Webgl gl_VertexID affected by drawArrays offset?

Question

When drawArrays is called with an offset, (the "first" argument being non zero), does the first gl_VertexID still start at 0, or does it start at the offset value?

Answer 1

update

This appears to be a bug in ANGLE on Windows. Filed a bug

https://github.com/KhronosGroup/WebGL/issues/2770

---

Let's try it

[...document.querySelectorAll('canvas')].forEach((canvas, ndx) => {
  const vs = `#version 300 es
  void main() {
    gl_Position = vec4(float(gl_VertexID) / 10., 0, 0, 1);
    gl_PointSize = 10.0;
  }`;
  const fs = `#version 300 es
  precision mediump float;
  out vec4 outColor;
  void main() {
    outColor = vec4(1, 0, 0, 1);
  }`;
  const gl = canvas.getContext('webgl2');
  if (!gl) {
    return alert('need webgl2');
  }
  const prg = twgl.createProgram(gl, [vs, fs]);
  gl.useProgram(prg);
  gl.drawArrays(gl.POINTS, ndx * 5, 5);
});

canvas {border: 1px solid black;}

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

Looks like the answer is it starts at the offset value.