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Export 1 Bit PNG with Alpha
Hello there,

in GIMP 2.8 it was easily possible to export images as true 1 bit PNG files with binary Alpha: You simply create a small transparent image, draw a few black pixels onto it, and turn it into indexed mode image ("black & white palette", 1 bit). Gimp 2.8 correctly recognizes this as an image with 1 color only and when exported as PNG (I always uncheck all export options), it is saved as 1 bit PNG (with transparent & black as the only available colors in the palette, because no white is used).

Whereas in GIMP 2.10, when I repeat exactly the same steps, it always turns out to be a 2 bit image (transparent + black + white), although no white pixel is set anywhere in the image. It seems it refuses to reduce the image into 1 BPP palette, because it insists to have at least black & white, and thus won't allow me to export transparent 1 bit PNGs at all.

Is there anything that I have to configure to allow me to use this GIMP 2.8 feature again? It may seem like no big deal to anyone else, but this is a drawback for me, which keeps me stuck with GIMP 2.8 once again.

Maybe some expert can help with this issue.
Thanks in advance.
I wonder if that is a bug in Gimp 2.10 or intentional or was an undiscovered bug in Gimp 2.8

Try your indexed conversion using a maximum number of colours 2 (the minimum possible)


and a regular export to a png


Seems to work here, the colormap just shows black.  I did open the exported file in a (linux) Gimp 2.8, still ok.  Let us know your findings.

Edit: A bit more exporting / reopening in Gimp 2.10 - weird results Get back 2 colours in the colormap sometimes black/black sometimes a-color/black
These look ok in the Gimp 2.8 colormap and no great difference using ImageMagick identify -verbose. Must be a Gimp 2.10 bug
Hi rich,

thank you very much for verifying that this isn't simply a problem on my side, but probably rather a GIMP 2.10 bug.

Also, now that you told me about weird GIMP behaviour after importing 1 bit alpha PNGs, I can remember that in some cases when I opened one of those images in GIMP 2.10, they appeared completely empty (transparent), although the files are perfectly ok in several other programs.

I guess I will just keep using GIMP 2.8 then, until some day it is fixed hopefully.
(09-08-2019, 04:13 PM)BileDemon Wrote: I guess I will just keep using GIMP 2.8 then, until some day it is fixed hopefully.

It won't be fixed if you don't report the problem (the last bug I reported was fixed in hours).

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