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WebGL drawArrays generates white background


WebGL drawArrays generates white background

By : aftab alam
Date : November 22 2020, 11:17 AM
hope this fix your issue In your example you just have to clear your color buffer every "frame". Your background is black at first, then gets "overwritten" by your triangle (that vertices buffer), like you mentioned. You actually just do one clear in your init function at startup.
code :
function initContext(context) {
    context.clearColor(0.0, 0.0, 0.0, 1.0);
    context.clear(context.COLOR_BUFFER_BIT);
}
setInterval(function() {
    context.clear(context.COLOR_BUFFER_BIT);
    renderTile(context, program);
}, 100);


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WebGL drawArrays with invalid mode is not generating a error

WebGL drawArrays with invalid mode is not generating a error


By : James DEWAR
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further Because it's not an error. You are telling it to draw 0 elements starting at vertex 3 (your geometry type, 381, may or may not be valid but I doubt it's checking that just yet.)
I would imagine that internally it looks at your request to draw 0 elements and says... "Okay, done! I drew nothing, just like you asked!" and then moves on. Seems like a logical place to early terminate.
code :
gl.drawArrays(-1,-1,2);
WebGL: drawArrays: attribs not setup correctly

WebGL: drawArrays: attribs not setup correctly


By : A.Z
Date : March 29 2020, 07:55 AM
wish helps you Here is a link to a gist with the files I updated to get the type attribute working.
If you search for //ADDED CODE you should be able to view every change I had to make to get it working.
code :
  triangleObjectTypeBuffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, triangleObjectTypeBuffer);
  objectTypes = [
    1.0, 1.0, 0.0
  ];
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(objectTypes), gl.STATIC_DRAW);
  triangleObjectTypeBuffer.itemSize = 1;
  triangleObjectTypeBuffer.numItems = 3;
  gl.bindBuffer(gl.ARRAY_BUFFER, triangleObjectTypeBuffer);
  gl.vertexAttribPointer(shaderProgram.objectTypeAttribute, triangleObjectTypeBuffer.itemSize, gl.FLOAT, false, 0, 0);
drawElements vs drawArrays in webgl

drawElements vs drawArrays in webgl


By : Chandrasekar
Date : March 29 2020, 07:55 AM
Hope that helps The advantage of glDrawElements() is in theory simple: you use fewer vertexes, and you get to use the post-T&L cache. The penalty is that you have to use an index array and cache locality might be worse.
Comparisons are complicated by the fact that the post-T&L cache will vary in size depending on (for example) how old your graphics card is. Newer cards might have huge caches, older cards might have smaller ones.
Why can't I delay my gl.drawArrays function in WebGL?

Why can't I delay my gl.drawArrays function in WebGL?


By : Krishnamurthy Palani
Date : March 29 2020, 07:55 AM
it should still fix some issue This has nothing to do with WebGL. It has to do with how javascript treats i in a timeout loop.
By the time the anonymous function inn the setTimeout gets called, i is already 2. The solution I used is to create a closure so that i retains the value it had in the loop when the setTimeout function was called. The correct, and working code, looks like this:
code :
  for (var index = 0; index < 3; index++) {
    (function (i) {
      setTimeout (function () {
        gl.vertexAttrib3f(vPosition, vertices[i * 2], vertices[i * 2 + 1], 0.0);
        gl.drawArrays(gl.POINTS, i, 1);
      }, 1000 * (i + 1));      
    })(index);
  }
Webgl gl_VertexID affected by drawArrays offset?

Webgl gl_VertexID affected by drawArrays offset?


By : newbie
Date : March 29 2020, 07:55 AM
will be helpful for those in need 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? , update
This appears to be a bug in ANGLE on Windows. Filed a bug
code :
[...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>
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