Yeah. There's definitely some unstated assumptions in that number. I think he is implying a linear scale, with the ability to represent details similar in magni

Yeah. There's definitely some unstated assumptions in that number. I think he is implying a linear scale, with the ability to represent details similar in magnitude to the minimum brightness. Without some assumptions like those, all it takes to represent that contrast is a single bit to choose between 1 and 1,000,000. EDIT: I might as well link to the author's clarification. I missed it in the noise: https://news.ycombinator.com/item?id=15538738 https://news.ycombinator.com/item?id=15538738 It is a little odd. I don't think a linear, fixed-precision scale for luminance is used much, though I could be quite mistaken. I'm not very familiar with HDR. Still, I might as well explain a bit further... Light accumulation is linear. If you have two lights, the amount of light reflecting off the surface is twice as much. Twice as many photons. Human perception, however, is logorithmic. Something that is twice as bright in terms of photons emitted actually only looks a little brighter to the human eye. To get the same perceived brightness increment again, you need to double the photon count again. With that in mind, what's the best numeric representation to use? Well, you want a simple way to add up the total amount of light from all sources. The amount of light may vary by orders of magnitude, but it's manageable because your need for precision scales with the magnitude you're dealing with. Basically, you want floating point. In practice, a float for each color channel means 3 floats (96 bits) per pixel, which is a lot. So, while that might be used during calculations, it's probably stored compressed. For example, RGBE is a 32 bit per pixel format. You use 8 bits per color channel and multiply them all by 2^(E+128), where E is an 8 bit signed integer. It's kind of inaccurate for the dim channels if luminance varies significantly between colors, but the brightest one is probably going to overwhelm the other channels anyways.