Unity shader(2) discard

正交相机下的结果

Shader "Cg  discard" {
   SubShader {
      Pass {
         Cull Off // turn off triangle culling, alternatives are:
         // Cull Back (or nothing): cull only back faces 
         // Cull Front : cull only front faces
 
         CGPROGRAM 
 
         #pragma vertex vert  
         #pragma fragment frag 
         #include "UnityCG.cginc"
         struct vertexInput {
            float4 vertex : POSITION;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 posInObjectCoords : TEXCOORD0;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            output.pos =  mul(UNITY_MATRIX_MVP, input.vertex);//本地坐标转到屏幕空间
            //output.posInObjectCoords=mul(_Object2World,input.vertex); //本地坐标转化到世界坐标  在世界坐标系进行discard
             output.posInObjectCoords= output.pos //裁剪空间进行discard
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR 
         {
            if (input.posInObjectCoords.y < 0.0) 
            {
               discard; // drop the fragment if y coordinate > 0
            }
            return float4(0.0, 1.0, 0.0, 1.0); // green
         }
 
         ENDCG  
      }
   }
}