How do I convert a vec4 rgba value to a float?

Question

I packed some float data in a texture as an unsigned_byte, my only option in webgl. Now I would like unpack it in the vertex shader. When I sample a pixel I get a vec4 whi

Answer 1

I tried Arjans solution, but it returned invalid values for 0, 1, 2, 4. There was a bug with the packing of the exponent, which i changed so the exp takes one 8bit float and the sign is packed with the mantissa:

//unpack a 32bit float from 4 8bit, [0;1] clamped floats
float unpackFloat4( vec4 _packed)
{
    vec4 rgba = 255.0 * _packed;
    float sign =  step(-128.0, -rgba[1]) * 2.0 - 1.0;
    float exponent = rgba[0] - 127.0;    
    if (abs(exponent + 127.0) < 0.001)
        return 0.0;           
    float mantissa =  mod(rgba[1], 128.0) * 65536.0 + rgba[2] * 256.0 + rgba[3] + (0x800000);
    return sign *  exp2(exponent-23.0) * mantissa ;     


}

//pack a 32bit float into 4 8bit, [0;1] clamped floats
vec4 packFloat(float f) 
{
    float F = abs(f); 
    if(F == 0.0)
    {
        return  vec4(0,0,0,0);
    }
    float Sign =  step(0.0, -f);
    float Exponent = floor( log2(F)); 

    float Mantissa = F/ exp2(Exponent); 
    //std::cout << "  sign: " << Sign << ", exponent: " << Exponent << ", mantissa: " << Mantissa << std::endl;
    //denormalized values if all exponent bits are zero
    if(Mantissa < 1.0)
        Exponent -= 1;      

    Exponent +=  127;

    vec4 rgba;
    rgba[0] = Exponent;
    rgba[1] = 128.0 * Sign +  mod(floor(Mantissa * float(128.0)),128.0);
    rgba[2] = floor( mod(floor(Mantissa* exp2(float(23.0 - 8.0))), exp2(8.0)));
    rgba[3] = floor( exp2(23.0)* mod(Mantissa, exp2(-15.0)));
    return (1 / 255.0) * rgba;
}