c++ - Return floats from fragmentshader -

for application need create depth-image of mesh. want image, each pixel tells distance between camera center , intersection point. choose task opengl, of computation can done on gpu instead of cpu.

here code vertex-shader. computes real-world coordinate of intersection point , stores coordinates in varying vec4, have access in fragment-shader.

varying vec4 verpos; void main(void) {     gl_position = ftransform();     verpos = gl_modelviewmatrix*gl_vertex; } 

and here code fragment-shader. in fragment-shader use these coordinates compute eulidean distance origin (0,0,0) using pythagoras. access computed value on cpu, plan store distance color using gl_fragcolor, , extract color values using glreadpixels(0, 0, width, height, gl_red, gl_float, &distances[0]);

varying vec4 verpos; void main()  {      float x = verpos.x;     float y = verpos.y;     float z = verpos.z;      float depth = sqrt(x*x + y*y + z*z) / 5000.0;     gl_fragcolor = vec4(depth, depth, depth, 1.0); } 

the problem is, receive results in precision of 0.003921569 (=1/255). resulting values contain instance 0.364705890, 0.364705890, 0.364705890, 0.364705890, 0.368627459, 0.368627459, 0.368627459. of values in row same, , there jump of 1/255. somewhere inbetween gl_fragcolor , glreadpixels opengl converts floats 8 bits , afterwards converts them float.

how can extract distance-values float without loosing precision?

attach r32f or r16f texture fbo , render it. extract pixels glgetteximage2d() glreadpixels().