WebGL does not support index drawing using index 255
<p>This is the simplest WebGL test program, which creates an index buffer with only one uint8 value of 255. It should draw a red square of size 64px, but it doesn't (the index value doesn't matter for drawing). </p>
<p>Somehow WebGL ignores the element at index 255 even though it is in the unsigned byte range. Using 254 or other values, the red square appears as expected. </p>
<p>Is this a bug in WebGL or expected behavior? I can't find any relevant information. </p>
<pre class="brush:php;toolbar:false;"><canvas width="800" height="600"></canvas>
<script type="module">
let canvas = document.querySelector("canvas");
let gl = canvas.getContext("webgl2", {antialias: false});
let vert = gl.createShader(gl.VERTEX_SHADER);
let frag = gl.createShader(gl.FRAGMENT_SHADER);
let prog = gl.createProgram();
let index_buf = gl.createBuffer();
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, index_buf);
gl.bufferData(
gl.ELEMENT_ARRAY_BUFFER, new Uint8Array([255]), gl.STATIC_DRAW
);
gl.shaderSource(vert, `#version 300 es
void main()
{
gl_Position = vec4(pos, 1);
gl_PointSize = 64.0;
}
`);
gl.shaderSource(frag, `#version 300 es
precision highp float;
out vec4 color;
void main()
{
color = vec4(1,0,0,1);
}
`);
gl.compileShader(vert);
gl.compileShader(frag);
gl.attachShader(prog, vert);
gl.attachShader(prog, frag);
gl.linkProgram(prog);
gl.clearColor(0, 0, 0, 1);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.useProgram(prog);
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, index_buf);
gl.drawElements(gl.POINTS, 1, gl.UNSIGNED_BYTE, 0);
</script></pre>
<p><br /></p>
