TYPO3 Core

Currently you need to specify both an bgImg (the one to overlay) as well as a mask that masks which parts should be overlayed. That mask is converted to a grayscale-image.

If you for example use a png with transparency as bgImg you are free to use a mask that instructs to use "all" pixels with full opacity. When the mask is applied the included transparency of the png still has an effect.

So when using a bgImg which has an alpha-channel itself or if you want to apply a simple "stamp" onto your image (square-image, no transparency at all) then it would make sense to be able to leave out the mask-parameter completely.

For basics how to use/test with bgImg and mask for example see #59392.