A 24-bit image is the same visually as a 32-bit image with a solid (opaque) alpha. You can import and render 24-bit images and ignore the alpha, but you can't "delete" the alpha from the internal processing - combustion already ignores the alpha when doing operations if it detects that the alpha is solid. Inverting the alpha to 100% black for 24-bit footage will remove the image from view, so you probably don't want to do that.
For new paint or solid layers, if you want to start with a black alpha channel, just check the "Transparent" button when creating the paint or solid, or click on the source solid footage and change the opacity to 0%.
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Greg Niles
combustion Product Specialist
Discreet