title: GLSL map gl_FragCoord.xy to coordinates in orthographic projection
body: <p>Hello I have an opengl Program that renders 2d polygons in an orthographic projection. At the start of the porgram, or when the window size changes, the function reshape is called. Here is the code for the reshape function:</p>
  
  <pre><code>    /* Call back when the windows is re-sized */
      void reshape(GLsizei width, GLsizei height) {
      // Compute aspect ratio of the new window
      if (height == 0) height = 1;                
      // To prevent divide by 0
      GLfloat aspect = (GLfloat)width / 
      (GLfloat)height;
  
      // Set the viewport to cover the new window
      glViewport(0, 0, width, height);
  
      // Set the aspect ratio of the clipping area to match the viewport
      glMatrixMode(GL_PROJECTION);  // To operate on the Projection matrix
      glLoadIdentity();             // Reset the projection matrix
      if (width &gt;= height) {
      clipAreaXLeft = -1.0 * aspect;
      clipAreaXRight = 1.0 * aspect;
      clipAreaYBottom = -1.0;
      clipAreaYTop = 1.0;
      }
      else {
      clipAreaXLeft = -1.0;
      clipAreaXRight = 1.0;
      clipAreaYBottom = -1.0 / aspect;
      clipAreaYTop = 1.0 / aspect;
      }
      clipAreaXLeft *= 600;
      clipAreaYBottom *= 600;
      clipAreaXRight *= 600;
      clipAreaYTop *= 600;
  
     gluOrtho2D(clipAreaXLeft, clipAreaXRight, 
  clipAreaYBottom, clipAreaYTop);
      glScissor(0, 0, width, height);
      glEnable(GL_SCISSOR_TEST);
  
      }
  </code></pre>
  
  <p>Here is some code for a GLSL fragment shader:</p>
  
  <pre><code>#version 420 core
  out vec4 color
  void main(){
  vec2 orthoXY =... need help here, should 
  convert window-space to ortho-space, 
  maybe use projection matrix from fixed 
  pipeline?
  color=vec4{1,1,1,1}
  }
  </code></pre>