title: OpenGL FXAA issues
body: <p>I am a Computer Science Student and I am currently doing a 2d Game for my Coursework.
  I know that it's kind of unnecessary, but I tried to implement the FXAA algorithm for the AA of the game. The shader I am using is this one, because I thought using just a simple frag shader would be alright. I can also be found <a href="https://github.com/demoscenepassivist/SocialCoding/blob/master/code_demos_jogamp/shaders/fxaa.fs" rel="nofollow">on Github</a>.</p>
  
  <pre><code>#version 120
  #define FXAA_REDUCE_MIN (1.0/128.0)
  #define FXAA_REDUCE_MUL (1.0/8.0)
  #define FXAA_SPAN_MAX 8.0
  uniform sampler2D sampler0;
  uniform vec2 resolution;
  
  void main(){
     vec2 inverse_resolution=vec2(1.0/resolution.x,1.0/resolution.y);
     vec3 rgbNW = texture2D(sampler0, (gl_FragCoord.xy + vec2(-1.0,-1.0)) * inverse_resolution).xyz;
     vec3 rgbNE = texture2D(sampler0, (gl_FragCoord.xy + vec2(1.0,-1.0)) * inverse_resolution).xyz;
     vec3 rgbSW = texture2D(sampler0, (gl_FragCoord.xy + vec2(-1.0,1.0)) * inverse_resolution).xyz;
     vec3 rgbSE = texture2D(sampler0, (gl_FragCoord.xy + vec2(1.0,1.0)) * inverse_resolution).xyz;
     vec3 rgbM  = texture2D(sampler0,  gl_FragCoord.xy  * inverse_resolution).xyz;
     vec3 luma = vec3(0.299, 0.587, 0.114);
     float lumaNW = dot(rgbNW, luma);
     float lumaNE = dot(rgbNE, luma);
     float lumaSW = dot(rgbSW, luma);
     float lumaSE = dot(rgbSE, luma);
     float lumaM  = dot(rgbM,  luma);
     float lumaMin = min(lumaM, min(min(lumaNW, lumaNE), min(lumaSW, lumaSE)));
     float lumaMax = max(lumaM, max(max(lumaNW, lumaNE), max(lumaSW, lumaSE))); 
     vec2 dir;
     dir.x = -((lumaNW + lumaNE) - (lumaSW + lumaSE));
     dir.y =  ((lumaNW + lumaSW) - (lumaNE + lumaSE));
     float dirReduce = max((lumaNW + lumaNE + lumaSW + lumaSE) * (0.25 * FXAA_REDUCE_MUL),FXAA_REDUCE_MIN);
     float rcpDirMin = 1.0/(min(abs(dir.x), abs(dir.y)) + dirReduce);
     dir = min(vec2( FXAA_SPAN_MAX,  FXAA_SPAN_MAX),max(vec2(-FXAA_SPAN_MAX, -FXAA_SPAN_MAX),dir * rcpDirMin)) * inverse_resolution;
     vec3 rgbA = 0.5 * (texture2D(sampler0,   gl_FragCoord.xy  * inverse_resolution + dir * (1.0/3.0 - 0.5)).xyz + texture2D(sampler0,   gl_FragCoord.xy  * inverse_resolution + dir * (2.0/3.0 - 0.5)).xyz);
     vec3 rgbB = rgbA * 0.5 + 0.25 * (texture2D(sampler0,  gl_FragCoord.xy  * inverse_resolution + dir *  - 0.5).xyz + texture2D(sampler0,  gl_FragCoord.xy  * inverse_resolution + dir * 0.5).xyz);
     float lumaB = dot(rgbB, luma);
     if((lumaB &lt; lumaMin) || (lumaB &gt; lumaMax)) {
        gl_FragColor = vec4(rgbA,1.0);
     } else {
        gl_FragColor = vec4(rgbB,1.0);
     }
  }
  </code></pre>
  
  <p>The problem I ran into is, that whenever I turn the shader on my screen is flipped upside down... How do I solve this? It's probably because of the Orthographic Projection, but I am just a student, so I can't come up with a solution.</p>