Re: PNG pictures have gamma correction twice applied

[David.Kastrup@t-online.de (David Kastrup)]

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Richard Stallman <rms@gnu.org> writes:

> We don't want to change the PNG library.  We want to use it
> unmodified.  Can you find a solution to this one problem
> that invoves changing Emacs and not the PNG library?

I have verified that the following patch will stop the double gamma
correction particular to PNG images.  I _strongly_ stress that this
is _not_ the proper way to fix Emacs' lack of proper color handling
here and elsewhere.

For that reason, I will not apply this patch myself.


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*** xfns.c.~1.565.~	2002-11-14 17:21:29.000000000 +0100
--- xfns.c	2002-12-05 17:35:07.000000000 +0100
***************
*** 9004,9010 ****
    screen_gamma = (f->gamma ? 1 / f->gamma / 0.45455 : 2.2);
  
    /* Tell the PNG lib to handle gamma correction for us.  */
! 
  #if defined(PNG_READ_sRGB_SUPPORTED) || defined(PNG_WRITE_sRGB_SUPPORTED)
    if (png_get_sRGB (png_ptr, info_ptr, &intent))
      /* The libpng documentation says this is right in this case.  */
--- 9004,9010 ----
    screen_gamma = (f->gamma ? 1 / f->gamma / 0.45455 : 2.2);
  
    /* Tell the PNG lib to handle gamma correction for us.  */
! #if 0
  #if defined(PNG_READ_sRGB_SUPPORTED) || defined(PNG_WRITE_sRGB_SUPPORTED)
    if (png_get_sRGB (png_ptr, info_ptr, &intent))
      /* The libpng documentation says this is right in this case.  */
***************
*** 9017,9023 ****
    else
      /* Use the standard default for the image gamma.  */
      png_set_gamma (png_ptr, screen_gamma, 0.45455);
! 
    /* Handle alpha channel by combining the image with a background
       color.  Do this only if a real alpha channel is supplied.  For
       simple transparency, we prefer a clipping mask.  */
--- 9017,9023 ----
    else
      /* Use the standard default for the image gamma.  */
      png_set_gamma (png_ptr, screen_gamma, 0.45455);
! #endif
    /* Handle alpha channel by combining the image with a background
       color.  Do this only if a real alpha channel is supplied.  For
       simple transparency, we prefer a clipping mask.  */

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-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum