[gs-bugs] [Bug 691922] Slightly wrong colors on RGBW model

bugzilla-daemon at ghostscript.com bugzilla-daemon at ghostscript.com
Sat Feb 12 18:59:40 UTC 2011


http://bugs.ghostscript.com/show_bug.cgi?id=691922

--- Comment #26 from Oldřich Jedlička <oldium.pro at seznam.cz> 2011-02-12 18:59:36 UTC ---
(In reply to comment #23)
> Second, the cups spec says that the W plane should only have values 0 and ~0.
> 0 where the pixel represents black text and ~0 everywhere else.  It is not
> spec'ed to be 1-C,1-M,1-Y,1-K. but rather RGB plus a key plate for
> non-ditherred, non-halftoned black text.  (The spec serendipitously came up on
> the PWG lists; I happened to read it yesterday.)

You are right, I've found it in

  http://www.cups.org/documentation.php/doc-1.4/spec-raster.html

The RGBW is pure sRGB with W channel as 0 and ~0 called "dedicated black text
channel". The specification says:

  The white channel is 0 for text (or "true") black, otherwise it must contain
the maximum color value: 1 for 1-bit, 3 for 2-bit, 15 for 4-bit, 255 for 8-bit,
or 65535 for 16-bit.

There is no word about the RGB values for text ("true" black), but I guess it
can be R=0, G=0, B=0, W=0. For other values it should be RGB=real value, W=255.

So my expectations were wrong and the old ghostscript's cups driver didn't do
the right job too. Anyway, there is some color correction in place that makes
the RGBW colors wrong according to what was originally in ghostscript 8.71.

I will generate some solid color boxes on one page to have some sample data.

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