Tileable Image - Rescaling Issue with Alpha Channel

[carstenj66483113]

Hi,

I work in game development, where it's common to use tileable textures that include additional information stored in the alpha channel of an image. For instance, we often assign the RGB channels to albedo data and reserve the alpha channel for roughness data.

Recently, we encountered an issue with the alpha channel of our textures. When downscaling the image in Photoshop (using "Bicubic Sharper" or other similar algorithms), the alpha channel displays visible seams along the edges. Interestingly, this problem does not affect the RGB channels, which scale down without any issues.

I suspect that the problem relates to the algorithm's handling of neighboring pixels, though it seems the alpha channel is processed differently than the RGB channels.
This poses a significant challenge, as our designers frequently need to downscale numerous textures. Is there a solution or an effective workaround for this issue? Unfortunately, using bilinear rescaling is not a viable option, as it results in overly blurred textures.

Thank you.

[Sameer K]

Hey, @carstenj66483113. Welcome to the Photoshop Community. I'll need more information to help you figure this out. Please share a video with this behavior and the system info from Photoshop Help > System info > Copy and paste into a text document > Upload and attach here.

Does it behave as mentioned after a specific set of steps? What troubleshooting steps have you tried already?

If this is a recent behavior change, you can try installing a previous version of Photoshop: https://helpx.adobe.com/download-install/using/install-previous-version.html

If it does help, please share the specific version of Photoshop where the behavior changed. It'll help us investigate this.

Also, please test and confirm if the issue exists with Photoshop (Beta). You can get Photoshop (beta) from the Creative Cloud > Apps tab > Beta Apps section.

Thanks! 
Sameer K

(Type '@' and type my name to mention me when you reply)

[carstenj66483113]

Hi @Sameer K, thanks for your response.
I've attached a video recording to reproduce the issue.
The beta and older versions have the same issue.

[c.pfaffenbichler]

Non-background Layers that completely fill the canvas seem to feature some artefacting in their transparency after resampling, too. 

If no solution should become available one could automate a work-around (at least for a file with one background layer and x alpha channels). 

[Sameer K]

Thanks for confirming this behavior, @c.pfaffenbichler, and for sharing the details, @carstenj66483113. 

To confirm again, what version of Photoshop did you notice this functionality changed? 

Sameer K

[carstenj66483113]

@Sameer KI’m currently using Photoshop Version 26.4.1. The same issue occurs in other versions of Photoshop as well, so I can’t confirm whether rescaling the alpha channel has worked in the past. I only recently noticed this problem while working on some textures.

[c.pfaffenbichler]

Please provide a sample file (pre- and post-scaling).

Do you mean an alpha channel or transparency? 

Edit: Never mind, I can reproduce in either case.