forced palette for RGBA images

thomas bonfort thomas.bonfort at GMAIL.COM
Wed Dec 12 03:46:19 EST 2007

A quick update on this.
I've now also added quantization for rgba images ( based on the median
cut implementation of pngquant / ppmquant: ).

examples here (the first two images are actually transparent. you'd
have to open them in an image editor to see that):

the full rgba png: : 129k

the quantized rgba png (may look funny under oldish MS IEs) : 39k

the output produced with the current gd quantization

I'd personally enable the pngquant quantization for rgb images to , as
it produces higher quality results, but there's a tradeoff on
rendering speed (rough times for quantization only: 1.5 sec for
pngquant vs 0.3 sec for gd, on a 2000x2000 image). Or this could be
yet another formatoption selectable option.

Dan: all this will be documented. We've talked with Assefa about
either writing a complete reference on the different formatoptions,
and/or a howto on reducing image size with agg (palette, quantization,
and tradeoffs beween speed/imagesize/flexibility)

Now for the questions:
* how should I add this? I'd go for a configure option
(--disable-experimental-png) with ifdef'd code as I don't know how
disruptive this would be on all platforms.
* can I add this soon? It's stable on my setup, but there might be
issues on other platforms. I'd like to have it tested as much as
possible before 5.2


On Dec 7, 2007 10:46 PM, Daniel Morissette <dmorissette at> wrote:
> Sounds like a cool feature. We'd need to make sure it is well documented
> and not just hidden in the source. A short RFC or ticket for this maybe?
> Daniel
> thomas bonfort wrote:
> > hello list,
> >
> > I've added the possiblity to force a color palette for RGBA images. As
> > gd does not support this kind of images, I had to resort to directly
> > use libpng functions for the actual saving of the file.
> >
> > The steps needed to actually use this would be:
> > - create a 24 bit rgba image representative of a typical image that
> > would be produced
> > - use an external program to create an rgba paletted version of the
> > image (pngquant, pngnq, photoshop...)
> > - extract the palette from the created image and save the values in
> > the form r,g,b,a in a text file (one per line)
> > - use palette_force and palette formatoptions in the outputformat block.
> >
> > This is rather complicated, but usefull on production setups to
> > minimize image size when using clients that overlay layers (eg
> > openlayers).
> >
> > There are still a few quirks I'd have to work on before the code is
> > robust enough for inclusion in the trunk, so I'd like to know if this
> > is a feature we'd definitely like to have before putting too much
> > effort into that.
> >
> > cheers,
> > thomas
> --
> Daniel Morissette

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