WebGL:循环索引不能与非常量表达式进行比较 [英] WebGL: Loop index cannot be compared with non-constant expression

问题描述

我有一个webgl模糊着色器:

I have the a webgl blur shader:

precision mediump float;
precision mediump int;

uniform sampler2D u_image;
uniform float blur;       
uniform int u_horizontalpass; // 0 or 1 to indicate vertical or horizontal pass
uniform float sigma;        // The sigma value for the gaussian function: higher value means more blur
                            // A good value for 9x9 is around 3 to 5
                            // A good value for 7x7 is around 2.5 to 4
                            // A good value for 5x5 is around 2 to 3.5
                            // ... play around with this based on what you need :)

varying vec4 v_texCoord;

const vec2 texOffset = vec2(1.0, 1.0);
// uniform vec2 texOffset;
const float PI = 3.14159265;

void main() {  
  vec2 p = v_texCoord.st;
  float numBlurPixelsPerSide = blur / 2.0; 

  // Incremental Gaussian Coefficent Calculation (See GPU Gems 3 pp. 877 - 889)
  vec3 incrementalGaussian;
  incrementalGaussian.x = 1.0 / (sqrt(2.0 * PI) * sigma);
  incrementalGaussian.y = exp(-0.5 / (sigma * sigma));
  incrementalGaussian.z = incrementalGaussian.y * incrementalGaussian.y;

  vec4 avgValue = vec4(0.0, 0.0, 0.0, 0.0);
  float coefficientSum = 0.0;

  // Take the central sample first...
  avgValue += texture2D(u_image, p) * incrementalGaussian.x;
  coefficientSum += incrementalGaussian.x;
  incrementalGaussian.xy *= incrementalGaussian.yz;

  // Go through the remaining 8 vertical samples (4 on each side of the center)
  for (float i = 1.0; i <= numBlurPixelsPerSide; i += 1.0) { 
    avgValue += texture2D(u_image, p - i * texOffset) * incrementalGaussian.x;         
    avgValue += texture2D(u_image, p + i * texOffset) * incrementalGaussian.x;         
    coefficientSum += 2.0 * incrementalGaussian.x;
    incrementalGaussian.xy *= incrementalGaussian.yz;
  }

  gl_FragColor = avgValue / coefficientSum;
}

构建时，出现以下错误消息:

When I build, I get the following error message:

webgl-renderer.js?2eb3:137未捕获的无法编译着色器:错误: 0:38:'i':循环索引无法与非恒定表达式进行比较

webgl-renderer.js?2eb3:137 Uncaught could not compile shader:ERROR: 0:38: 'i' : Loop index cannot be compared with non-constant expression

我还尝试使用 just 均匀浮点模糊来与i进行比较.有没有什么办法解决这一问题?

I have also tried to use just the uniform float blur to compare i to. Is there any way to fix this?

问题在此处进一步详细说明: https://www.khronos.org/webgl/public-mailing-list/archives/1012/msg00063.php

The problem is further detailed here: https://www.khronos.org/webgl/public-mailing-list/archives/1012/msg00063.php

我发现环顾四周的解决方案是仅在比较循环变量时使用常量表达式.这与我需要做的工作不符，该工作根据模糊半径的不同而有所不同.

The solution that I've found looking around is to only use a constant expression when comparing a loop var. This doesn't fit with what I need to do which is vary how many times I'm looping based on the blur radius.

对此有何想法?

推荐答案

尝试如下操作:

const float MAX_ITERATIONS = 100.0;

// Go through the remaining 8 vertical samples (4 on each side of the center)
for (float i = 1.0; i <= MAX_ITERATIONS; i += 1.0) { 
    if (i >= numBlurPixelsPerSide){break;}
    avgValue += texture2D(u_image, p - i * texOffset) * incrementalGaussian.x;         
    avgValue += texture2D(u_image, p + i * texOffset) * incrementalGaussian.x;         
    coefficientSum += 2.0 * incrementalGaussian.x;
    incrementalGaussian.xy *= incrementalGaussian.yz;
}

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