Convert specific faces on planar mesh to image mask

Question

Suppose I have a planar mesh like the following screenshot, with 4 faces (and face indices sloppily drawn on):

Is there a way to turn this mesh into an image texture, to use as a material mask? If so, is there a way to use only certain faces as the mask? Lastly, can the faces being used as the image mask be dynamically updated? Like, start with face 1 as the mask, then use face 1 and 2, or 3 and 4, etc.?

I am not familiar with geometry nodes, but I imagine that will have to be utilized. Any help would be appreciated. Thanks,

Answer 1

If you select a face you want to use as a mask, add it to a new vertex group. Then in shader editor add an Attribute node, set the type to geometry and enter the name of the vertex group. Add a color ramp to finetune the masking area. Plug the color ramp output into the Material output. Add an image texture, create a new image, select said texture node. Now navigate to properties>render>bake, set the bake type to emit and leave all other settings as they are and just hit bake. You now got an image texture of the Vertex group, don't forget to save it.
I hope this helps.

Oh,make sure you're using cycles. After baking you can switch back to eevee if you need to.

Answer 2

is this what you are trying to achieve?

or using as mask:

and yes, it works "dynamic":

https://github.com/luckychris/blender_gifs/blob/main/dynamic%20material.gif

still SE cannot upload gifs...so i have insert a link - sorry.