vec4 HdrEncode(vec3 value)
{
value = value / 65536.0;
vec3 exponent = clamp(ceil(log2(value)), -128.0, 127.0);
float commonExponent = max(max(exponent.r, exponent.g), exponent.b);
float range = exp2(commonExponent);
vec3 mantissa = clamp(value / range, 0.0, 1.0);
return vec4(mantissa, (commonExponent + 128.0)/256.0);
}
vec3 HdrDecode(vec4 encoded)
{
float exponent = encoded.a * 256.0 - 128.0;
vec3 mantissa = encoded.rgb;
return exp2(exponent) * mantissa * 65536.0;
}

Note that this snippet uses a rather strange scaling factor of 65536.0. I had to add this to avoid a bug with exponents near 0, and since my values where all below 65536.0, I just avoided and ignored this. And since I was able to change my MRT code to avoid the depth buffer at all (and therefore avoid this custom HDR coded), I’m not going to fix this. However, I still think it might be useful.