Get accurate integer modulo in WebGL shader - webgl

Question

I want to get an accurate modulo of x and y in a WebGL fragment shader. x and y are integers.
Graphing mod(x,y), we get the following:
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);
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.
Reading the chart, we see that although mod(6,6) is correctly 0, mod(7,7) is actually 7! How do I fix this?
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)));
}
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;
}
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.
(Update: It works perfectly on my phone. It's not my code in error now, it's just my problem…)

Answer 1

Here is a GLSL function that calculates MOD accurately with (float) parameters that should be integers:
/**
* Returns accurate MOD when arguments are approximate integers.
*/
float modI(float a,float b) {
float m=a-floor((a+0.5)/b)*b;
return floor(m+0.5);
}
Please note, if a<0 and b>0 then the return value will be >=0, unlike other languages' % operator.

Answer 2

You're not modding 7 by 7 you're modding 7/15ths by 7/15ths
Try
gl_FragColor = vec4(mod(
floor(v_texCoord[0] * 15.),
floor(v_texCoord[1] * 15.)
) / 15., 0, 0, 1);
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>
You should also note that mod by 0 is undefined meaning you'll get different results on different GPUs

Answer 3

Note that since webGL2, we now have int operations so this problem is now trivially solve by x % y .