https://zhuanlan.zhihu.com/p/411778623
// RGBM encode/decode
static const float kRGBMRange = 8.0;

half4 EncodeRGBM(half3 color)
{
    color *= 1.0 / kRGBMRange;
    half m = max(max(color.x, color.y), max(color.z, 1e-5));
    m = ceil(m * 255) / 255;
    return half4(color / m, m);
}

half3 DecodeRGBM(half4 rgbm)
{
    return rgbm.xyz * rgbm.w * kRGBMRange;
}

RGBM编码的核心思想就是进行归一化，把>1.0的数值转化到[0,1]区间内。