WebGL: How to bind values to a mat4 attribute?

Question

In some WebGL application, let's assume that we have a GLSL vertex shader which starts like this:

attribute vec4 foo1;
attribute vec4 foo2;
attribute vec4 foo3;
attribute vec4 foo4;

and some corresponding Javascript code for binding a data structure for those attributes:

var buf = gl.createBuffer(), loc;
gl.bindBuffer(gl.ARRAY_BUFFER, buf);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([…]));

loc = gl.getAttribLocation(program, 'foo1');
gl.enableVertexArray(loc);
gl.vertexAttribPointer(loc, 4, gl.FLOAT, false, 16, 0);

loc = gl.getAttribLocation(program, 'foo2');
gl.enableVertexArray(loc);
gl.vertexAttribPointer(loc, 4, gl.FLOAT, false, 16, 4);

loc = gl.getAttribLocation(program, 'foo3');
gl.enableVertexArray(loc);
gl.vertexAttribPointer(loc, 4, gl.FLOAT, false, 16, 8);

loc = gl.getAttribLocation(program, 'foo4');
gl.enableVertexArray(loc);
gl.vertexAttribPointer(loc, 4, gl.FLOAT, false, 16, 12);

Now, according to the GL ES 2.0 specs, a vertex shader attribute can be defined as either a float, vec2, vec3, vec4, mat2, mat3 or mat4.

So if I change the vertex shader code to define just one mat4 attribute, like so...

attribute mat4 foo;

... the question is what is the corresponding JS code to bind some pointers to a mat4 attribute?

I have found the question mat3 attribute in WebGL, but the answer is not explicit enough. Reading the answers and some other documentation, it seems that the correct solution is along the lines of:

loc = gl.getAttribLocation(program, 'foo');
gl.enableVertexArray(loc);
gl.vertexAttribPointer(loc  , 4, gl.FLOAT, false, 16, 0);
gl.vertexAttribPointer(loc+1, 4, gl.FLOAT, false, 16, 4);
gl.vertexAttribPointer(loc+2, 4, gl.FLOAT, false, 16, 8);
gl.vertexAttribPointer(loc+3, 4, gl.FLOAT, false, 16, 12);

Am I right in assuming that the locations of the 4 vec4 components of a mat4 are always adjacent and in increasing order? Is this documented somewhere?

Besides these locations counting towards the MAX_VERTEX_ATTRIBS limit (normally 16 in WebGL), is there any other good practice to be aware of?

Answer 1

You're correct. From the spec section 2.10.4

When an attribute variable is declared as a mat2, its matrix columns are taken from the (x, y) components of generic attributes i and i + 1. When an attribute variable is declared as a mat3, its matrix columns are taken from the (x, y, z) components of generic attributes i through i + 2. When an attribute variable is declared as a mat4, its matrix columns are taken from the (x, y, z, w) components of generic attributes i through i + 3.

stride and offsets in WebGL are in bytes so I suspect you wanted

gl.vertexAttribPointer(loc  , 4, gl.FLOAT, false, 64, 0);
gl.vertexAttribPointer(loc+1, 4, gl.FLOAT, false, 64, 16);
gl.vertexAttribPointer(loc+2, 4, gl.FLOAT, false, 64, 32);
gl.vertexAttribPointer(loc+3, 4, gl.FLOAT, false, 64, 48);

Let's check

var vs = `
attribute mat4 matrix;
attribute vec4 color;

varying vec4 v_color;

void main() {
  gl_PointSize = 10.0;
  gl_Position = matrix * vec4(0, 0, 0, 1);
  v_color = color;
}
`;
var fs = `
precision mediump float;

varying vec4 v_color;

void main() {
  gl_FragColor = v_color;
}
`;

var m4 = twgl.m4;
var gl = document.querySelector("canvas").getContext("webgl");
var program = twgl.createProgramFromSources(gl, [vs, fs]);

var matrixLoc = gl.getAttribLocation(program, "matrix");
var colorLoc = gl.getAttribLocation(program, "color");

function r(min, max) {
  if (max === undefined) {
    max = min;
    min = 0;
  }
  return Math.random() * (max - min) + min;
}

var numPoints = 100;
var matrices = [];
var colors = [];
for (var ii = 0; ii < numPoints; ++ii) {
  matrices.push.apply(matrices, m4.translation([r(-1,1), r(-1,1), 0]));
  colors.push(r(1), r(1), r(1), 1);
}

function makeBuffer(gl, array) {
  const buf = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, buf);
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(array), gl.STATIC_DRAW);
  return buf;
}

var buffers = {
  matrices: makeBuffer(gl, matrices),
  colors: makeBuffer(gl, colors),
};

gl.useProgram(program);

gl.bindBuffer(gl.ARRAY_BUFFER, buffers.matrices);
for (var ii = 0; ii < 4; ++ii) {
  gl.enableVertexAttribArray(matrixLoc + ii);
  gl.vertexAttribPointer(matrixLoc + ii, 4, gl.FLOAT, 0, 64, ii * 16);
}

gl.bindBuffer(gl.ARRAY_BUFFER, buffers.colors);
gl.enableVertexAttribArray(colorLoc);
gl.vertexAttribPointer(colorLoc, 4, gl.FLOAT, 0, 0, 0);

gl.drawArrays(gl.POINTS, 0, numPoints);

canvas { border: 1px solid black; }

<script src="https://twgljs.org/dist/4.x/twgl-full.js" crossorigin></script>
<canvas></canvas>