An implementation of Photoshop's "Screen" blend mode for Unity3D.

gistfile1.txt

Shader "Sunspots" {
	Properties {
		_MainTex ("Texture1 (RGB)", 2D) = "white" {}
		_Color ("Main Color", Color) = (1,1,1,1)
	}

	Category {
		Tags {"RenderType"="Opaque" "Queue"="Transparent"}
		Lighting Off
		BindChannels {
			Bind "Color", color
			Bind "Vertex", vertex
			Bind "Texcoord", Texcoord
		}
		
		//screen 1 - (1 - a) * (1 - b)
		
		SubShader {
			Pass {
				ZWrite Off
				Cull Off
				Blend One One
				BlendOp Sub
				SetTexture [_MainTex] {
					constantColor (1,1,1,1)
					Combine constant
				}
			}
			
			Pass {
				ZWrite Off
				Cull Off
				Blend Zero OneMinusSrcColor
				BlendOp Add
				SetTexture [_MainTex] {
					constantColor [_Color]
					Combine texture * constant
				}
			}
			Pass {
				ZWrite Off
				Cull Off
				Blend One One
				BlendOp Sub
				SetTexture [_MainTex] {
					constantColor [_Color]
            	}
				SetTexture [_MainTex] {
					constantColor (1,1,1,1)
					Combine constant, previous
				}
			}
		}
	}
}