pack.c

// packing a float in glsl with multiplication and fract
vec4 packFloat( float depth ) {
  const vec4 bit_shift = vec4(
    256.0 * 256.0 * 256.0, 256.0 * 256.0, 256.0, 1.0 );
  const vec4 bit_mask  = vec4(
    0.0, 1.0 / 256.0, 1.0 / 256.0, 1.0 / 256.0 );

  // fract is the problem
  vec4 res = fract( depth * bit_shift );

  res -= res.xxyz * bit_mask;
  return res;
}