Comment by bscphil | Hacker News Reader

Comment by bscphil | Hacker News Reader

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> To me, the biggest limitation is the limited range in luminance. The difference in light intensity between sunlit snow and shadowed rock is often far, far more than 8 stops.

The bit depth of a photo or display device is not the same as the dynamic range at which that photo is displayed. With traditional SDR image formats, on some screens a "0" pixel will be displayed at about 1 nit brightness, and a "255" pixel will be displayed at about 100 nits. But on other screens, the same pixel will be shown at 500 nits or more. Thus with SDR images, the luminance range depends on the characteristics of the display device in use, and not the fact that it's only an "8 bit" photo. "8 bit" ≠ "8 stops". On the other hand, on displays with very high peak brightness, you may get banding even with images that are properly dithered to take advantage of the full capabilities of the 8 bit range.

The point of 10 bit and higher is that you can accurately capture all the details in between a very dark portion of an image and a very bright one without banding. The point of the new HDR standards (which are just now being introduced to images) is that luma is now on an absolute scale, so that you are no longer dependent on the characteristics of the individual display. (You have to have both together for either to be very useful.) But whether you're working with SDR or HDR, 8 bit or 10 bit, fundamentally the question of how many stops you can get (the display's true contrast) is a question of what the display is capable of reproducing, not the underlying technology you're working with.

I'd contend that a display that spans a dynamic range of a factor of 100 is only a 6.7-bit display. It is impossible to calibrate such a display to span 8 bits of intensity.

It isn't the maximal intensity that matters, (to the extent that the viewer's eye can adapt to the differences in intensity between displays), but rather the number of subdivisions available between peak intensity and darkness.

Totally agreed that improved bit-depth improves gradations/banding. An N+1-bit display has N bits between half-intensity and full intensity, twice as many bits as an N-bit display has.

Another way to see that this matters is that imaging sensors are generally linear in their response to impinging light. The optimal way to display the output from a 14-bit sensor is a 14-bit (or higher) display.

> I'd contend that a display that spans a dynamic range of a factor of 100 is only a 6.7-bit display. It is impossible to calibrate such a display to span 8 bits of intensity.

That's not what a nit is. A nit is a measurement of absolute brightness. You can have 256 gradations (8 bits) on a 100 nit display. You can have 1024 gradations (10 bits). You could have 65536. What's being measured here are two different things. Imagine I have a flashlight with adjustable brightness: if I turn it all the way up, that's what we're measuring when we talk about the nits of a display with with all white pixels. The bit depth is how many gradations the flashlight can display between full brightness and no light at all. So in fact a 100 nit display can have any bit depth. A stop is not a nit is not bit depth.

(This is a simplification that is only completely accurate for SDR screens with sRGB gamut. With HDR and tagged images, part of the range created by the extra bit depth is for "impossible" values, stuff that is too bright or colorful for the screen to display, and has to be tonemapped by the display into the possible range.)

First off -- thank you for this conversation. It has caused me to think more-deeply about the meaning of pixel-values in a display context. It has sent me into the standards, too. In particular, I found this document useful:

http://www.color.org/WP40-Black_Point_Compensation_2010-07-2...

We agree that bit-depth is independent of maximum light output. Where we appear to disagree is on the meaning of an ideal bit, in particular whether or not it is acceptable for the (gamma-corrected) intensity difference between pixel-value 0 and pixel-value 1 to be greater than the base intensity of the display at pixel-value 0. I believe that I'm arguing for the concept depicted in Figure 3 of the document above (relative colorimetric), while you are arguing for the concept in Figure 4 (black-point compensation).

There is an important pragmatism to your approach -- high contrast ratios are technically difficult, while higher-resolution gradations sitting atop a pedestal of minimum intensity may be more technically-feasible. Increased subdivisions between minimum intensity and maximum intensity do reduce banding, as you suggest. Such a rendering choice can portray the content of an image at the expense of the creator's intended contrast.

A linear display output with no offset will yield an intensity proportional to the intensity value assigned to that pixel. Higher bit-depth does not make for greater intensity, but rather more gradations between maximum intensity and zero.

As a scientist or as an artist, when specifying pixel values I expect the intensity, on a calibrated display, to reproduce a calibrated fraction of the maximum light intensity. It is helpful to understand that not all systems render with this intent.

In the original reply, the thought-experiment display had a minimum brightness of 1 nit and a maximum brightness of 100 nits and 255 brightness levels between them. That spans a factor of 2^6.7. The discrepancy between our opinions represents differences in a choice of rendering intent. If I had to specify the rendering strategy for portraying 8-bit pixel values on such a display, it would have less contrast in the shadows than the highlights but equal contrast in the highlights when compared with an 8-bit display with a minimum brightness of 0.4 nits and a maximum brightness of 100 nits (which is capable of 2^8 dynamic range). Your strategy would yield uniform contrast spanning the shadows and highlights, but less contrast on the 1-100 nit display than the 0.4-100 nit display.

In that situation, I suspect that most people would prefer your black-point compensated rendering strategy, as the shadows would not be crushed. I simply wouldn't market/sell it as a true 8-bit display, though I understand why a manufacturer would choose to do so.

Thank you -- I learned something useful.

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