Re: PNG with indexed colors and tranparency

On Fri, 28 Jun 2019, Przemysław Sobala wrote:
>
> And it turns out that GraphicsMagick can't do that because of Cannot write 
> image as indexed PNG, writing RGBA message.
> Look for transparent colormap entry reveals num_alpha = 47and in coders/png.c 
> there's an if
>
>                       if (num_alpha == 1)
>
>                       else
>                       {
> ping_colortype=PNG_COLOR_TYPE_RGB_ALPHA;
>
> I tried some other PNG images with transparency and they all have 
> num_alpha>1.
>
> Does that mean that GraphicsMagick can't handle creating palette PNG images 
> with transparency? How can I obviate that? Is reducing number of transparent 
> colors in palette to 1 the best solution?

I am not sure about PNG but it is not unusual (e.g. GIF) that palette 
formats only support one transparent color.  I also know that there 
are some weaknesses in the implementation that Glenn did not address.

In GraphicsMagick, colormap entries do have an opacity property 
(inverse of transparency/alpha channel) but opacity is not considered 
for color reduction except that one color can be 'transparent' if the 
specified colorspace (-colorspace) is Transparent.

It may be necessary to assure that there is only one transparent color 
when attempting to write an indexed PNG with transparency.

We could really use a PNG expert to join the development team and work 
on improving the PNG writer.

Bob
-- 
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
Public Key,     http://www.simplesystems.org/users/bfriesen/public-key.txt

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