        /* How many bits we still need to add to 'pixel',
           as we build it up to the
           full amount we have to return.  The bits are right justified in
           it -- additional bits will shift in from the right.
        */

    pixel = 0;
    nBitsStillNeeded = nBits;

    while (nBitsStillNeeded > 0) {
        if (rdrP->nBitsLeft == 0)
            /* Buffer's empty.  Have to go back to the well for
               rdrP->bitsPerItem more bits.
            */
            readItem(rdrP);

        {
            unsigned int const nBitsToTake =
                MIN(rdrP->nBitsLeft, nBitsStillNeeded);
            unsigned long const bitsToTakeMask = lsbmask[nBitsToTake];
                /* E.g. if nbitsToTake is 4, this is 0x0000000F */

            unsigned long bitsToTake;
                /* The actual bits we take, in the 'nBitsToTake' low bits */

            assert(nBitsToTake <= 32);
            
            if (rdrP->bitOrder == MSBFirst) {
                unsigned int const nBitsToLeave =
                    rdrP->nBitsLeft - nBitsToTake;
                bitsToTake =
                    (rdrP->itemBuffer >> nBitsToLeave) & bitsToTakeMask;
            } else {
                bitsToTake = rdrP->itemBuffer & bitsToTakeMask;
                rdrP->itemBuffer >>= nBitsToTake;
            }                
            /* Shift the bits into the right end of the accumulator */
            pixel <<= nBitsToTake;
            pixel |= bitsToTake;

            rdrP->nBitsLeft -= nBitsToTake;
            nBitsStillNeeded -= nBitsToTake;
        }
    }
    return pixel;
}



static unsigned long
pixelReader_getpix(pixelReader * const rdrP) {
/*----------------------------------------------------------------------------
   Get a pixel from the input image.

   A pixel is a bit string.  It may be either an rgb triplet or an index
   into the colormap (or even an rgb triplet of indices into the colormaps!).
   We don't care -- it's just a bit string.

   We return an integer.  It's the integer that the pixel represents as
   pure binary cipher, with the first bit the most significant bit.
   
   The basic unit of storage in the input file is an "item."  An item
   can be 1, 2, or 4 bytes, and 'bits_per_item' tells us which.  Each
   item can have its bytes stored in forward or reverse order, and
   'byte_order' tells us which.  We have seen a Direct Color 24 bpp/32 bpi
   image which said 'byte_order' == LSBFirst, but the byte order is
   nonetheless MSB first.

   Each item can contain one or more pixels, and may contain
   fractional pixels.  'bits_per_pixel' tells us how many bits each
   pixel has, and 'bits_per_pixel' is always less than or equal to
   'bits_per_item', but not necessarily a factor of it.  Within an item,
   after taking care of the endianness of its storage format, the pixels
   may be arranged from left to right or right to left.  'bit_order' tells
   us which.  We have also seen images in which the pixels are arranged
   from left to right within the items, but the RGB components within
   each pixel are right to left and 'bit_order' is LSBFirst.

   But it's not that simple.  Sometimes dummy bits are added to the
   right edge of the image in order to make an integral number of
   items in each row of the raster.  And we've even seen images where
   there are a ridiculous number of padding bits on the right so as to
   make the number of items per line equal the number of pixels per
   line, even though items are 32 bits and pixels are 24 bits!  The
   XWD header has a field that tells how many bytes there are per XWD
   raster line, so that's the final word on how much padding there is.

   We maintain a 32 bit buffer to decouple reading of whole items from
   the file and reading of an arbitrary number of bits from the
   pixelReader.
-----------------------------------------------------------------------------*/
    assert(rdrP->bitsPerPixel <= 32);

    return pixelReader_getbits(rdrP, rdrP->bitsPerPixel);
}



static void
reportInfo(int              const cols, 
           int              const rows, 
           unsigned int     const padright, 
           xelval           const maxval, 
           enum visualclass const visualclass,
           int              const format, 
           int              const bits_per_pixel,
           int              const bits_per_item, 
           struct compMask  const compMask,
           enum byteorder   const byte_order, 
           enum byteorder   const bit_order) {
    
    const char *visualclass_name;
    const char *byte_order_name;
    const char *bit_order_name;
    switch (visualclass) {
    case StaticGray:  visualclass_name="StaticGray";  break;
    case GrayScale:   visualclass_name="Grayscale";   break;
    case StaticColor: visualclass_name="StaticColor"; break;
    case PseudoColor: visualclass_name="PseudoColor"; break;
    case TrueColor:   visualclass_name="TrueColor";   break;
    case DirectColor: visualclass_name="DirectColor"; break;
    default:          visualclass_name="(invalid)";    break;
    }
    switch (byte_order) {
    case MSBFirst: byte_order_name = "MSBFirst";  break;
    case LSBFirst: byte_order_name = "LSBFirst";  break;
    default:       byte_order_name = "(invalid)"; break;
    }
    switch (bit_order) {
    case MSBFirst: bit_order_name = "MSBFirst";  break;
    case LSBFirst: bit_order_name = "LSBFirst";  break;
    default:       bit_order_name = "(invalid)"; break;
    }
    pm_message("%d rows of %d columns with maxval %d",
               rows, cols, maxval);
    pm_message("padright=%u bits.  visualclass = %s.  format=%d (%c%c)",
               padright, visualclass_name, 
               format, format/256, format%256);
    pm_message("bits_per_pixel=%d; bits_per_item=%d",
               bits_per_pixel, bits_per_item);
    pm_message("byte_order=%s; bit_order=%s",
               byte_order_name, bit_order_name);
    pm_message("component mask: red=0x%.8lx; grn=0x%.8lx; blu=0x%.8lx",
               compMask.red, compMask.grn, compMask.blu);
}



static void 
convertRowSimpleIndex(pixelReader *  const pixelReaderP,
                      int            const cols,
                      const xel *    const colors,
                      xel *          const xelrow) {
    
    unsigned int col;
    for (col = 0; col < cols; ++col)
        xelrow[col] = colors[pixelReader_getpix(pixelReaderP)];
}



static void
convertRowDirect(pixelReader *   const pixelReaderP,
                 int             const cols,
                 const xel *     const colors,
                 struct compMask const compMask,
                 xel *           const xelrow) {
        
    unsigned int col;

    for (col = 0; col < cols; ++col) {
        unsigned long pixel;
            /* This is a triplet of indices into the color map, packed
               into this bit string according to compMask
            */
        unsigned int red_index, grn_index, blu_index;
            /* These are indices into the color map, unpacked from 'pixel'.
             */
            
        pixel = pixelReader_getpix(pixelReaderP);

        red_index = (pixel & compMask.red) >> zero_bits(compMask.red);
        grn_index = (pixel & compMask.grn) >> zero_bits(compMask.grn); 
        blu_index = (pixel & compMask.blu) >> zero_bits(compMask.blu);

        PPM_ASSIGN(xelrow[col],
                   PPM_GETR(colors[red_index]),
                   PPM_GETG(colors[grn_index]),
                   PPM_GETB(colors[blu_index])
            );
    }
}



static void
convertRowTrueColor(pixelReader *   const pixelReaderP,
                    int             const cols,
                    pixval          const maxval,
                    const xel *     const colors,
                    struct compMask const compMask,
                    xel *           const xelrow) {

    unsigned int col;
    unsigned int red_shift, grn_shift, blu_shift;
    unsigned int red_maxval, grn_maxval, blu_maxval;

    red_shift = zero_bits(compMask.red);
    grn_shift = zero_bits(compMask.grn);
    blu_shift = zero_bits(compMask.blu);

    red_maxval = compMask.red >> red_shift;
    grn_maxval = compMask.grn >> grn_shift;
    blu_maxval = compMask.blu >> blu_shift;

    for (col = 0; col < cols; ++col) {
        unsigned long pixel;

        pixel = pixelReader_getpix(pixelReaderP);

        /* The parsing of 'pixel' used to be done with hardcoded layout
           parameters.  See comments at end of this file.
        */
        PPM_ASSIGN(xelrow[col],
                   ((pixel & compMask.red) >> red_shift) * maxval / red_maxval,
                   ((pixel & compMask.grn) >> grn_shift) * maxval / grn_maxval,
                   ((pixel & compMask.blu) >> blu_shift) * maxval / blu_maxval
            );

    }
}



static void
convertRow(pixelReader *    const pixelReaderP,
           FILE *           const ofP,
           unsigned int     const padright, 
           int              const cols, 
           xelval           const maxval,
           int              const format, 
           struct compMask  const compMask,
           const xel*       const colors, 
           enum visualclass const visualclass) {
/*----------------------------------------------------------------------------
   Read a row from the XWD pixel input stream 'pixelReaderP' and write
   it to the PNM output stream 'ofP'.

   The row is 'cols' pixels.

   After reading the 'cols' pixels, we read and discard an additional
   'padright' bits from the input stream, so as to read the entire
   input line.
-----------------------------------------------------------------------------*/
    xel* xelrow;
    xelrow = pnm_allocrow(cols);

    switch (visualclass) {
    case StaticGray:
    case GrayScale:
    case StaticColor:
    case PseudoColor:
        convertRowSimpleIndex(pixelReaderP, cols, colors, xelrow);
        break;
    case DirectColor: 
        convertRowDirect(pixelReaderP, cols, colors, compMask, xelrow);
        
        break;
    case TrueColor: 
        convertRowTrueColor(pixelReaderP, cols, maxval, colors,
                            compMask, xelrow);
        break;
            
    default:
        pm_error("unknown visual class");
    }
    pixelReader_getbits(pixelReaderP, padright);
    
    pnm_writepnmrow(ofP, xelrow, cols, maxval, format, 0);
    pnm_freerow(xelrow);
}



static void
reportOutputType(int const format) {

    switch (PNM_FORMAT_TYPE(format)) {
    case PBM_TYPE:
        pm_message("writing PBM file");
    break;

    case PGM_TYPE:
        pm_message("writing PGM file");
    break;
    
    case PPM_TYPE:
        pm_message("writing PPM file");
    break;
    
    default:
        pm_error("shouldn't happen");
    }
}



int
main(int argc, char *argv[]) {

    struct cmdlineInfo cmdline;
    FILE * ifP;
    int rows, cols, format;
    unsigned int padright;
        /* Number of bits of padding on the right of each row */
    unsigned int row;
    int bitsPerPixel;
    int bitsPerItem;
    struct compMask compMask;
    xelval maxval;
    enum visualclass visualclass;
    enum byteorder byteOrder, bitOrder;
    xel * colors;  /* the color map */
    pixelReader pixelReader;

    pnm_init(&argc, argv);

    parseCommandLine(argc, argv, &cmdline);

    debug = cmdline.debug;
    verbose = cmdline.verbose;

    if (cmdline.inputFilename != NULL) 
        ifP = pm_openr(cmdline.inputFilename);
    else
        ifP = stdin;

    getinit(ifP, &cols, &rows, &padright, &maxval, &visualclass, &format, 
            &colors, &bitsPerPixel, &bitsPerItem, 
            &compMask, &byteOrder, &bitOrder,
            cmdline.headerdump);
    
    if (verbose) 
        reportInfo(cols, rows, padright, maxval, visualclass,
                   format, bitsPerPixel, bitsPerItem, compMask,
                   byteOrder, bitOrder);

    pixelReader_init(&pixelReader, ifP, bitsPerPixel, bitsPerItem,
                     byteOrder, bitOrder);

    pnm_writepnminit(stdout, cols, rows, maxval, format, 0);

    reportOutputType(format);

    for (row = 0; row < rows; ++row) {
        convertRow(&pixelReader, stdout,
                   padright, cols, maxval, format, compMask,
                   colors, visualclass);
    }

    pixelReader_term(&pixelReader);
    
    pm_close(ifP);
    pm_close(stdout);
    
    return 0;
}



/*
   This used to be the way we parsed a direct/true color pixel.  I'm 
   keeping it here in case we find out some application needs it this way.

   There doesn't seem to be any reason to do this hard-coded stuff when
   the header contains 32 bit masks that tell exactly how to extract the
   3 colors in all cases.

   We know for a fact that 16 bit TrueColor output from XFree86's xwd
   doesn't match these hard-coded shift amounts, so we have replaced
   this whole switch thing.  -Bryan 00.03.01

   switch (bits_per_pixel) {
                
   case 16:
       PPM_ASSIGN( *xP,
                   ( ( ul & red_mask )   >> 0    ),
                   ( ( ul & green_mask ) >> 5  ),
                   ( ( ul & blue_mask )  >> 11) );
       break;

   case 24:
   case 32:
       PPM_ASSIGN( *xP, ( ( ul & 0xff0000 ) >> 16 ),
                   ( ( ul & 0xff00 ) >> 8 ),
                   ( ul & 0xff ) );
       break;

   default:
       pm_error( "True/Direct is valid only with 16, 24, and 32 bits" );
   }
*/