lcms-user

[Lcms-user] How does a transformation work

[flap <fb...@oh...>]

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

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

[flap <fb...@oh...>]

Am Freitag, 29. Dezember 2023, 14:01:40 CET schrieb 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.

Hmm. It just was a test to understand how it (may) works.
The profile I create is used in the muPDF library to create a greyscale out 
from a PDF document. I would expect, any kind of PDF could be rasterized into 
a greyscale raster this way.

Cheers,
Jürgen

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

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

Ok, 

The procedure to create a gray profile is simple. 
First measure the transfer function of your gray device to XYZ. That could be a gamma curve or anything.
Then measure the white point of your gray space. The adaptation to D50 is already done by lcms.
Then create a tone curve with your transfer function. 
Finally, by using white point and curve, create a profile.

Please note you cannot map contone values for input to output since the output profile is unknown. So, you cannot map 0 to 40 and 255 to 240. You can only characterize which XYZ you get with gray = 0 and gray = 255.

Example:

    cmsToneCurve* tone;
    cmsUInt16Number curve[256];
    int i;
    cmsHPROFILE hProfile;

    for (i = 0; i < 256; i++)
    {        
        curve[i] = (cmsUInt16Number) round(65535.0 * pow(i / 255.0, 2.2));
    }

    tone = cmsBuildTabulatedToneCurve16(0, 256, curve);

    hProfile = cmsCreateGrayProfile(cmsD50_xyY(), tone);
    cmsFreeToneCurve(tone);

    cmsSaveProfileToFile(hProfile, "gray_test.icc");
    cmsCloseProfile(hProfile);

Regards

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


> -----Original Message-----
> From: flap <fb...@oh...>
> Sent: Friday, December 29, 2023 2:21 PM
> To: lcm...@li...
> Subject: Re: [Lcms-user] How does a transformation work
> 
> Am Freitag, 29. Dezember 2023, 14:01:40 CET schrieb
> 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.
> 
> Hmm. It just was a test to understand how it (may) works.
> The profile I create is used in the muPDF library to create a greyscale out from
> a PDF document. I would expect, any kind of PDF could be rasterized into a
> greyscale raster this way.
> 
> Cheers,
> Jürgen
> 
> 
> 
> 
> 
> _______________________________________________
> Lcms-user mailing list
> Lcm...@li...
> https://lists.sourceforge.net/lists/listinfo/lcms-user

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
> 
> 
> 
> 
> _______________________________________________
> Lcms-user mailing list
> Lcm...@li...
> https://lists.sourceforge.net/lists/listinfo/lcms-user