纹理拼接后的Wrap寻址

sampler2D atlasTexture;
float4 texRect;   //(left, top, width, height) of uv
float2 invSize;   //(1/width, 1/height)
float4 uvRange;   //(left, top, right, bottom) of uv
float4 ps_main( float2 tex : TEXCOORD0 ) : COLOR
{
   // convert original uv to atlas uv's unit
   tex *= texRect.zw;
   
   float2 uv = tex - texRect.xy;
   // bring coordinates into normalized local texture coord [0..1]
   uv *= invSize;
   // if texture repeats then coords are > 1, use frc to bring 
   // these coords back into [0, 1) interval.
   uv = frac(uv);
   // transform coords back to texture atlas coords
   uv = uv * texRect.zw + texRect.xy;
   // clamp to inside texture (to avoid bi-linear filter pulling in foreign texels)
   uv = clamp(uv, uvRange.xy, uvRange.zw);
   // use the original coords for mip-map calculation
   return tex2Dgrad(atlasTexture, uv, ddx(tex), ddy(tex));
}