MDenham wrote:

> From what I'm reading elsewhere, Y/Cr/Cb/K can be folded down to Y/Cr/Cb by just
> subtracting K from Y.  (Yes, this means you'd need to potentially support a
> negative luma component in the process of generating the colors at any given
> spot...  but shouldn't the internal calculation not have an issue with that
> anyway, given that negative channel values can crop up in OpenEXR files
> already?)
> 

You are confusing things here. Negative luminance is just impossible 
while having one or two negative RGB components (with at least one 
positive) is perfectly valid and is indeed needed to describe colors out 
of the gamut of the current RGB color system. In practice this means 
e.g. colors that are not displayable on your CRT/LCD monitor but do 
exist in the real world. Usually such colors are highly saturated 
'neon'-colors.

The transformation from YCCK to RGB would require 2 steps.
First transform from Y/Cb/Cr to CMY in the same way as the YCbCr -> RGB 
conversion is done (but actually you will get CMY values if 
jpeg_color_space is JCS_YCCK ) either by using a set of pre-build lookup 
tables like libjpeg does for speed reasons or by actually calculating 
the transformation in place.
Leave the K untouched and you get CMYK.
But the transformation from CMYK (being a color separation) to RGB is 
impossible without also knowing the used CMYK color space definition as 
defined e.g. within an ICC profile. POV-Ray does currently not support 
such color profiles.
And I know that there exists this simple 'formula' for converting CMYK 
to RGB but believe me, this 'formula' should have never existed in the 
first place. It once even found its way into the wikipedia article about 
CMYK that was a mess at this time anyway and it was quite some fighting 
involved to remove it. This happened a few years ago and meanwhile the 
article is a quite good one. So if you are interested you may want to 
look at the CMYK to RGB conversion section there for more details.


> Am I reading stuff from people who are grossly misinformed?

Might be ;)

-Ive

Post a reply to this message