MacOS Finder Image Thumbnail and QuickLook Preview Limited to 20-21 Alpha Channels?

[Chris.S]

Issue Summary:
MacOS Finder does not generate image previews for images with more than 21 Alpha Channels.

Steps to Reproduce:

1. Create or obtain an RGB image with more than 21 Alpha Channels (or use the sample files attached).
2. Save the image in .tif, .psb, or .psd format with "Maximize PSD and PSB File Compatibility" enabled in Adobe Photoshop (for greatest file compatibility; though this doesn't impact results).
3. Observe the lack of a preview/thumbnail from a Finder window (any view, i.e., list, icon, etc.) or via QuickLook.

Expected Behavior:

MacOS should generate and display an image preview/thumbnail regardless of the number of Alpha Channels.

Observed Behavior:

• This issue presents itself regardless of the view in the Finder window (i.e., list, icon, etc.).
• The same issue occurs with QuickLook, whether via the simple approach of using the space bar on the image, when viewing the preview in the Preview Pane, or using the 'qlmanage' command.
• The default file icon (tif/psb/psd) is shown instead of an actual image thumbnail in these cases.
• The `qlmanage` thumbnail command (below) hangs when attempting to generate previews for these images.
• Image size is not a limiting factor; tests failed on images less than 150 KB.
• This behavior occurs regardless if the image has a transparent background (transparent background + more than 20 Alpha Channels) or has been flattened (more than 21 Alpha Channels).

• Notably, another observation is that when the image layers are flattened, but the image has 21 Alpha Channels, an image thumbnail will be visible from the Finder window, but the QuickLook Preview will only show the full image if the top Alpha Channel is White... if it's Black, QuickLook Preview will not show anything. If more than 21 Alpha Channels, the image thumbnail reverts to the generic Photoshop thumbnail.

• Having a White Alpha Channel at the top of the Alpha Channels is a general workaround to avoid only part of the image from being displayed, but this does not fix the issue when the number of Alpha Channels is in the 21+ range.

Use Case:
In a custom application on MacOS, the following shell script is used to generate image previews:

qlmanage -t [sourcePath] -s 512 -o [outputPath]

This script hangs if the image has more than 21 alpha channels, even if the image is small in size.

Testing Environment:
While not limited to this specific environment, testing was performed on the latest software/hardware:

• MacOS: Sonoma 14.5
• Adobe Photoshop: 2024 (25.6.0); "Maximize PSD and PSB File Compatibility" enabled
• Device: 2023 MacBookPro 16-inch M3 Max 128GB

Attachments:
Screenshot attached for a quick visual of the missing thumbnails along with the simplified test files.

Suggestions Requested:
Any insights or solutions to ensure image previews are generated correctly for images with more than 21 alpha channels would be greatly appreciated.

* Note: This question has been posted to Apple Community & Developers Forums as well.

[Lumigraphics]

I would ask on the Apple Developer forums. Just be aware that most Apple engineers are busy with prep with the developer conference coming up soon.

[Chris.S]

Thanks. Will post there as well.