Photo: Export transparent PNG with auto palette broken

[dPRogerWilco]

If I want to export a layer with transparency in Photo 2.5.5 as PNG with palette "auto", then the preview (exported file) just shows an empty picture.
PNG without palette and not automatic palettes seems to work.
I remember that some minor version before had already a similar bug. Was it introduced again?

[walt.farrell]

Welcome to the Affinity forums.

Do you really mean to be exporting a 32-bit (RGB-32) PNG file? More commonly RGB-8 or RGB-16 would be used, and I'm not sure it makes sense to be using RGB-32 and Palettized. For RGB-8 and 16 Palettized works fine for me.

You can change the Pixel Format in your Export Settings:

[dPRogerWilco]

Thanks, didn't noticed it (I assume I tested the different presets). 

But sadly it's also not working with any of the other pixel formats (including keeping document format).
Weird that it works for you, maybe something wrong with my picture? I attached it now to this post in the afphoto format.
What I also noticed: picture is 403 pixels high in the main view, but on the export dialog it changes to 404 pixels...

example.afphoto

[walt.farrell]

45 minutes ago, dPRogerWilco said:

What I also noticed: picture is 403 pixels high in the main view, but on the export dialog it changes to 404 pixels...

Your document height is actually 403.334501px, and since exported images must use whole pixels, it's rounded up.

48 minutes ago, dPRogerWilco said:

Weird that it works for you, maybe something wrong with my picture? I attached it now to this post in the afphoto format.

Thanks for sharing the image. Your Curves Adjustment is what causes the problem with the Palletization, but I have no idea why. Hiding that layer allows the Preview to work with Palletization specified.

[NotMyFault]

Palettized option seem to need matting enabled, or an document with fully opaque content. Then it works.

Algorithm seems to use alpha value of transparent pixels which are the majority in you file.

 

if you merge visible, or deactivate curves adjustment layer, palettised export works. So workaround is simple. But probably a bug.

[dPRogerWilco]

Thanks both for your help.

Interesting, that a picture can have non natural numbers as dimension.

Back to my export issue:
I use the curve adjustment layer to manual control the "binary" mask, because indexed PNG doesn't have semi transparency and the export dialog does not have a adjustment for this.
I tried now your suggestions and merged the layers. I also exported the file as non indexed PNG (see attachment) to be sure that nothing is left of my layers and half pixels. But still, export as indexed PNG with palette does not work for me.

[NotMyFault]

you curves adjustment does not fully mitigate the issues. When I compare the rasterized layer against the source by blend mode difference, it shows minor differences.

I get better results (no differences) when using a levels adjustment, set both black and white level to 50%.

[NotMyFault]

2 hours ago, dPRogerWilco said:

But still, export as indexed PNG with palette does not work for me

You curve is not correct. the nodes must be exactly 0.5 / 0.0 and 0.5 / 1.0.

Both nodes are sligthly off, leading to alpha of 254 instead of 255.

If you set the nodes by numeric input to the exact position, it will work.

Thos minor details matter.

 

[NotMyFault]

To summarize, i don't think we have a bug.

Like in some other functions (e.g. stack), alpha channel is treated "binary". only alpha = 1 gets exported, all smaller values are treated as 0.

The wrong export was caused by a curves adjustment which lead to alpha = 254 / 255, thus treated as zero at export.

The info panel will show the issue of partial alpha.

[dPRogerWilco]

Thanks a lot for your help, that was the reason it was not working... such a minimal mistake.  

It probably only sees exact 1.0 as opaque, because it wouldn't know what background color it should use to calculate the RGB result otherwise. But from usability point of view a threshold of 0.5 (or even selectable) would be better.