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This is a part of my Audiosurf 2 scripting library.

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Manual Post-Processing Example

With custom shaders it has always been possible to use special vertex shaders to position geometry directly in screen space without using AddPostEffect. This can be used to draw a full-screen quad and we can sample the screen using GrabPass. It can also be used with BuildMesh for custom HUD elements, which I have used for debugging my skins. Here is the essence of what I have been doing for post-processing:

RevealAlpha.shader

Shader "RevealAlpha" {
Properties {
}

SubShader {
	Tags { "Queue"="Overlay" "IgnoreProjector"="True" "ForceNoShadowCasting"="True" }
	
	GrabPass { Tags { "LightMode" = "Always" } }
	
	Pass {
		Cull Off ZWrite Off ZTest Always Fog { Mode Off } Tags { "LightMode" = "Always" }
		CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			
			#include "UnityCG.cginc"
			
			sampler2D _GrabTexture;

			struct appdata_t {
				float4 vertex : POSITION;
				float2 texcoord : TEXCOORD0;
			};
			struct v2f {
				float4 vertex : SV_POSITION;
				float2 texcoord : TEXCOORD0;
			};
			v2f vert (appdata_t v)
			{
				v2f o;
				// place each vertex on the screen according to its UV position
				o.vertex.xy = v.texcoord * 2.0 - 1.0; // screen is -1 to 1 and UVs are 0 to 1
				o.vertex.z = UNITY_NEAR_CLIP_VALUE;
				o.vertex.w = 1.0; // coordinates will be divided by w
				o.texcoord = v.texcoord;
				#if UNITY_UV_STARTS_AT_TOP
				o.texcoord.y = 1.0 - o.texcoord.y;
				#endif
				#ifdef UNITY_HALF_TEXEL_OFFSET
				o.vertex.xy -= 0.5 / _ScreenParams.xy;
				#endif
				return o;
			}
			
			fixed4 frag (v2f i) : COLOR
			{
				fixed a = tex2D(_GrabTexture, i.texcoord).a;
				return fixed4(a,a,a,a);
			}
		ENDCG
	}
}
}

postprocess.lua

CreateObject{
	railoffset = 0, -- follow the player so it is never culled for being out of the camera's view
	floatonwaterwaves = false,
	gameobject = {
		mesh = "quad.obj",
		--mesh = "quad_right_half.obj",
		shader = "RevealAlpha.shader",
		renderqueue = 4000, -- RenderQueue.Overlay
		layer = 13, -- foreground camera
	}
}

quad.obj

# Blender v2.64 (sub 0) OBJ File: 'quad.blend'
# www.blender.org
mtllib quad.mtl
v 0.500000 -0.500000 -0.000000
v -0.500000 -0.500000 -0.000000
v 0.500000 0.500000 0.000000
v -0.500000 0.500000 0.000000
vt -0.010000 -0.010000
vt 1.010000 -0.010000
vt 1.010000 1.010000
vt -0.010000 1.010000
usemtl 
s off
f 2/1 1/2 3/3 4/4

quad_right_half.obj

# Blender v2.64 (sub 0) OBJ File: 'quad.blend'
# www.blender.org
mtllib quad.mtl
v 0.500000 -0.500000 -0.000000
v -0.500000 -0.500000 -0.000000
v 0.500000 0.500000 0.000000
v -0.500000 0.500000 0.000000
vt 0.500000 -0.010000
vt 1.010000 -0.010000
vt 1.010000 1.010000
vt 0.500000 1.010000
usemtl 
s off
f 2/1 1/2 3/3 4/4