[Shader Library] Posterization Post Processing Effect (GLSL)

Shader Library - Posterization Post Processing Effect

More shaders are available here: Geeks3D Shader Library.

Image posterization is the tranformation of an image with a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another. The left part of the image shows the posterization effect while the right part shows the normal image.

You can download the demo source code here:
To see the shader in real time, you need GeeXLab 0.1.13. This demo does not use Python so you can use the version of GeeXLab without Python.

Unzip the source code somewhere, start GeeXLab and drop the file Posterize_Effect_Demo.xml in GeeXLab.

Some results:

  • AMD X2 3800+ / Radeon HD 5770 (Cat9.11 beta) / WinXP SP2: 710 FPS (640×480) and 690 FPS (1920×1080)

Shader desription (for NVIDIA and ATI)

Language: OpenGL 2 – GLSL

Type: Post processing filter


  • sceneBuffer (sampler2D): the scene buffer.
  • numColors (float): number of tone regions. Default=8.0
  • gamma (float): gamma factor. Default=0.6

Ouputs: color buffer

Shader code:

void main(void)
  gl_Position = ftransform();
  gl_TexCoord[0] = gl_MultiTexCoord0;
uniform sampler2D sceneTex; // 0
uniform float gamma; // 0.6
uniform float numColors; // 8.0
void main() 
  vec3 c = texture2D(sceneTex, gl_TexCoord[0].xy).rgb;
  c = pow(c, vec3(gamma, gamma, gamma));
  c = c * numColors;
  c = floor(c);
  c = c / numColors;
  c = pow(c, vec3(1.0/gamma));
  gl_FragColor = vec4(c, 1.0);


3 thoughts on “[Shader Library] Posterization Post Processing Effect (GLSL)”

  1. n00body

    Why didn’t the shader author just smear the gamma value? (ie “gamma.rrr” vs “vec3(gamma, gamma, gamma)”)

  2. JeGX Post Author

    you mean:
    c = pow(c, gamma.rrr);
    in place of
    tc = pow(tc, vec3(gamma, gamma, gamma)) ?

    If so, it’s just for the sake of clarity.

    c = pow(c, gamma.rrr) does not compile with ATI Radeon:
    “Fragment shader failed to compile with the following errors:
    WARNING: 0:13: warning(#153) Field selection requires structure, vector, or matrix on left hand side rrr”

    c = pow(c, vec3(gamma.rrr))
    is okay on Radeon. Funny!

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