How to debug a GLSL shader

Question

I need to debug a GLSL program but I don t know how to output intermediate result  Is it possible to make some debug traces  like with printf  with GLSL

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The existing answers are all good stuff  but I wanted to share one more little gem that has been valuable in debugging tricky precision issues in a GLSL shader  With very large int numbers represented as a floating point  one needs to take care to use floor n  and floor n   0 5  properly to implement round   to an exact int  It is then possible to render a float value that is an exact int by the following logic to pack the byte components into R  G  and B output values        Break components out of 24 bit float with rounded int value      scaledWOB    offset  gt  gt  8   amp  0xFFFF   float scaledWOB   floor offset   256 0        c2    scaledWOB  gt  gt  8   amp  0xFF   float c2   floor scaledWOB   256 0        c0   offset -  scaledWOB  lt  lt  8    float c0   offset - floor scaledWOB   256 0        c1   scaledWOB -  c2  lt  lt  8    float c1   scaledWOB - floor c2   256 0         Normalize to byte range   vec4 pix      pix r   c0   255 0    pix g   c1   255 0    pix b   c2   255 0    pix a   1 0    gl FragColor   pix

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You can try this  https   github com msqrt shader-printf which is an implementation called appropriately  Simple printf functionality for GLSL    You might also want to try ShaderToy  and maybe watch a video like this one  https   youtu be EBrAdahFtuo  from  The Art of Code  YouTube channel where you can see some of the techniques that work well for debugging and visualising  I can strongly recommend his channel as he writes some really good stuff and he also has a knack for presenting complex ideas in novel  highly engaging and and easy to digest formats  His Mandelbrot video is a superb example of exactly that   https   youtu be 6IWXkV82oyY   I hope nobody minds this late reply  but the question ranks high on Google searches for GLSL debugging and much has of course changed in 9 years  -   PS  Other alternatives could also be NVIDIA nSight and AMD ShaderAnalyzer which offer a full stepping debugger for shaders

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I am sharing a fragment shader example  how i actually debug    version 410 core  uniform sampler2D samp  in VS OUT       vec4 color      vec2 texcoord    fs in   out vec4 color   void main void        vec4 sampColor      if  texture2D samp  fs in texcoord  x  gt  0 8f     Check if Color contains red         sampColor   vec4 1 0f  1 0f  1 0f  1 0f      If yes  set it to white     else         sampColor   texture2D samp  fs in texcoord     else sample from original     color   sampColor

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If you want to visualize the variations of a value across the screen  you can use a heatmap function similar to this  I wrote it in hlsl  but it is easy to adapt to glsl    float4 HeatMapColor float value  float minValue  float maxValue         define HEATMAP COLORS COUNT 6     float4 colors HEATMAP COLORS COUNT                  float4 0 32  0 00  0 32  1 00           float4 0 00  0 00  1 00  1 00           float4 0 00  1 00  0 00  1 00           float4 1 00  1 00  0 00  1 00           float4 1 00  0 60  0 00  1 00           float4 1 00  0 00  0 00  1 00              float ratio  HEATMAP COLORS COUNT-1 0  saturate  value-minValue   maxValue-minValue        float indexMin floor ratio       float indexMax min indexMin 1 HEATMAP COLORS COUNT-1       return lerp colors indexMin   colors indexMax   ratio-indexMin       Then in your pixel shader you just output something like     return HeatMapColor myValue  0 00  50 00     And can get an idea of how it varies across your pixels       Of course you can use any set of colors you like

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You can t easily communicate back to the CPU from within GLSL   Using glslDevil or other tools is your best bet     A printf would require trying to get back to the CPU from the GPU running the GLSL code   Instead  you can try pushing ahead to the display   Instead of trying to output text  output something visually distinctive to the screen   For example you can paint something a specific color only if you reach the point of your code where you want add a printf   If you need to printf a value you can set the color according to that value

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I have found Transform Feedback to be a useful tool for debugging vertex shaders  You can use this to capture the values of VS outputs  and read them back on the CPU side  without having to go through the rasterizer    Here is another link to a tutorial on Transform Feedback

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At the bottom of this answer is an example of GLSL code which allows to output the full float value as color  encoding IEEE 754 binary32  I use it like follows  this snippet gives out yy component of modelview matrix    vec4 xAsColor toColor gl ModelViewMatrix 1  1    if bool 1      put 0 here to get lowest byte instead of three highest     gl FrontColor vec4 xAsColor rgb 1   else     gl FrontColor vec4 xAsColor a 0 0 1     After you get this on screen  you can just take any color picker  format the color as HTML  appending 00 to the rgb value if you don t need higher precision  and doing a second pass to get the lower byte if you do   and you get the hexadecimal representation of the float as IEEE 754 binary32   Here s the actual implementation of toColor     const int emax 127     Input  x gt  0    Output  base 2 exponent of x if  x  0  amp  amp   isnan x   amp  amp   isinf x              -emax if x  0            emax 1 otherwise int floorLog2 float x        if x  0   return -emax         NOTE  there exist values of x  for which floor log2 x   will give wrong         off by one  result as compared to the one calculated with infinite precision         Thus we do it in a brute-force way      for int e emax e gt  1-emax --e          if x gt  exp2 float e    return e         If we are here  x must be infinity or NaN     return emax 1        Input  any x    Output  IEEE 754 biased exponent with bias emax int biasedExp float x    return emax floorLog2 abs x          Input  any x such that   isnan x   amp  amp   isinf x      Output  significand AKA mantissa of x if  isnan x   amp  amp   isinf x             undefined otherwise float significand float x           converting int to float so that exp2 genType  gets correctly-typed value     float expo float floorLog2 abs x         return abs x  exp2 expo         Input  x in 0 1            N gt  0    Output  Nth byte as counted from the highest byte in the fraction int part float x int N           All comments about exactness here assume that underflow and overflow don t occur     const float byteShift 256          Multiplication is exact since it s just an increase of exponent by 8     for int n 0 n lt N   n          x  byteShift          Cut higher bits away          q  in  0 1   cap  mathbb Q        float q fract x           Shift and cut lower bits away  Cutting lower bits prevents potentially unexpected        results of rounding by the GPU later in the pipeline when transforming to TrueColor        the resulting subpixel value          c  in  0 255   cap  mathbb Z          Multiplication is exact since it s just and increase of exponent by 8     float c floor byteShift q       return int c         Input  any x acceptable to significand      Output  significand of x split to  8 8 8 -bit data vector ivec3 significandAsIVec3 float x        ivec3 result      float sig significand x  2      shift all bits to fractional part     result x part sig 0       result y part sig 1       result z part sig 2       return result        Input  any x such that  isnan x     Output  IEEE 754 defined binary32 number  packed as ivec4 byte3 byte2 byte1 byte0  ivec4 packIEEE754binary32 float x        int e   biasedExp x          sign to bit 7     int s   x lt 0    128   0       ivec4 binary32      binary32 yzw significandAsIVec3 x          clear the implicit integer bit of significand     if binary32 y gt  128  binary32 y- 128         put lowest bit of exponent into its position  replacing just cleared integer bit     binary32 y  128 int mod float e  2            prepare high bits of exponent for fitting into their positions     e  2         pack highest byte     binary32 x e s       return binary32     vec4 toColor float x        ivec4 binary32 packIEEE754binary32 x          Transform color components to  0 1  range         Division is inexact  but works reliably for all integers from 0 to 255 if        the transformation to TrueColor by GPU uses rounding to nearest or upwards         The result will be multiplied by 255 back when transformed        to TrueColor subpixel value by OpenGL      return vec4 binary32  255

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The GLSL Shader source code is compiled and linked by the graphics driver and executed on the GPU  If you want to debug the shader  then you have to use graphics debugger like RenderDoc or NVIDIA Nsight

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void main      float bug 0 0    vec3 tile texture2D colMap  coords st  xyz    vec4 col vec4 tile  1 0      if something  bug 1 0     col x  bug     gl FragColor col

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Do offline rendering to a texture and evaluate the texture s data  You can find related code by googling for  render to texture  opengl Then use glReadPixels to read the output into an array and perform assertions on it  since looking through such a huge array in the debugger is usually not really useful     Also you might want to disable clamping to output values that are not between 0 and 1  which is only supported for floating point textures   I personally was bothered by the problem of properly debugging shaders for a while  There does not seem to be a good way - If anyone finds a good  and not outdated deprecated  debugger  please let me know

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GLSL Sandbox has been pretty handy to me for shaders    Not debugging per se  which has been answered as incapable  but handy to see the changes in output quickly

[opengl] How to debug a GLSL shader?

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