Re: Importing GIF looses transparency (w/ConstituteImage)

On Thu, 15 Oct 2020, [email protected] wrote:

> Hello Bob,
>
> On Wed, 14 Oct 2020 at 18:52, Bob Friesenhahn
> <[email protected]> wrote:
>> Libvip's implementation of magick_import_pixels() does not appear to
>> be very efficient since it is creating a temporary image.
>
> It's copied from this compatibility wrapper I found:
>
> https://sourceforge.net/p/graphicsmagick/code/ci/default/tree/wand/magick_compat.c#l315

This is likely code that I wrote.

> Something quicker would be great.
>
> I added:
>
>    image->matte = constitute_image->matte;
>
> And GM is now saving transparency! Thanks!
>
> It's still writing 16-bit images though. Is there something I can set
> to get StorageType passed along?

The image components will always be scaled up/down to the size of 
Quantum because GraphicsMagick's image model is exceedingly simple.

The way to influence the default depth when writing is image->depth 
(e.g. image->depth=8).

The StorageType argument does imply a particular size, which may or 
may not fit directly into Quantum.  The maximum depth value supported 
by Quantum may be obtained from the QuantumDepth macro, which is fixed 
for any given GraphicsMagick build.

If QuantumDepth is 16, then ShortPixel will fit exactly, and CharPixel 
will be scaled up (but could be claimed to have a 'depth' of 8).  So 
it may make sense to set the default depth to the maximum bits given 
QuantumDepth and the size denoted by StorageType.

I would like to be able to say that the solution is just to add

   image->depth = constitute_image->depth;

but I see that ConstituteImage() has a bug in that it is not 
automatically setting the image depth parameter based on the size 
denoted by StorageType.  I can fix this but it would not help you for 
a very long time.  Sorry about that!

Bob
-- 
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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