Blendable RGBM
What is RGBM? Using a fat buffer(16 bit-per-channel) is the most intuitive way to store a HDR render target, but it was too slow and it used too much memory, so we decided to encode(pack) it into a 8 bit-per-channel buffer. There are different packing methods, but I believe the most widely used method was RGBM. (also this link has a nice summary of LogLUV too, so give it a read if you are not familiar with this topic at all)