miscellaneous update

git-svn-id: http://svn.code.sf.net/p/netpbm/code/userguide@2313 9d0c8265-081b-0410-96cb-a4ca84ce46f8

1 files changed, 26 insertions, 5 deletions

diff --git a/pamtogif.html b/pamtogif.html
index 48f8168e..61fe6ec7 100644
--- a/pamtogif.html
+++ b/pamtogif.html
@@ -64,7 +64,9 @@ readers conceivably could not recognize GIF89.
 <p>The GIF format is not capable of representing an image with more than
 256 colors in it (it contains a color map with a maximum size of 256).
 If the image you want to convert has more colors than that (<b>ppmhist</b>
-can tell you), you can use <b>pnmquant</b> to reduce it to 256.
+can tell you), you can use <b>pnmquant</b> to reduce it to 256.  Or
+use the more complex but faster method described under the <b>-mapfile</b>
+option.
 
 <p>If your input image is a PAM with transparency information, <b>ppmtogif</b>
 uses one entry in the GIF colormap specifically for the transparent pixels,
@@ -121,10 +123,29 @@ duplication of colors in the image is irrelevant.
 
 <p>The map file's depth must match the number of color components in
 the input (which is not necessarily the same as the input's depth --
-the input might have an alpha plane in addition).  If your map file
+the input might have a transparency plane in addition).  If your map file
 does not, or it <em>might</em> not, run your input through
 <b>pnmremap</b> using the same map file so that it does.
 
+<p>You can use <b>-mapfile</b> to speed up conversion of an image where you
+already have a map file because of earlier processing of your image.  For
+example, it is common to start with an image that has more than 256 colors
+and remap its colors to a set of 256 colors so that <b>pamgtogif</b> can
+convert it (a GIF can have only 256 colors; <b>pamtogif</b> without
+<b>-mapfile</b> fails on any image that has more than that) with
+<b>pnmquant</b>.  When you do this, <b>pnmquant</b> generates a palette to do
+the color quantization, then <b>pamtogif</b> generates an identical palette
+from the quantized image.  You can save computation by generating the palette
+once:
+
+<pre>
+<kbd>
+    $ pnmcolormap 256 myimage.ppm &gt;/tmp/colormap.ppm
+    $ pamtogif myimage.ppm -mapfile=/tmp/colormap.ppm &gt;output.gif
+</kbd>
+</pre>
+
+
 <DT><B>-transparent=</B><I>color</I>
 
 <DD>
@@ -161,9 +182,9 @@ such an option because it was not capable of taking PAM input that contains
 a transparency (alpha) plane, so one used this option to supply a
 transparency plane as a separate PGM file.
 
- This option names a PGM file that contains an alpha mask for the
+ This option names a PGM file that contains a transparency mask for the
 image.  <B>pamtogif</B> creates fully transparent pixels wherever the
-alpha mask indicates transparency greater than 50%.  The color of
+transparency mask indicates transparency greater than 50%.  The color of
 those pixels is that specified by the <B>-alphacolor</B>
 option, or black by default.
 
@@ -172,7 +193,7 @@ addition to the entries for colors that are actually in the image.  It
 marks that colormap entry as transparent and uses that colormap index
 in the output image to create a transparent pixel.
 
-<P> The alpha image must be the same dimensions as the input
+<P> The transparency image must be the same dimensions as the input
 image, but may have any maxval.  White means opaque and black means
 transparent.