clarify applicable standard, add more precision

git-svn-id: http://svn.code.sf.net/p/netpbm/code/trunk@3164 9d0c8265-081b-0410-96cb-a4ca84ce46f8

2 files changed, 17 insertions, 9 deletions

diff --git a/lib/pm_gamma.h b/lib/pm_gamma.h
index 3c48e52e..00d551fc 100644
--- a/lib/pm_gamma.h
+++ b/lib/pm_gamma.h
@@ -15,13 +15,23 @@ extern "C" {
 static __inline__ float
 pm_gamma709(float const intensity) {
 
-    /* Here are parameters of the gamma transfer function
-       for the Netpbm formats.  This is CIE Rec 709.
+    /* Here are parameters of the gamma transfer function for the Netpbm
+       formats.  This is ITU-R Recommendation BT.709, FKA CIE Rec 709.  It is
+       also ITU-R Recommendation BT.601, FKA CCIR 601.
 
        This transfer function is linear for sample values 0 .. .018
        and an exponential for larger sample values.
        The exponential is slightly stretched and translated, though,
        unlike the popular pure exponential gamma transfer function.
+
+       The standard actually defines the linear expansion as 4.500, which
+       means there is a discontinuity at linear intensity .018.  We instead
+       use ~4.514 to make a continuous function.  This may have been simply
+       a mistake when this code was written or based on an actual benefit
+       to having a continuous function -- The history is not clear.
+
+       Note that the discrepancy is below the precision of a maxval 255
+       image.
     */
     float const gamma = 2.2;
     float const oneOverGamma = 1.0 / gamma;
diff --git a/lib/ppm.h b/lib/ppm.h
index 5b94af9e..6f1f21b5 100644
--- a/lib/ppm.h
+++ b/lib/ppm.h
@@ -211,9 +211,7 @@ PPM_DISTANCE(pixel const p1,
    combination of intensities, whereas luma is a linear combination of
    gamma-adjusted intensities, as you would find in a Netpbm image.
 
-   These are from ITU-R BT.601.5.  That means they probably aren't technically
-   right for use with PPM images, because the PPM spec says ITU-R BT.709.
-   The two are similar, though.
+   These are from ITU-R BT.601.5.
 */
 #define PPM_LUMINR (0.2989)
 #define PPM_LUMING (0.5866)
@@ -222,12 +220,12 @@ PPM_DISTANCE(pixel const p1,
 #define PPM_LUMIN(p) ( PPM_LUMINR * PPM_GETR(p) \
                        + PPM_LUMING * PPM_GETG(p) \
                        + PPM_LUMINB * PPM_GETB(p) )
-#define PPM_CHROM_B(p) ( -0.16874 * PPM_GETR(p) \
-                         - 0.33126 * PPM_GETG(p) \
+#define PPM_CHROM_B(p) ( -0.168736 * PPM_GETR(p) \
+                         - 0.331264 * PPM_GETG(p) \
                          + 0.5 * PPM_GETB(p) )
 #define PPM_CHROM_R(p) ( 0.5 * PPM_GETR(p) \
-                         - 0.41869 * PPM_GETG(p) \
-                         - 0.08131 * PPM_GETB(p) )
+                         - 0.418688 * PPM_GETG(p) \
+                         - 0.081312 * PPM_GETB(p) )
 
 pixel
 ppm_color_from_ycbcr(unsigned int const y,