Accessing Pixel Coordinates in a Fragment Shader

This JavaScript program demonstrates how to use the fragment shader to access pixel coordinates in a WebGL program.

FragmentShaderCoordinates.html

<!DOCTYPE html>
<html>
  <head>
    <title>XoaX.net's WebGL</title>

    <script  id="idVertexShader" type="c">
      attribute vec2 av2Vertex;
      void main() {
        gl_Position = vec4(av2Vertex, 0.0, 1.0);
      }
    </script>

    <script  id="idFragmantShader" type="c">
      precision mediump float;
      uniform float ufCanvasWidth;
      uniform float ufCanvasHeight;
      void main() {
        gl_FragColor = vec4(gl_FragCoord.x/ufCanvasWidth, gl_FragCoord.x/ufCanvasWidth, gl_FragCoord.y/ufCanvasHeight, 1.0);
      }
    </script>

    <script type="text/javascript">
    var gqProgram = null;
    var gqGL = null;
    function CreateProgramAndContext() {
      // Get the WebGL Context
      var qCanvas = document.querySelector("#idCanvasWebGL");
      gqGL = qCanvas.getContext("webgl");

      // Compile the vertex shader
      var sVertexShaderCode = document.querySelector("#idVertexShader").text;
      var qVertexShader = gqGL.createShader(gqGL.VERTEX_SHADER);
      gqGL.shaderSource(qVertexShader, sVertexShaderCode);
      gqGL.compileShader(qVertexShader);

      // Compile the fragment shader
      var sFragmentShaderCode = document.querySelector("#idFragmantShader").text;
      var qFragmentShader = gqGL.createShader(gqGL.FRAGMENT_SHADER);
      gqGL.shaderSource(qFragmentShader, sFragmentShaderCode);
      gqGL.compileShader(qFragmentShader);

      // Compile and link the program
      gqProgram = gqGL.createProgram();
      gqGL.attachShader(gqProgram, qVertexShader);
      gqGL.attachShader(gqProgram, qFragmentShader);
      gqGL.linkProgram(gqProgram);
      gqGL.useProgram(gqProgram);
    }

    function CreateBuffers() {
      var faVertices = new Float32Array([1.0, 1.0,  -1.0, 1.0,  1.0, -1.0,  -1.0, -1.0]);

      var qVerticesBuffer = gqGL.createBuffer();
      gqGL.bindBuffer(gqGL.ARRAY_BUFFER, qVerticesBuffer);
      gqGL.bufferData(gqGL.ARRAY_BUFFER, faVertices, gqGL.STATIC_DRAW);

      var qVertexLoc = gqGL.getAttribLocation(gqProgram, 'av2Vertex');
      gqGL.vertexAttribPointer(qVertexLoc, 2, gqGL.FLOAT, false, 0, 0);
      gqGL.enableVertexAttribArray(qVertexLoc);

      // Unbind the buffer
      gqGL.bindBuffer(gqGL.ARRAY_BUFFER, null);
    }

    function Initialization() {
      CreateProgramAndContext();
      CreateBuffers();
      var fCanvasWidth = gqGL.getUniformLocation(gqProgram, 'ufCanvasWidth');
      var fCanvasHeight = gqGL.getUniformLocation(gqProgram, 'ufCanvasHeight');
      gqGL.uniform1f(fCanvasWidth, gqGL.drawingBufferWidth);
      gqGL.uniform1f(fCanvasHeight, gqGL.drawingBufferHeight);

      gqGL.clearColor(0.0, 0.0, 0.0, 1.0);
      gqGL.clear(gqGL.COLOR_BUFFER_BIT);
      gqGL.drawArrays(gqGL.TRIANGLE_STRIP, 0, 4);
    }
    </script>
  </head>
  <body onload="Initialization();">
    <canvas id="idCanvasWebGL" width="400", height="400" style="border:1px solid black"></canvas>
  </body>
</html>

WebGL JavaScript

Accessing Pixel Coordinates in a Fragment Shader

FragmentShaderCoordinates.html

Output