c - What does the constant 0.0039215689 represent? -

i keep seeing constant pop in various graphics header files

0.0039215689 

it seems have color maybe?

here first hit on google:

void rdp_g_setfogcolor(void) {     gfx.fogcolor.r = _shiftr(w1, 24, 8) * 0.0039215689f;     gfx.fogcolor.g = _shiftr(w1, 16, 8) * 0.0039215689f;     gfx.fogcolor.b = _shiftr(w1, 8, 8) * 0.0039215689f;     gfx.fogcolor.a = _shiftr(w1, 0, 8) * 0.0039215689f; }  void rdp_g_setblendcolor(void) {     gfx.blendcolor.r = _shiftr(w1, 24, 8) * 0.0039215689f;     gfx.blendcolor.g = _shiftr(w1, 16, 8) * 0.0039215689f;     gfx.blendcolor.b = _shiftr(w1, 8, 8) * 0.0039215689f;     gfx.blendcolor.a = _shiftr(w1, 0, 8) * 0.0039215689f;      if(opengl.ext_fragmentprogram && (system.options & brdp_combiner)) {         glprogramenvparameter4farb(gl_fragment_program_arb, 2, gfx.blendcolor.r, gfx.blendcolor.g, gfx.blendcolor.b, gfx.blendcolor.a);     } }  //...more 

what number represent? why no 1 seem declare const?

i couldn't find on google explained it.

0.0039215689 approximately equal 1/255.

seeing opengl, performance important. it's safe guess done performance reasons.

multiplying reciprocal faster repeatedly dividing 255.

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side note:

if you're wondering why such micro-optimization isn't left compiler, it's because unsafe floating-point optimization. in other words:

x / 255  !=  x * (1. / 255) 

due floating-point round-off errors.

so while modern compilers may smart enough optimization, not allowed unless explicitly tell them via compiler flag.

related: why doesn't gcc optimize a*a*a*a*a*a (a*a*a)*(a*a*a)?