MacOS Finder Preview/Thumbnail Generation Limited to 20-21 Alpha Channels?

Question

Chris-S OP

Created May ’24

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I am using the following shell script to return an image preview for use in FileMaker:

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

This usually works well, but it hangs if the RGB image (.tif, .psb, or .psd) has too many Alpha Channels ( >20 if on transparent background; >21 if flattened).

This issue can be also be seen when looking at the image thumbnail or preview in the Finder. It appears MacOS won't create a thumbnail when the image has over 21 Alpha Channels... it just shows the default tif/psb/psd thumbnail, even if the image is very small.

Environment

MacOS Sonoma 14.4.1
Adobe Photoshop 2024 (25.6.0)
Maximize PSD and PSB File Compatibility is enabled when saved from Photoshop

Since I'm only able to upload a screenshot to this post, the original test files can be found in the Adobe Forum with the Title: "MacOS Finder Preview Limited to 20-21 Alpha Channels?"

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Answer 1

Chris-S OP

Jun ’24

Additional Notes:

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 (above) 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.

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