Unity Shader：Projective Texture Mapping

Projective texture mapping in Unity Shader is a technique used to project a texture onto an object in a way that simulates the effect of a slide projector. It can be used to create various effects like projecting a spotlight texture onto a surface, projecting a decal onto a curved surface, or creating a shadow map for an object.

To implement projective texture mapping in Unity Shader, you need to use a combination of vertex and fragment shaders. The vertex shader calculates the projective transformation matrix, which is used to transform the texture coordinates in the fragment shader. The fragment shader samples the texture and applies it to the object's surface.

Here's a simple example of a vertex shader that calculates the projective transformation matrix:

Shader "Custom/ProjectiveTextureMapping"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Projector ("Projector", 3D) = "" {}
    }

    SubShader
    {
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            uniform sampler2D _MainTex;
            uniform float4x4 unity_Projector;
            float4x4 unity_ObjectToWorld;

            struct appdata
            {
                float4 vertex : POSITION;
                float3 normal : NORMAL;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = mul(unity_Projector, v.vertex).xy;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                return tex2D(_MainTex, i.uv);
            }

            ENDCG
        }
    }
}

In this example, the `_MainTex` property is the texture to be projected, and the `_Projector` property is the object that will project the texture. The `unity_Projector` variable is a matrix that transforms the texture coordinates from object space to clip space. The `vert` function calculates the transformed texture coordinates and passes them to the fragment shader in the `v2f` struct.

The fragment shader simply samples the texture using the transformed texture coordinates and returns the color.

You can use this shader by assigning it to a material and setting the `_MainTex` and `_Projector` properties to appropriate textures and objects, respectively.