Re: [Lcms-user] Unexpected gray to RGB result

[<mar...@li...>]

Hello,

>As I am trying to make sense of it, I notice that the XYZ numbers you cited
are off by two digits. According to ICC.1:2022, chapter 6.3.4.3, they should
be:

 >> XYZ values for the PCS perceptual black point (X = 0,003357, Y =
0,003479, Z = 0,002869)

ICC PCS uses XYZ values divided by 100, so Y in white point is represented
by 1.0 instead of 100. Is just a matter of encoding.

Regards
Marti Maria
The LittleCMS Project
https://www.littlecms.com

-----Original Message-----
From: Ralf Junker <ral...@gm...> 
Sent: Wednesday, November 30, 2022 11:01 AM
To: lcm...@li...
Subject: Re: [Lcms-user] Unexpected gray to RGB result

On 29.11.2022 20:00, mar...@li... wrote:

 > ICC spec expects a "good" V4 perceptual profile to return "perceptual
black"
 > on darkest colorant (gray=0). This is around Lab=(3.14, 0, 0) or XYZ=(  >
0.3357, 0.3479, 0.2869).

Thanks for explaining!

As I am trying to make sense of it, I notice that the XYZ numbers you cited
are off by two digits. According to ICC.1:2022, chapter 6.3.4.3, they should
be:

 > XYZ values for the PCS perceptual black point (X = 0,003357, Y =
0,003479, Z = 0,002869)


I further noticed that lcms2.h uses slightly different values:

// V4 perceptual black
#define cmsPERCEPTUAL_BLACK_X  0.00336
#define cmsPERCEPTUAL_BLACK_Y  0.0034731 #define cmsPERCEPTUAL_BLACK_Z
0.00287

https://github.com/mm2/Little-CMS/blob/caab4c07e60022a0f776b543eaa30785e2bb4
2ed/include/lcms2.h#L280-L283


I also noticed that src/cmscam02.c contains the floating-point literal
3.141592654 three times, all of which could probably be replaced by M_PI for
clarity, simplicity and accuracy.

Ralf


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