where are chrominance and luminance parameters?

[Xavier Brochard]

Hi

I am looking for the chrominance and luminance quality parameters in the code. I've read some files and the 2 available APIs but didn't find them. Note that I am not a C programmer, only a modest sysadmin who try to understand some pieces of code.

Can someone could point me to the related files?

Also, I would like to know if they are in the API?

Any help welcome

Regards

[DRC]

If you are referring to the ability to set the JPEG quality
independently for chrominance and luminance components, that is only
available in the libjpeg API at the moment.

This code:

https://github.com/libjpeg-turbo/libjpeg-turbo/blob/c5db99e1aafe302598708c02a8bb9b7dcd5344a4/rdswitch.c#L80-L82
https://github.com/libjpeg-turbo/libjpeg-turbo/blob/c5db99e1aafe302598708c02a8bb9b7dcd5344a4/rdswitch.c#L282-L358

shows how cjpeg accomplishes it.

DRC

[Xavier Brochard]

Thanks a lot.

So, if I have well understood, you don't use jpeg_set_quality at all, but rather jpeg_add_quant_table ?

[DRC]

Yes.  jpeg_set_quality(cinfo, quality, force_baseline) is just a wrapper for

  jpeg_add_quant_table(cinfo, 0, std_luminance_quant_tbl, jpeg_quality_scaling(quality), force_baseline);
  jpeg_add_quant_table(cinfo, 1, std_chrominance_quant_tbl, jpeg_quality_scaling(quality), force_baseline);

You can simply copy those two tables (which are directly from the JPEG spec: https://www.w3.org/Graphics/JPEG/itu-t81.pdf, Annex K) into your own code and call:

  jpeg_add_quant_table(cinfo, 0, std_luminance_quant_tbl, jpeg_quality_scaling(luminance_quality), force_baseline);
  jpeg_add_quant_table(cinfo, 1, std_chrominance_quant_tbl, jpeg_quality_scaling(chrominance_quality), force_baseline);

(force_baseline is documented in libjpeg.txt.)

DRC

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