For an intuitive painting workflow it's necessary to have an eraser at your finger tips at all times.
This has been a shortcoming in Blender for painting, since the erasing of alpha channels has only been available as a blending mode for brushes.
This is very hard to access via the UI and shortcuts.
The ideal solution is to have a very simple brush tool in the toolbar. Aside from the typical texture, stroke, fallof and cursor settings, it can only erase alpha.
This tool can then have the shortcut E assigned by default.
Pure Emissive Colors
When painting float or premultiplied images, the alpha is treated differently. Aany color with an alpha 0 would ahve the RGB colors displayed as pure emmissive colors and still visible.
The only way to erase the colors completely is to also erase the RGB channels to 0.
As a result the design for the brush should emphasise this: The Erase brush is a destructive tool!
If you erase the alpha directly via painting tools you should expect to likely lose the full RGBA color information.
The alpha 0 areas can be painted over with a new color afterwards.
For non destructive erasing of alpha values it is recommended to use a separate image as a mask. This can be done right now via nodes.
This non-destructive approach might not be very accessible right now but that can be addressed separately (Especially with the layered textures design)
There are and will be various other tools that will affect color. It will be beneficial to have the opportunity to erase colors with them as well.
- Gesture Tools - Box/Lasso/Line Erase
- Color Filter - Erase Filter Type
An alternative approach that would be good to make possible is to make the "Erase Alpha" blending mode in the regular painting tools more accessible.
In other painting application such as Krita every tools and brush has an "Erase Mode" button that can be easily toggled with a shortcut and applies globally to all tools and brushes.
We could implement this design as an alternative to the Erase Tools for users that prefer this workflow.
In that case the Erase Alpha blending mode would be a button in the header that can be toggled and have a shortcut assigned, instead of being part of the blending modes popup.
This toggle should be available for for all brushes and tools that affect colors.
This could be easily accessible with the shortcut Alt E so both workflows of erasing are supported by default.