5 ms·

OK but that doesn't explain this sentence from the original blog post : In the real world, the total contrast ratio between the brightest highlights and

OK but that doesn't explain this sentence from the original blog post : In the real world, the total contrast ratio between the brightest highlights and darkest shadows during a sunny day is on the order of 1,000,000:1. We would need 20 bits of just luminance to represent those illumination ranges

oppositelock 9y ago
Let's call that the writer's artistic license.
- ttoinou 9y ago
  Well it's kinda accepted in photography / computer vision to say this. Another argument for this formula might come from Shannon information entropy but I'm not 100% sure
jacobolus 9y ago
Theoretically you can have arbitrary contrast with 1 bit (white = looking at the sun, black = 1 photon per square centimeter per year, or whatever). The author probably has some specific amount of brightness discrimination in mind that he wants to adequately capture. For example, with brightness levels corresponding to just-noticeable differences in terms of human perception, after adapting to looking at that specific portion of the scene. In practice modeling vision precisely gets really complicated because there are several types of spatial and temporal adaptation and contrast effects that the human visual system can undergo / be affected by, and something like a video game definitely doesn’t have the computation available to handle it perfectly (not to mention the programmers don’t want to implement anything that complex and finicky). There is also a complication that display black is never especially close to zero light, and depends a lot on the light hitting the display. Etc. So for practical purposes you just have to come up with some kind of “good enough” heuristic.
- ttoinou 9y ago
  So for practical purposes you just have to come up with some kind of “good enough” heuristic Ok I agree ; that's what I'm saying to chrisseaton
- dragonwriter 9y ago
  > The author probably has some specific amount of brightness discrimination in mind that he wants to adequately capture. The assumptions seems to be a linear illumination scale with the darkest value at 1 (with true absence of illumination at zero); 1,000,000:1 needs about 20 bits for that.
baq 9y ago
2^20 is just above 1M, so that's how many bits you need to capture the darkest and the brightest pixel without losing information (brightest/darkest = 1M). the assumption is that you tone map those 20 bits onto however many bits you have available (8 for standard display, 10 for HDR, 14 for DSLR sensor, etc.) - at least that's my understanding after consulting wikipedia.
- chrisseaton 9y ago
  Either I'm crazy or everyone else is :) You still have lost information with 20 bits. There are an infinite number of levels of light between the darkest and the brightest. With 20 bits you can represent 1,000,000 steps between darkest and brightest. Why do you need to represent 1,000,000 steps? Why does it matter that the brightest is 1,000,000 times brighter than the darkest? 20 bits lets you represent ones times brighter, two times bright, etc up to 1,000,000 times brighter, but what about one-and-a-half times brighter? You can't represent that in 20 bits. Why doesn't that matter? The article says that the highest light level in a scene is 1,000,000 times brighter than the lowest. It does not say, or even imply, that you can see the difference between those levels of light to a one in a million, which is what 20 bits needs. Can you see what I mean? The factor between the lowest and the highest is entirely separate from the question of how many levels between you can see, which is what directs how many bits of resolution you need.
  - balls187 9y ago
    To express a ratio of 1000000:1, you need at least 20bits. I think that's all the article is saying, and that far more bits than the current contrast ratio that is supported--essentially show that we're far off from supporting actual contrast ratios. I didn't interpret the articles premise that if you had 20 bits, you could 100% reproduce the light in a scene.
    - chrisseaton 9y ago
      Sorry I still don't get it :) > To express a ratio of 1000000:1, you need at least 20 bits. You mean to express all the integer factors in the range 1000000:1, you need at least 20 bits. With 20 bits you can represent 1 times brighter, 2 times brighter ... 1000000 times brighter. But there's nothing special about those coefficients. 20 bits does not allow you to represent 1.5 times brighter. That's still in the range 1000000:1 but 20 bits isn't enough to represent it. If we're happy to skip 1.5 times brighter, why can't we skip all the even integer times brighter values, and use 19 bits?
stefs 9y ago
what he meant, technically: 2^19 = 524,288 2^20 = 1,048,576

oppositelock 9y ago

Let's call that the writer's artistic license.

ttoinou 9y ago
Well it's kinda accepted in photography / computer vision to say this. Another argument for this formula might come from Shannon information entropy but I'm not 100% sure

jacobolus 9y ago

Theoretically you can have arbitrary contrast with 1 bit (white = looking at the sun, black = 1 photon per square centimeter per year, or whatever). The author probably has some specific amount of brightness discrimination in mind that he wants to adequately capture. For example, with brightness levels corresponding to just-noticeable differences in terms of human perception, after adapting to looking at that specific portion of the scene. In practice modeling vision precisely gets really complicated because there are several types of spatial and temporal adaptation and contrast effects that the human visual system can undergo / be affected by, and something like a video game definitely doesn’t have the computation available to handle it perfectly (not to mention the programmers don’t want to implement anything that complex and finicky). There is also a complication that display black is never especially close to zero light, and depends a lot on the light hitting the display. Etc. So for practical purposes you just have to come up with some kind of “good enough” heuristic.

ttoinou 9y ago
So for practical purposes you just have to come up with some kind of “good enough” heuristic Ok I agree ; that's what I'm saying to chrisseaton
dragonwriter 9y ago
> The author probably has some specific amount of brightness discrimination in mind that he wants to adequately capture. The assumptions seems to be a linear illumination scale with the darkest value at 1 (with true absence of illumination at zero); 1,000,000:1 needs about 20 bits for that.

baq 9y ago

2^20 is just above 1M, so that's how many bits you need to capture the darkest and the brightest pixel without losing information (brightest/darkest = 1M). the assumption is that you tone map those 20 bits onto however many bits you have available (8 for standard display, 10 for HDR, 14 for DSLR sensor, etc.) - at least that's my understanding after consulting wikipedia.

chrisseaton 9y ago
Either I'm crazy or everyone else is :) You still have lost information with 20 bits. There are an infinite number of levels of light between the darkest and the brightest. With 20 bits you can represent 1,000,000 steps between darkest and brightest. Why do you need to represent 1,000,000 steps? Why does it matter that the brightest is 1,000,000 times brighter than the darkest? 20 bits lets you represent ones times brighter, two times bright, etc up to 1,000,000 times brighter, but what about one-and-a-half times brighter? You can't represent that in 20 bits. Why doesn't that matter? The article says that the highest light level in a scene is 1,000,000 times brighter than the lowest. It does not say, or even imply, that you can see the difference between those levels of light to a one in a million, which is what 20 bits needs. Can you see what I mean? The factor between the lowest and the highest is entirely separate from the question of how many levels between you can see, which is what directs how many bits of resolution you need.
- balls187 9y ago
  To express a ratio of 1000000:1, you need at least 20bits. I think that's all the article is saying, and that far more bits than the current contrast ratio that is supported--essentially show that we're far off from supporting actual contrast ratios. I didn't interpret the articles premise that if you had 20 bits, you could 100% reproduce the light in a scene.
  - chrisseaton 9y ago
    Sorry I still don't get it :) > To express a ratio of 1000000:1, you need at least 20 bits. You mean to express all the integer factors in the range 1000000:1, you need at least 20 bits. With 20 bits you can represent 1 times brighter, 2 times brighter ... 1000000 times brighter. But there's nothing special about those coefficients. 20 bits does not allow you to represent 1.5 times brighter. That's still in the range 1000000:1 but 20 bits isn't enough to represent it. If we're happy to skip 1.5 times brighter, why can't we skip all the even integer times brighter values, and use 19 bits?

stefs 9y ago

what he meant, technically: 2^19 = 524,288 2^20 = 1,048,576