Re: [Lcms-user] How does a transformation work

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

Hi,

>From your explanation, it seems to me that the transformation you want has nothing to do with color management. You know nothing about the L* of your grayscale and need only some scaling. I would rather use simple math in this case:

Gray_out = (Gray_in * 40) /51 + 40;

You could just use a for() loop. This is going to be faster that using any color management routines.

Regards

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




> -----Original Message-----
> From: flap <fb...@oh...>
> Sent: Friday, December 29, 2023 12:06 PM
> To: lcm...@li...
> Subject: [Lcms-user] How does a transformation work
> 
> Hi list,
> 
> what I want: an 8 bit grey-scale output for my printer, where the input dot
> values (0 … 255) are scaled and limited to values of 40 … 240 (…just an
> example).
> 
> So I created a simple test input data (256 elements of unsigned char) with a
> linear ramp from 0 (at index 0) … 255 (at index 255).
> 
> And I created an array with 4096 short entries beginning with 40 (* 256 for
> unsigned short) and ramp up linear to 240 (* 256 for unsigned short) and feed
> it into cmsBuildTabulatedToneCurve16() and cmsCreateGrayProfile() with
> cmsD50_xyY() as its whitepoint (for the output profile).
> 
> For the input profile I used cmsBuildGamma() with a value of 1.0 and
> cmsCreateGrayProfile() with cmsD50_xyY().
> 
> After creating the transformation with cmsCreateTransform() with
> input/output types both TYPE_GRAY_8 and an intent of INTENT_PERCEPTUAL
> and applying this transformation with cmsDoTransformLineStride () to my
> simple input data, the output result is still almost linear from 0 to 255 and not
> limited to 40 … 240.
> 
> So, I think I'm using the tone curves in a wrong way. But where is my mistake?
> 
> Cheers,
> Jürgen
> 
> 
> 
> 
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