I like add paint to game walls, but using decals is a overkill, and i see the portal 2 technical: http://www.valvesoftware.com/publications/2011/gdc_2011_grimes_nonstandard_textures.pdf
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But no idea about how make this material and change in-game, any idea or tutorial for this ?
The material part should not be particularly hard, except that you’ll have to make sure that every object that can be hit by it has a material that can blend to a painted state. Look up one of the many vertex blending examples for guidance. However, unlike those you’ll want to use a mask texture that uses a mesh’s lightmap coordinates instead of its vertex colors to blend between regular and painted material states.
The more difficult part will be the generation of the mask textures. You’ll have to generate these in blueprints and/or code. A basic implementation would be to generate a low resolution mask for each object you paint. However, if you want to do this on a large scale you’ll end up with a very large number of mask textures. Instead you might have to dive deeper into the code and generate a set of packed mask atlases that behave like the packed lightmaps Unreal uses.
I personally believe that the easiest thing to do would be to make a blueprint at the emitter that spawns actors semi-randomly, with variation in size. These will be individual blobs. These actors will also have a decal attached, and upon impact to specific mesh, they will die, leaving behind the decal. Not sure how decal can be used to validate a surface upon which portal placement is ok, but this will be the easiest thing to do for aesthetic purposes. The gel itself should really just be a well -divided icosphere with an animated displacement map. Doesn’t seem too hard… You could do tesselation on the sphere.