Unity: Apply an Image Effect on one layer only

Question

Please consider this simple scene.

It’s composed of 3 different game objects. 1 background image + 1 static character image + 1 small part (with the pet) that is changed at runtime to perform an animation.

The Background is rendered using a dedicated camera. The character (static + anim) is rendered using another dedicated camera. Both camera are set to Depth Only with a different culling mask (each camera renders only its part). Note that the camera in charge of the character has a greater depth than the one in charge of the background (i.e. Character is rendered after the background).

I need to apply an image effect only on the character camera, but I can’t manage to find a way for this effect to not affect the background.

Example here with a Vortex effect on the character camera.

How would you do such a trick?

Answer 1

The problem is that the image effect grab the current screen after render.

Since your camera render directly to backbuffer (they only clear depth), the image effect is apply to everything.

Solution can be :

• Setup a Camera first that clear to a transparent color, with a setup RenderTarget (you can create it by script so it is the same size as your screen) and the wanted postprocess.
• Setup a Camera second that render your background, that clear to background color/skybox
• Setup a Camera third, that render nothing (culling mask set to Nothing), clear nothing and have the AlphaBlit.cs script attached to it. AlphaBlit need two info : a material that use the AlphaBlit.shader and the rendertarget to blend (you'll need to assign it by script if you created it by script in the 1st step)
• Setup a Camera fourth that render your forward character.

CAREFUL: this is a painter algorithm. Meaning everything the third camera render is rendered on top of what already there, and the fourth camera will render on top that.

I'm not sure there is a way to do better, as it would require to also distort the depth buffer, which is not possible... (not without directly rendering with a vortex effect which is more difficult)...

Of course the more optimised way to do it would be to not apply the postprocess on the 1st camera, but just make it render in a rendertarget and modify the AlphaBlit.shader to do the vortex effect as it blit that RenderTarget. But that's more complexe.