clarify applicable standard, add more precision

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

1 files changed, 12 insertions, 2 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;