bug#74476: [PATCH] Explore JPEG loading without quantization

[Alan Third <alan@idiocy.org>]

On Sat, Nov 30, 2024 at 12:19:16PM +0200, Eli Zaretskii wrote:
> > Date: Fri, 22 Nov 2024 15:53:18 +0100
> > From:  Manuel Giraud via "Bug reports for GNU Emacs,
> >  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> > 
> > While trying to speed up "emacs as an image viewer", I found that emacs
> > is using libjpeg with color quantization and it seems that removing this
> > quantization could speed up JPEG loading a bit.
> > 
> > My simple limited benchmark:
> > 
> >           - M-: (clear-image-cache)
> >           - Open an image in folder with some large enough pictures in
> >             it (4000x3000 here)
> >           - M-: (benchmark-run 10 (image-next-file 1))
> > 
> > Here are the results I get:
> > 
> > without this path: (5.415405491 1 0.09232176400000025)
> > with: (3.079911418 1 0.0751190459999993)
> > 
> > I don't think that this patch could be applied as is (it is rather
> > ugly).  And I also think that I probably have missed some (many?) use
> > case (where color quantization is mandatory).  But I'm submitting this
> > patch anyway as a conversation starter on the subject.
> 
> Alan, any comments?  I know nothing about this "color quantization"
> aspect of JPEG images.

Hi Eli,

I'm not an expert on this part of the image code, but I think the jpeg
colour quantization path exists for colour mapped environments. For
example you can pass it an array of 256 colours and it will map all
the colours in the jpeg to those colours.

I don't think we need to do this. In most cases we're not in a colour
mapped environment, and the existing code doesn't ask the jpeg code to
map to the existing colour map anyway.

AFAICT Emacs provides its own functions for colour mapping, so I think
we should try turning off this quantization and rely on our own
methods. That works for PNGs and others, after all.

Manuel, I think you want to use lookup_rgb_color when setting the
pixels in the final image buffer. This should do the right thing. You
presumably also need to call init_color_table before using it, and I
see calls to colors_in_color_table and free_color_table which look
necessary, but I think they're already in the jpeg code.
-- 
Alan Third