[Lcms-user] Crafting a YCbCr profile with LittleCMS

[L. E. S. <am...@am...>]

Hi,

I am writing to this list in hope of getting some help for crafting 
profiles with LittleCMS. My objective is to craft a Rec.709 YCbCr to XYZ 
profile for Krita. The YCbCr -> XYZ conversion goes as this:

1. YCbCr -> normalized R'G'B. Source: Ford & Roberts (1998), eq. 105. 
<http://poynton.ca/PDFs/coloureq.pdf>

2. Normalized R'G'B -> linear RGB. Source: ITU-R BT.2087

3. Linear RGB -> XYZ. Source: Ford & Roberts (1998), eq. 102. 
<http://poynton.ca/PDFs/coloureq.pdf>


And for the XYZ -> YCbCr side of things,

1. XYZ -> Linear RGB. Source: Ford & Roberts (1998), eq. 103. 
<http://poynton.ca/PDFs/coloureq.pdf>

2. Linear RGB -> Normalized R'G'B. Source: ITU-R BT.2087

3. Normalized R'G'B -> YCbCr. Source: Ford & Roberts (1998), eq. 104. 
<http://poynton.ca/PDFs/coloureq.pdf>



My questions are as follows:

1. Are the pipelines correct? If not, what are the errors?

2. According to the ICC 4.3 standard, sections 10.10.1 and 10.11.1, to 
use the CLUT I must wrap it in two triplets of (identity) tone curves. 
But doing so simply renders the pipeline into an empty tag, so I need to 
downgrade to 2.2 and remove the tone curves from the pipeline. Why is 
this happening?



If anyone wants to check my code out, I have uploaded it at 
<https://github.com/amyspark/krita-ycbcr-profiles>. Please reply to this 
email with any questions you may have.

Best regards,

amyspark

-- 
amyspark 🌸 https://www.amyspark.me