4 ms·

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

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?
- dragonwriter 9y ago
  > 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? Things get wonky if you don't have a linear scale with a true zero; in such a scale the low end of your N:1 contrast ratio (in the smallest representation) has a value of 1, and the high end has a value of N.
  - comex 9y ago
    Hmm… but the actual display’s representation of 0 cannot be darker than the minimum brightness value it supports. If 1 really is 1x the minimum, then 0 and 1 have to be displayed identically. But then your scale isn’t even linear, nor does it have a true zero. How does that help? edit: And aren’t output color spaces already highly nonlinear due to gamma correction?
    - colejohnson66 9y ago
      0 is the lowest and x-1 is the highest for a ratio of 1:x. It’s 1 all the way to x; You just subtract one because computers start counting from 0
      - comex 9y ago
        But in that case, skipping "even integer times brighter values" wouldn't exclude 0 - because 0 is 1x brighter, an odd integer.
  - colejohnson66 9y ago
    Is that the reason you can’t just chop off the two least significant bits when converting from 10-bit to 8-bit?
  - tripzilch 9y ago
    Good thing in real life there's no such thing as truly zero photons, then (which is a detail the article actually shortly touches upon). Also, if that type of "wonky" throws off your rendering pipeline, you're bound to get something else wrong. Such as ever having a linear scale with a small number of bits in your pipeline. The linear scaled brightness stays afloat all the way through (cause floats have this handy feature of being transparently sorta-logarithmic in the way they use their bits, even 16-bit floats beat 20-bit ints for that purpose), only at the very end you apply the tonemap+gamma function(s), then dither, then truncate to fixed (8) bit integer.
robotresearcher 9y ago
> To express a ratio of 1000000:1, you need at least 20bits. You only need 1 bit if you declare an encoding scheme where 1 == 1 million. You need 20 bits to define 1 million DIFFERENT luminances. Their ratio is subject to an arbitrary scaling.

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?

dragonwriter 9y ago
> 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? Things get wonky if you don't have a linear scale with a true zero; in such a scale the low end of your N:1 contrast ratio (in the smallest representation) has a value of 1, and the high end has a value of N.
- comex 9y ago
  Hmm… but the actual display’s representation of 0 cannot be darker than the minimum brightness value it supports. If 1 really is 1x the minimum, then 0 and 1 have to be displayed identically. But then your scale isn’t even linear, nor does it have a true zero. How does that help? edit: And aren’t output color spaces already highly nonlinear due to gamma correction?
  - colejohnson66 9y ago
    0 is the lowest and x-1 is the highest for a ratio of 1:x. It’s 1 all the way to x; You just subtract one because computers start counting from 0
    - comex 9y ago
      But in that case, skipping "even integer times brighter values" wouldn't exclude 0 - because 0 is 1x brighter, an odd integer.
- colejohnson66 9y ago
  Is that the reason you can’t just chop off the two least significant bits when converting from 10-bit to 8-bit?
- tripzilch 9y ago
  Good thing in real life there's no such thing as truly zero photons, then (which is a detail the article actually shortly touches upon). Also, if that type of "wonky" throws off your rendering pipeline, you're bound to get something else wrong. Such as ever having a linear scale with a small number of bits in your pipeline. The linear scaled brightness stays afloat all the way through (cause floats have this handy feature of being transparently sorta-logarithmic in the way they use their bits, even 16-bit floats beat 20-bit ints for that purpose), only at the very end you apply the tonemap+gamma function(s), then dither, then truncate to fixed (8) bit integer.

robotresearcher 9y ago

> To express a ratio of 1000000:1, you need at least 20bits. You only need 1 bit if you declare an encoding scheme where 1 == 1 million. You need 20 bits to define 1 million DIFFERENT luminances. Their ratio is subject to an arbitrary scaling.