在WebGL着色器中获取准确的整数模 [英] Get accurate integer modulo in WebGL shader

问题描述

我想在WebGL片段着色器中获得x和y的准确模. x和y是整数.

I want to get an accurate modulo of x and y in a WebGL fragment shader. x and y are integers.

绘制mod(x,y)，我们得到以下信息:

Graphing mod(x,y), we get the following:

用于生成红色和黑色矩形的实际代码为:

The actual code used to generate the red-and-black rectangle is:

gl_FragColor = vec4(mod(
  float(int(v_texCoord[0]*15.))/15.,
  float(int(v_texCoord[1]*15.))/15.
), 0, 0, 1);

其中v_texCoord是一个vec2，范围从左上角的0,0到右下角的1,1.将float和int的精度都设置为mediump.

Where v_texCoord is a vec2 ranging from 0,0 at the top-left to 1,1 at the bottom-right. Precision is set to mediump for both float and int.

阅读图表，我们看到尽管mod(6,6)正确地是0，但是mod(7,7)实际上是7！ 我该如何解决?

Reading the chart, we see that although mod(6,6) is correctly 0, mod(7,7) is actually 7! How do I fix this?

我试图实现自己的mod()函数.但是，它具有相同的错误，并生成相同的图.

I tried to implement my own mod() function. However, it has the same errors, and produces the same graph.

int func_mod(int x, int y) {
    return int(float(x)-float(y)*floor(float(x)/float(y)));
}

在Java脚本中，我可以对其进行调试，该功能可以完美运行.然后，我尝试了一种迭代方法，因为我担心自己会发疯，而且我也不信任浮点除法.

In Javascript, where I can debug it, the function works perfectly. I then tried an iterative approach, because I was worried I was going insane and I didn't trust the floating-point division anyway.

int iter_mod(int x, int y) {
    x = int(abs(float(x))); y = int(abs(float(y)));
    for(int i=0; i>-1; i++) {
        if(x < y) break;
        x = x - y;
    }
    return x;
}

这行得通，但是我无法用图形表示出来，因为当我尝试使用ring 0时，它使linux崩溃并出现错误.对于我需要的Spritesheet计算来说，它工作得很好，但我真的觉得这是不正确的解决方案.

This worked, but I can't graph it because it crashes linux with an error in ring 0 when I try. It works fine for the spritesheet calculations I need it for, but I really feel it's an incorrect solution.

(更新:它可以在我的手机上正常工作.现在不是我的代码有误，这只是我的问题……)

(Update: It works perfectly on my phone. It's not my code in error now, it's just my problem…)

推荐答案

您不是将7乘7乘以7/15乘7/15乘以

You're not modding 7 by 7 you're modding 7/15ths by 7/15ths

尝试

gl_FragColor = vec4(mod(
  floor(v_texCoord[0] * 15.),
  floor(v_texCoord[1] * 15.)
) / 15., 0, 0, 1);

您可以在此处看到运行的两个版本

You can see the 2 versions running here

function render(num) {
  var gl = document.getElementById("c" + num).getContext("webgl");
  var programInfo = twgl.createProgramInfo(gl, ["vs", "fs"]);

  var arrays = {
    position: [-1, -1, 0, 1, -1, 0, -1, 1, 0, -1, 1, 0, 1, -1, 0, 1, 1, 0],
  };
  var bufferInfo = twgl.createBufferInfoFromArrays(gl, arrays);

  var uniforms = {
    resolution: [gl.canvas.width, gl.canvas.height],
    intMod: num == 1,
  };

  gl.useProgram(programInfo.program);
  twgl.setBuffersAndAttributes(gl, programInfo, bufferInfo);
  twgl.setUniforms(programInfo, uniforms);
  twgl.drawBufferInfo(gl, bufferInfo);
}

render(0)
render(1);

canvas { margin: 1em; height: 100px; width: 150px; }
div { display: inline-block; }
pre { text-align: center; }

<script src="https://twgljs.org/dist/3.x/twgl.min.js"></script>
  <script id="vs" type="notjs">
attribute vec4 position;

void main() {
  gl_Position = position;
}
  </script>
  <script id="fs" type="notjs">
precision mediump float;

uniform vec2 resolution;
uniform bool intMod;

void main() {
  vec2 v_texCoord = gl_FragCoord.xy / resolution.xy;

  if (!intMod) {
  
    gl_FragColor = vec4(mod(
      float(int(v_texCoord[0]*15.))/15.,
      float(int(v_texCoord[1]*15.))/15.
    ), 0, 0, 1);

  } else {

    gl_FragColor = vec4(mod(
      floor(v_texCoord[0]*15.),
      floor(v_texCoord[1]*15.)
    )/15., 0, 0, 1);
    
  }
  
}
  </script>
<div><canvas id="c0"></canvas><pre>mod with fractions</pre></div>
<div><canvas id="c1"></canvas><pre>mod with ints</pre></div>

您还应该注意，mod by 0是未定义的，这意味着您将在不同的GPU上获得不同的结果

You should also note that mod by 0 is undefined meaning you'll get different results on different GPUs

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