ShaderLab: legacy texture combining

Note: The ShaderLabUnity’s language for defining the structure of Shader objects. More info
See in Glossary functionality on this page is legacy, and is documented for backwards compatibility only. If your shaderA program that runs on the GPU. More info
See in Glossary source file includes HLSL code, Unity ignores these commands completely. If your shader source file does not include HLSL code, Unity compiles these commands into regular shader programs on import.

Render pipeline compatibility

Feature name	Built-in Render PipelineA series of operations that take the contents of a Scene, and displays them on a screen. Unity lets you choose from pre-built render pipelines, or write your own. More info See in Glossary	Universal Render Pipeline (URP)	High Definition Render Pipeline (HDRP)	Custom SRP
Legacy texture combining	Yes	No	No	No

Overview

After the basic vertex lighting has been calculated, textures are applied. In ShaderLab this is done using SetTexture command.

Fixed function texturing is the place to do old-style combiner effects. You can have multiple SetTexture commands inside a pass - all textures are applied in sequence, like layers in a painting program. SetTexture commands must be placed at the end of a Pass.

Syntax

SetTexture [TextureName] {Texture Block}

Assigns a texture. TextureName must be defined as a texture property. How to apply the texture is defined inside the TextureBlock.

The texture block controls how the texture is applied. Inside the texture block can be up to two commands: combine and constantColor.

Texture block combine command

combine src1 * src2: Multiplies src1 and src2 together. The result will be darker than either input.

combine src1 + src2: Adds src1 and src2 together. The result will be lighter than either input.

combine src1 - src2: Subtracts src2 from src1.

combine src1 lerp (src2) src3: Interpolates between src3 and src1, using the alpha of src2. Note that the interpolation is opposite direction: src1 is used when alpha is one, and src3 is used when alpha is zero.

combine src1 * src2 + src3: Multiplies src1 with the alpha component of src2, then adds src3.

All the src properties can be either one of previous, constant, primary or texture.

• Previous is the the result of the previous SetTexture.
• Primary is the color from the lighting calculation or the vertex color if it is bound.
• Texture is the color of the texture specified by TextureName in the SetTexture (see above).
• Constant is the color specified in ConstantColor.

Modifiers:

• The formulas specified above can optionally be followed by the keywords Double or QuadA primitive object that resembles a plane but its edges are only one unit long, it uses only 4 vertices, and the surface is oriented in the XY plane of the local coordinate space. More info
  See in Glossary to make the resulting color 2x or 4x as bright.
• All the src properties, except lerp argument, can optionally be preceded by one - to make the resulting color negated.
• All the src properties can be followed by alpha to take only the alpha channel.

Texture block constantColor command

ConstantColor color: Defines a constant color that can be used in the combine command.

Functionality removed in Unity 5.0

Unity versions before 5.0 did support texture coordinate transformations with a matrix command inside a texture block. If you need this functionality now, consider rewriting your shader as a programmable shader instead, and do the UV transformation in the vertex shaderA program that runs on each vertex of a 3D model when the model is being rendered. More info
See in Glossary.

Similarly, 5.0 removed signed add (a+-b), multiply signed add (a*b+-c), multiply subtract (a*b-c) and dot product (dot3, dot3rgba) texture combine modes. If you need them, do the math in the pixelThe smallest unit in a computer image. Pixel size depends on your screen resolution. Pixel lighting is calculated at every screen pixel. More info
See in Glossary shader instead.

Details

Before fragment programs existed, older graphics cards used a layered approach to textures. The textures are applied one after each other, modifying the color that will be written to the screen. For each texture, the texture is typically combined with the result of the previous operation. These days it is advisable to use actual fragment programs.

Note that each texture stage may or might not be clamped to 0..1 range, depending on the platform. This might affect SetTexture stages that can produce values higher than 1.0.

Separate Alpha & Color computation

By default, the combiner formula is used for calculating both the RGB and alpha component of the color. Optionally, you can specify a separate formula for the alpha calculation. This looks like this:

SetTexture [_MainTex] { combine previous * texture, previous + texture }

Here, we multiply the RGB colors and add the alpha.

Specular highlights

By default the primary color is the sum of the diffuse, ambient and specular colorsThe color of a specular highlight.
See in Glossary (as defined in the Lighting calculation). If you specify SeparateSpecular On in the pass options, the specular color will be added in after the combiner calculation, rather than before. This is the default behavior of the built-in VertexLit shader.

Graphics hardware support

Modern graphics cards with fragment shader support (“shader model 2.0” on desktop, OpenGL ES 2.0 on mobile) support all SetTexture modes and at least 4 texture stages (many of them support 8). If you’re running on really old hardware (made before 2003 on PC, or before iPhone3GS on mobile), you might have as low as two texture stages. The shader author should write separate SubShaders for the cards they want to support.

Examples

Alpha Blending Two Textures

This small examples takes two textures. First it sets the first combiner to just take the _MainTex, then is uses the alpha channel of _BlendTex to fade in the RGB colors of _BlendTex

Shader "Examples/2 Alpha Blended Textures" {
    Properties {
        _MainTex ("Base (RGB)", 2D) = "white" {}
        _BlendTex ("Alpha Blended (RGBA) ", 2D) = "white" {}
    }
    SubShader {
        Pass {
            // Apply base texture
            SetTexture [_MainTex] {
                combine texture
            }
            // Blend in the alpha texture using the lerp operator
            SetTexture [_BlendTex] {
                combine texture lerp (texture) previous
            }
        }
    }
}

Alpha Controlled Self-illumination

This shader uses the alpha component of the _MainTex to decide where to apply lighting. It does this by applying the texture to two stages; In the first stage, the alpha value of the texture is used to blend between the vertex color and solid white. In the second stage, the RGB values of the texture are multiplied in.

Shader "Examples/Self-Illumination" {
    Properties {
        _MainTex ("Base (RGB) Self-Illumination (A)", 2D) = "white" {}
    }
    SubShader {
        Pass {
            // Set up basic white vertex lighting
            Material {
                Diffuse (1,1,1,1)
                Ambient (1,1,1,1)
            }
            Lighting On

            // Use texture alpha to blend up to white (= full illumination)
            SetTexture [_MainTex] {
                constantColor (1,1,1,1)
                combine constant lerp(texture) previous
            }
            // Multiply in texture
            SetTexture [_MainTex] {
                combine previous * texture
            }
        }
    }
}

We can do something else for free here, though; instead of blending to solid white, we can add a self-illumination color and blend to that. Note the use of ConstantColor to get a _SolidColor from the properties into the texture blending.

Shader "Examples/Self-Illumination 2" {
    Properties {
        _IlluminCol ("Self-Illumination color (RGB)", Color) = (1,1,1,1)
        _MainTex ("Base (RGB) Self-Illumination (A)", 2D) = "white" {}
    }
    SubShader {
        Pass {
            // Set up basic white vertex lighting
            Material {
                Diffuse (1,1,1,1)
                Ambient (1,1,1,1)
            }
            Lighting On

            // Use texture alpha to blend up to white (= full illumination)
            SetTexture [_MainTex] {
                // Pull the color property into this blender
                constantColor [_IlluminCol]
                // And use the texture's alpha to blend between it and
                // vertex color
                combine constant lerp(texture) previous
            }
            // Multiply in texture
            SetTexture [_MainTex] {
                combine previous * texture
            }
        }
    }
}

And finally, we take all the lighting properties of the vertexlit shader and pull that in:

Shader "Examples/Self-Illumination 3" {
    Properties {
        _IlluminCol ("Self-Illumination color (RGB)", Color) = (1,1,1,1)
        _Color ("Main Color", Color) = (1,1,1,0)
        _SpecColor ("Spec Color", Color) = (1,1,1,1)
        _Emission ("Emmisive Color", Color) = (0,0,0,0)
        _Shininess ("Shininess", Range (0.01, 1)) = 0.7
        _MainTex ("Base (RGB)", 2D) = "white" {}
    }

    SubShader {
        Pass {
            // Set up basic vertex lighting
            Material {
                Diffuse [_Color]
                Ambient [_Color]
                Shininess [_Shininess]
                Specular [_SpecColor]
                Emission [_Emission]
            }
            Lighting On

            // Use texture alpha to blend up to white (= full illumination)
            SetTexture [_MainTex] {
                constantColor [_IlluminCol]
                combine constant lerp(texture) previous
            }
            // Multiply in texture
            SetTexture [_MainTex] {
                combine previous * texture
            }
        }
    }
}