Bug 686750

Summary:	color components are systematically too low
Product:	Ghostscript	Reporter:	Felix Pahl <fpahl>
Component:	General	Assignee:	Dan Coby <dan.coby>
Status:	NOTIFIED WONTFIX
Severity:	minor	Keywords:	bountiable
Priority:	P4
Version:	7.04
Hardware:	PC
OS:	Windows XP
Customer:		Word Size:	---

Description Felix Pahl 2003-02-26 06:55:40 UTC

I've looked at the color components in various intermediate
stages in gsciemap.c, especially in gx_cie_remap_finish.
Whenever I had values of my own to compare with, I found
that the ghostscript values were systematically below the
ones I had calculated, by something around 0.003. Note that
this is near 1/256, so this might be related to incorrect
rounding or to a difference in scaling 1.0 to 256 or to 255.
I'm not familiar enough with the ghostscript color mapping code
to investigate this myself, but if someone wants to investigate
it, I could provide examples of systematic differences with
respect to "correct" values calculated in double precision.

Comment 1 Jack Moffitt 2003-02-27 10:02:45 UTC

Assigning to Dan.

Comment 2 Dan Coby 2003-02-27 10:19:52 UTC

Note to self - reference bug #465921 - Retest if changes made.

Comment 3 Dan Coby 2005-04-13 15:43:11 UTC

Note:  I have been checking the math in the color look up logic.

There seem to be two sources of error.  There is no interpolation done with 
table entries which are less than 0.01 apart.  This is controlled by a value 
in gscie.h.

The table entries are clamp to the output range.  As a result, there can be 
errors in the interpolation if one table entry is clamped and the other is not.