games202-hw/hw4/TODO.md

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>