title: Convert WebGL fragment shader to GLES
body: <p>I am a beginner in OpenGL, trying to run some tests. There are plenty of fragment shaders available at <a href="https://glsl.heroku.com/" rel="nofollow">GLSL Sandbox Gallexy</a>, and I would like to try them in GLES, reusing the code. But, for most of the shaders, it does not seem to work.</p>
  
  <p>Among the shaders I tried to run, the only one that worked in GLES is <a href="https://glsl.heroku.com/e#11602.0" rel="nofollow">this one</a>, for some reason. And I had to eliminate the time dependence of the shader, in order to accomplish that.</p>
  
  <p>It seems that some variable names are different in WebGL and GLES. If that's the case, which ones, exactly? If not, what exactly is the procedure of translation from the former to the later?</p>
  
  <p><a href="https://glsl.heroku.com/e#11726.1" rel="nofollow">One example</a> of simple fragment shader that gives a black screen only. In my particular case, I am running it along this minimal vertex shader:</p>
  
  <pre><code>precision mediump float;
  
  uniform mat4 uMVPMatrix;
  
  attribute vec4 aPosition;
  attribute vec2 aTextureCoord;
  
  varying vec2 vTextureCoord;
  
  void main() {
      vTextureCoord = aTextureCoord;
      gl_Position = uMVPMatrix * aPosition;
  }
  </code></pre>