Painting a Mask in the Compositor

i am trying to paint an alpha mask in the compositor.
Similar to painting layer masks in Photoshop.
I have two images which I mix together with a mix node.
I would like to use a black and white image in the “Fac” input of the mix node.
It would be ideal if I could create this black and white image directly in Blender.

What I have done so far:

  • open UV/Image Editor
  • create a new image by clicking the “Create new image” button.
  • set the same size as my comp and naming it “myMask”
  • created image node in compositor
  • set image node to “myMask”
  • started drawing on the “myMask” texture in the UV/Image Editor with the grease pencil.

The problem is, it doesn´t work. I can not see the grease pencis strokes on the image in the compositor window.

Am I doing something wrong? Is there a different way to do this?

You can do it but not with the grease pencil. You have to use the paint mode of the image editor and use this image in your comp without any problem.

Cool, thank you.
Is there a way to put the image of the Viewernode underneath the mask Layer? It would be helpful directly to see where i am painting the mask.

You could…Right in a 3d view. Material for the background image - Emission, 1.
I’d stay with Gimp if you ask me…

Well, I can not use Gimp because the company I currently work at doesn´t allow it for whatever reasons.
However, I have Photoshop here and have been using that for this unitl I discovered that Blender is a lot faster and less annoying with 32bit images than PS.
There doesn´t appear to be a possibility to quickly draw a quick mask with Blender in the compositor, though. Too bad.
Perhaps I can script something some time.

Of course you can use what you paint (even with Blender image editor paint) in the compositor. Load the image with the Image node and that’s it. What’s wrong with it?

Make sure to save your mask image first so that Blender doesn’t have to try and regenerate paint decisions.