WebGL: Loop index cannot be compared with non-constant expression

问题

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;
}

When I build, I get the following error message:

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

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

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

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.

Any thoughts on this?

回答1:

Try something like this:

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;
}

回答2:

This happens because on some hardware, GLSL loops are un-rolled into native GPU instructions. This means there needs to be a hard upper limit to the number of passes through the for loop, that governs how many copies of the loop's inner code will be generated. If you replace numBlurPixelsPerSide with a const float or even a #define directive, and the shader compiler can then determine the number of passes at compile time, and generate the code accordingly. But with a uniform there, the upper limit is not known at compile time.

There's an interesting wrinkle in this rule: You're allowed to break or call an early return out of a for loop, even though the max iterations must be discernible at compile time. For example, consider this tiny Mandelbrot shader. This is hardly the prettiest fractal on GLSL Sandbox, but I chose it for its small size:

precision mediump float;
uniform float time;
uniform vec2 mouse;
uniform vec2 resolution;
varying vec2 surfacePosition;

const float max_its = 100.;

float mandelbrot(vec2 z){
    vec2 c = z;
    for(float i=0.;i<max_its;i++){     // for loop is here.
        if(dot(z,z)>4.) return i;      // conditional early return here.
        z = vec2(z.x*z.x-z.y*z.y,2.*z.x*z.y)+c;
    }
    return max_its;
}


void main( void ) {
    vec2 p = surfacePosition;
    gl_FragColor = vec4(mandelbrot(p)/max_its);
}

In this example, max_its is a const so the compiler knows the upper limit and can un-roll this loop if it needs to. Inside the loop, a return statement offers a way to leave the loop early for pixels that are outside of the Mandelbrot set.

You still don't want to set the max iterations too high, as this can produce a lot of GPU instructions and possibly hurt performance.

回答3:

I've had similar problem with image downsampling shader. The code is basically the same:

for (int dx = -2 * SCALE_FACTOR; dx < 2 * SCALE_FACTOR; dx += 2) {
    for (int dy = -2 * SCALE_FACTOR; dy < 2 * SCALE_FACTOR; dy += 2) {
        /* accumulate fragment's color */
    }
}

What I've ended up doing is using preprocessor and creating separate shader programs for every SCALE_FACTOR used (luckily, only 4 was needed). To achieve that, a small helper function was implemented to add #define ... statements to shader code:

function insertDefines (shaderCode, defines) {
    var defineString = '';

    for (var define in defines) {
        if (defines.hasOwnProperty(define)) {
            defineString +=
                '#define ' + define + ' ' + defines[define] + '\n';
        }
    }

    var versionIdx = shaderCode.indexOf('#version');

    if (versionIdx == -1) {
        return defineString + shaderCode;
    }

    var nextLineIdx = shaderCode.indexOf('\n', versionIdx) + 1;

    return shaderCode.slice(0, nextLineIdx) +
        defineString +
        shaderCode.slice(nextLineIdx);
}

The implementation is a bit tricky because if the code already has #version preprocessor statement in it, all other statements have to follow it.

Then I've added a check for SCALE_FACROR being defined:

#ifndef SCALE_FACTOR
#   error SCALE_FACTOR is undefined
#endif

And in my javascript code I've done something like this:

var SCALE_FACTORS = [4, 8, 16, 32],
    shaderCode, // the code of my shader
    shaderPrograms = SCALE_FACTORS.map(function (factor) {
        var codeWithDefines = insertDefines(shaderCode, { SCALE_FACTOR: factor });
        /* compile shaders, link program, return */
    });

回答4:

I use opengl es3 on android and solve this problem by using extension above the beginning of program like this:

#extension GL_EXT_gpu_shader5 : require

I don't know whether it work on webGL, but you can try it. Hope it can help.

来源：https://stackoverflow.com/questions/38986208/webgl-loop-index-cannot-be-compared-with-non-constant-expression