Export bicubic or higher resampling introduces artifact

[TwoKoalas]

On export to PNG or TIF I'm noticing a solid transition line between two overlapping layers that's not visible in the viewport.
This starts appearing when exporting with bicubic or higher. 

This can be fixed if I manually rasterize the layers before export. I thought of filing this because might expose some error under the hood.

Bicubic:

Bilinear

 

export resampling error.afphoto

Edited September 9, 2022 by TwoKoalas

[David in Яuislip]

Yes, I see the same but I note that your pix layer does not have consistent colour whereas the lowest layer does
If the pix layer is recoloured to be 231 230 228 then it works for any resample algorithm
This is only an observation, I have no explanation

[TwoKoalas]

Thanks for checking this David. I know the values are not the same, but close. It's something I extrapolated and prepared from a real file I was working on.

In any case, I don't see why it should create such a hard transition at the transparency point. Maybe it's an alpha pre-multiplication issue.

[David in Яuislip]

Blimey I'm slow, just noticed that the bottom pixel row of the pix layer is partially transparent. I can't paint over it to make it solid colour, I tried a curve layer set to Alpha but got nowhere so I deleted it and got a surprise. That layer seems to be well and truly buggered and it may well turn me to drink. Good luck

ps I finally outnerded myself and used a rectangle to clip off the lowest two rows of the pix layer and it worked
I shall now have a beer