问编写非常简单的HLSL像素着色器时出错

EN

float4 DitherByChance(float2 coords : TEXCOORD0) : COLOR0
{
    float4 newColor;    // The pixel color to return
    float4 oldColor = tex2D(s0, coords);

    // Note I know that as the code stands now rCutOff = gCutOff = bCutOff I will sort a fix for this out later
    float rCutOff = random(coords);     // A random float that determines if the red channel will be rounded up or down
    float gCutOff = random(coords);     // A random float that determines if the green channel will be rounded up or down
    float bCutOff = random(coords);     // A random float that determines if the blue channel will be rounded up or down

    float rPercentChance = frac(oldColor.r * 255);  //Chance to round red channel up
    float gPercentChance = frac(oldColor.g * 255);  //Chance to round green channel up
    float bPercentChance = frac(oldColor.b * 255);  //Chance to round blue channel up

    //In the code below (1/255) is the floating point represntaion of an incress of one on the 0-255 RGB scale

    if (rCutOff <= rPercentChance) newColor.r = oldColor.r + ((1 - rPercentChance) / 255);  //Bump up one r value
    else newColor.r = oldColor.r - rPercentChance * (1 / 255);                  //Bump down to ensure r is not rounded up

    if (gCutOff <= gPercentChance) newColor.g = oldColor.g + ((1 - gPercentChance) / 255);  //Bump up one g value
    else newColor.b = oldColor.b - bPercentChance * (1 / 255);                  //Bump down to ensure g is not rounded up

    if (bCutOff <= bPercentChance) newColor.b = oldColor.b + ((1 - bPercentChance) / 255);  //Bump up one b value
    else newColor.b = oldColor.b - bPercentChance * (1 / 255);                  //Bump down to ensure b is not rounded up

    return newColor;
}

// Input: It uses texture coords as the random number seed.
// Output: Random number: [0,1), that is between 0.0 and 0.999999... inclusive.
// Author: Michael Pohoreski
// Copyright: Copyleft 2012 :-)
float random( vec2 p )
{
  // We need irrationals for pseudo randomness.
  // Most (all?) known transcendental numbers will (generally) work.
  const vec2 r = vec2(
    23.1406926327792690,  // e^pi (Gelfond's constant)
     2.6651441426902251); // 2^sqrt(2) (Gelfond–Schneider constant)
  return fract( cos( mod( 123456789., 1e-7 + 256. * dot(p,r) ) ) );  
}

sampler2D Tex0;
...
.... 
.....
float4 oldColor = tex2D(Tex0, coords);