3.3.1. Cg / HLSL variables and connection vectors

Continuing with the USB_simple_color shader, notice that within our program there is a variable of sampler2D type and a vector of four dimensions for the definition of the _MainTex texture.

sampler2D _MainTex;
float4 _MainTex_ST;

These connection variables have been declared “global” within the Cg program and correspond to a_MainTex property reference, previously included in the shader Properties. 

The connection variables are used to connect the values ​​or parameters of the properties with our program variables or internal vectors. In the case of _MainTex, we can assign a texture from the Unity Inspector and use it as a texture in the shader. 

To understand this concept better, let’s assume that we want to create a shader that has the ability to can change colour. To do this, we would have to go to our properties, create a “Color” type and then generate a connection variable within the CGPROGRAM field, in this way we can generate a link between them.

Properties    
{
    // first declare the property
    _Color (“Tint”, Color) = (1, 1, 1, 1)
}
    ...
    CGPROGRAM  
    ...  
    // then declare the connection variable
    sampler2D _MainTex;
    float4 _Color;
    ...
    ENDCG

Generally, the global variables in Cg or HLSL are declared with the word “uniform” (e.g. uniform float4 _Color) however Unity ignores this step since this declaration is included within the shader.