I cannot get the depth image. The particle system will not be seen outside the water; otherwise, I would have to place trails when the object is in the water. I bet when you did this test you were probably in Forward mode, but most other examples you saw were from people using Deferred or from people on OpenGL platforms. Graphics Textures Render Texture. This could be useful for example if you render a half-resolution depth texture in script using a secondary camera and want to make it available to a post-process shader.
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