<p>HDR doesn't relate to color space at all, as it stands for "High <em>Dynamic</em> Range", ie. brightness, not color, contrast.</p>

<p>Traditional TV signals had a rather low contrast ratio, as old TV sets had a contrast of only <a href="http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html#contrast_ratio" rel="nofollow noreferrer">30:1</a>. Modern displays can produce a lot more, 1000:1 or more with dynamic backlight (local dimming).</p>

<p>"HDR" displays are just a marketing term for displays with a very high contrast ratio.</p>

<p>But using this high contrast will require a higher bit depth signal or you'll have only a few steps spread out over the whole range which may introduce a kind of banding just like in the color domain.</p>

<p>You can't squeeze 1000:1 into an 8 bit brightness signal, so you need a higher bit depth. 10 bit can encode 1024 steps, which still isn't that high really, but better than 256.</p>

<p>Calling those displays "HDR" is, from a photography perspective, still laughable, as HDR photos are encoded in 32 bit floating point values with near infinite steps, but the dynamic range <em>is</em> higher than that of a traditional CRT TV or the standard NTSC/PAL signal. (Note that you need an "HDR" signal source to actually enjoy that capacity.)</p>

CC BY-SA 3.0
2017-06-13T22:09:37.067Z
@Vikram With two dimensional color gamut maps both the *x* and *y* coordinates specify a particular hue (color). Brightness is not represented at all. If brightness were included the map would need a third *z* axis perpendicular to the plane formed by the *x* and *y* axes.Keep in mind that most color gamut maps you see aren't the *actual* colors. If that were the case you couldn't view the edges of an Adobe RGB color map on an sRGB device, much less a printed piece of paper. You wouldn't be able to see anything outside the gamut lines even on an Adobe RGB monitor.
Michael C
CC BY-SA 3.0
2017-06-13T11:52:05.630Z
2017-06-13T11:58:34.737Z
ths
<p>HDR in the modern TV sense refers to one of several different HDR specifications for color.  To understand what High Dynamic Range means, we first need to understand what dynamic range is.</p>

<p>Dynamic Range is the difference between the highest and lowest values in a signal or display.  A high dynamic range TV generally means that there is a large amount of brightness levels between the darkest and lightest possible portions of the display that can be shown at the same time.</p>

<p>The HDR specifications specify more than just the max and min brightness levels that must be able to be displayed though.  They also specify that a relatively high bit depth must be possible.  This allows for a video to have a greater number of levels of brightness within it, which also means that more subtle variations in color can be created and thus better use of the increased range of brightness that can be produced.  The higher the dynamic range, the more change in color between each value at a particular bit depth, but you can have fairly low dynamic range displays or fairly high dynamic range displays that still have a high bit depth.</p>

<p>There is also requirements for contrast ratio.  Contrast ratio is similar to dynamic range, but not exactly the same thing.  Contrast ratio is the relationship, in absolute light level, between the darkest and brightest values the display can simultaneously display.  It is dependent both on how bright the display can get and how well it can block light.  The contrast ratio determines how well a bright and dark portion of a scene can co-exist at the same time.  Static contrast ratio is the meaningful value as it is what the display can really do.  Dynamic doesn't mean a whole lot because it depends on being able to do adjustments that aren't at a pixel level and don't really produce an ideal image quality.</p>

CC BY-SA 3.0
2017-06-13T22:59:30.450Z
2017-06-13T22:59:30.450Z
AJ Henderson
<p>HDR is high dynamic range. The older technology is SDR - standard dynamic range. When we talk about colorspaces, there are also colorspaces specifically designed for HDR like ICtCp. ICtCp is derived from IPT (a colorspace for SDR) by using a different function for lightness.</p>
<p>Standard colorspaces are usually poorly defined for highlights and dark tones. This means highlights would be most likely encoded as a small region around pure white (i.e. loose precision/data/information there). Similar with shadows.</p>
<p>So, HDR in the colorspace terms is: a different lightness function (HLG - hybrid log or PQ function) + better defined color information for highlights and shadows.</p>
<p>Using HDR colorspace is only relevant for editing/storing when you deal with SDR content (that is why you can recover shadows/highlights when editing RAW files). When you deal with HDR content - you would have to use a colorspace for HDR content.</p>
<p>A lot of research papers clearly say that SDR colorspaces (i.e. CIECAM02, Lab) were only tested for illuminant around 100 nits. Higher illuminant or extreme changes in lightness and SDR colorspaces enter an &quot;uncharted&quot; territory. They could work fine, but less optimal as colorspaces defined for HDR.</p>

CC BY-SA 4.0
2024-01-29T10:26:35.217Z
2024-01-29T10:37:37.403Z
2024-01-29T10:37:37.403Z
Colorplane
<p>To comment two well-known statements here:</p>

<p>"You could also have a fairly low bit depth in a very wide color space where the steps between each gradation are much coarser."</p>

<p>"Color space and bit depth is only related to HDR in the sense that they are part of the same standards. Yes, you need more bits to represent a higher dynamic range without banding."</p>

<p>Both don't take dithering into account. In the absence of dither, the statements are correct but with dither applied, in theory, a smooth gradient would be even possible with only one bit, although it's extremely noisy when applying spatial dithering.</p>

<p>If one accepts a lower SNR, there won't be banding regardless of the bit depth.</p>

<p>If it is true that bit depth and colour space are not related (which makes sense as any colour space could exist in a purely analog domain as well without any bits to consider at all), then the logical conclusion would be: theoretical perfect dithering techniques assumed, a given bit depth - analog to audio - only limits the achievable signal to noise ratio and thereas the dynamic range as the noise will mask the lower end, however not the possible amount of colors, not any banding and no brightest whites or whatever.</p>

CC BY-SA 3.0
2017-07-24T13:45:55.140Z
Sure, but then you are sacrificing resolution for color and it only works to a point because if you don't have enough pixels to dither over, it becomes noticeable to the eye.  Also,. to the best of my knowledge, most displays aren't applying dithering on the fly to compensate for something like this, they are just displaying pixel values provided, so dithering doesn't make much of a practical difference in relation to things like the various HDR tv specs.
AJ Henderson
CC BY-SA 3.0
2017-07-27T02:55:39.590Z
@AJ Henderson:

thanks for your reply. You are right that many practical implementations lead to effects, which the oversimplified and - from the signal theory's point of view incorrect - assumptions suggest.

However, the fact alone that practical implementations are so often so bad, doesn't make the oversimplified assumptions correct vice-versa, either.

I can't prove it yet either, but I doubt that dithering in fact reduces resolution as I think most people confuse resolution and signal-to-noise ratio and these are two different parameters determining the quality.
little-endian
CC BY-SA 3.0
2017-07-27T02:57:27.897Z
One can beautifully test this on one's own by using the renderer "madVR" under Windows (recommended in conjunction with the player MPC, but it should work with others, too) which supports several great implementations of dither.

Using "randomised dither", one can reduce the bit depth to even only 1-bit per color channel (leading to 8 distinct colors only in total) and still have all the colors and no banding.
little-endian
CC BY-SA 3.0
2017-07-27T02:59:00.877Z
The image is very noisy (just like a bad analog TV signal back in the days) and certain detail is lost in the noise of course, but the nominal frequency response of the image (resolution) seems to remain unaffected which can be indirectly proven by the absence of introduced aliasing which would be to be expected if the real resolution frequency-wise would be lowered by the dithering.

In the audio domain, it's also exactly the same. There dither definitely does NOT reduce or limit the frequency response in any way, but depending on the bit depth it will raise the overall noise floor.
little-endian
CC BY-SA 3.0
2017-07-27T03:00:13.027Z
Video resolution equals audio frequency response.

Hence from my understanding, it is correct to talk about a higher dynamic range and lower noise floor with a higher bit depth (one bit giving about 6dB of additional SNR), but not about "finer resolution of colors" or even a higher color space as all these assumptions are wrong in the audio domain and a video signal could always also be interpreted as a high frequency audio signal too so all parameters logically have to have a 1:1 expression (which they seem to have in practise too).
little-endian
CC BY-SA 3.0
2017-07-27T03:00:37.327Z
Hence, we might not necessarily need more data, but a better implementation in practise when it comes to video applications.
little-endian
CC BY-SA 3.0
2017-07-27T03:11:17.857Z
Sorry, forgot another important remark. You wrote "Also, to the best of my knowledge, most displays aren't applying dithering on the fly to compensate for something like this". Actually, that isn't the display's task anyway just like it wouldn't be a D/A-converter for audio to apply dither either. The dither is supposed to be part of the image so to say. The only exception is that if some render device accepts a higher bit depth and has to reduce it "on the fly" due to its own limitations. Otherwise, the dither is part of the image so the pixels are displayed 1:1 at the given color depth.
little-endian
CC BY-SA 3.0
2017-07-24T10:33:40.040Z
2017-07-24T10:56:26.743Z
2017-07-24T10:56:26.743Z
little-endian
<p>TV technology and video encoding is in principle outside the scope of this site. 
Still, as photographers we are presumably interested in new display standards that promise both more dynamic range and wider color gamut. HDR computer monitors <a href="http://www.pcworld.com/article/3154466/displays/jaw-droppingly-gorgeous-hdr-explodes-onto-pc-monitors-at-ces-2017.html" rel="noreferrer">are coming</a>, so it probably won't be long before photographers too will be interested in HDR displays.</p>

<p>HDR by itself just means High Dynamic Range. Dynamic range is the brightness ratio between the brightest and the darkest parts. "High" is relative to what the sensor can capture or to what a standard display can show.</p>

<p><a href="https://en.wikipedia.org/wiki/High-dynamic-range_video" rel="noreferrer">HDR video</a> is one of several standards for video encoding. They have names like HDR10, Dolby Vision, and Hybrid Log-Gamma. They all support higher bit depth (10-12 bits), wider color gamut (<a href="https://en.wikipedia.org/wiki/Rec._2020" rel="noreferrer">Rec. 2020</a> and/or <a href="https://en.wikipedia.org/wiki/DCI-P3" rel="noreferrer">DCI-P3</a>), higher dynamic range, and 4k resolution. They are not compatible with each other, but some devices support more than one HDR standard.</p>

<p><a href="http://www.trustedreviews.com/opinions/hdr-tv-high-dynamic-television-explained" rel="noreferrer">"HDR TV"</a> doesn't necessarily mean much, except that it will accept a HDR video signal and display an image. It does <em>not</em> necessarily mean that it can <em>display</em> the wider gamut and higher dynamic range, just that it can read the signals and show something, even if what it shows is plain sRGB with a mundane dynamic range.</p>

<p><a href="https://www.cnet.com/news/what-is-uhd-alliance-premium-certified/" rel="noreferrer">"UHD Premium"</a> is a certification standard for displays that can actually <em>show</em> HDR. Requirements include:</p>

<ul>
<li>minimum 10 bit color</li>
<li>minimum 90% of the DCI-P3 color gamut</li>
<li>use a HDR transfer function <em>(The transfer function for sRGB is called 'gamma', it translates bit value to brightness value. HDR uses a different function so it can cover a larger dynamic range with a small increase in the number of bits)</em></li>
<li>either: Minimum 1,000 nits brightness and max 0.05 nits black level, contrast ratio 20,000:1 or better (for LCD displays)</li>
<li>or: Minimum 540 nits brightness and max 0.0005 nits black level, contrast ratio 1,080,000:1 or better (for OLED displays, they can't go as bright but have better black levels)</li>
</ul>

<p>Standard displays are 200-400 nits and rarely go beyond 1,000:1 contrast ratio. So a "Premium" HDR display has significantly higher peak brightness as well as an order of magnitude better contrast. </p>

<p>Brighter highlights, higher contrast ratios, and a different "bits to brightness" encoding/decoding all combine to enable the "high dynamic range" part of a HDR TV.</p>

<p>Color space and bit depth is only related to HDR in the sense that they are part of the same standards. Yes, you need more bits to represent a higher dynamic range without banding. But the larger color space is a different decision, it's more a matter of including enough improvements to make a new video standard worth the trouble. (4k is also part of the standard, and 4k is not related to HDR either.)</p>

<p>"The sun looks brighter on a HDR display" means that a) the display emits more light in the brightest parts of the scene, and b) it may look even brighter because the rest of the scene can be dimmer without losing detail. </p>

<p>So it's a real improvement; the wider color gamut and higher dynamic range can  contribute to more vivid photos and video. </p>

<p>The caveat is that TVs/monitors may be sold as "HDR compatible" without any actual HDR capability in the display. The content needs to be made specifically for HDR to make use of the new capabilities. And for computer monitors, you need a graphics card that supports HDR output. </p>

CC BY-SA 3.0
2017-06-13T20:37:28.627Z
2017-06-13T20:46:02.233Z
j-g-faustus
<p>I'm not a photographer, but I'm struggling to understand HDR technology as it relates to TVs and monitors, so I thought this would be a good place to ask.</p>

<p>First, I got a good understanding of the terms "color space" and "bit depth" by reading this <a href="https://photo.stackexchange.com/a/50551/63891">excellent answer</a>.</p>

<p>How does HDR technology relate to these terms? Is HDR simply a combination of a 10-bit color depth and really wide color space, or is there something more to it? </p>

<p>In the marketing language, certain areas of the screen (such as the rising sun) are said to be "brighter" than other areas of the screen on a HDR display. What does "brighter" actually mean here — does it just mean "farther to the edge of the color space"?</p>

<p>I currently have a DELL U2410 monitor, which, as I understand, has a 12-bit internal processor and an 8-bit panel with FRC dithering, and supports an Adobe RGB color space. Even though this is not true 10-bit, will it produce a result similar to HDR if I use a device which can output HDR? I currently set the color space to "sRGB" since Adobe RGB looks too oversaturated. Could this be because my graphics card does not support a 10-bit output?</p>

<p>EDIT: I am talking about "HDR" specifically as it relates to how the term is used in modern 4K TVs. </p>

CC BY-SA 3.0
2017-06-13T09:52:54.413Z
"HDR" means a lot of different things to a lot of different people. In terms of bit depth, HDR usually refers to a 32-bit floating point light map that isn't viewable on any monitor in existence. To view an HDR image it must be tone mapped (or otherwise interpolated) to squeeze those 32 bits into an 8 or 10 bit capable output device.
Michael C
CC BY-SA 3.0
2017-06-13T09:55:24.497Z
The problem with bit depth is that it has absolutely nothing to do with the size of a color space. You could have a very high bit depth in a very narrow color space where the steps between each gradation are very fine. You could also have a fairly low bit depth in a very wide color space where the steps between each gradation are much coarser.
Michael C
CC BY-SA 3.0
2017-06-13T09:59:48.373Z
"Could this be because my graphics card does not support a 10-bit output?" It's probably much more likely your monitor's saturation is just turned up too high. Most screens come from the factory way too bright, way too saturated, and way too cool (high color temperature). Have you calibrated and profiled your system with a colorimeter?
Michael C
CC BY-SA 3.0
2017-06-13T10:05:19.940Z
Related: [What's the difference between “Fake HDR” and real, bracketed exposure HDR?](https://photo.stackexchange.com/q/80591/15871)
Michael C
CC BY-SA 3.0
2017-06-13T10:07:19.943Z
Related: [What's the point of capturing 14 bit images and editing on 8 bit monitors?](https://photo.stackexchange.com/q/72116/15871)
Michael C
CC BY-SA 3.0
2017-06-13T10:26:30.297Z
HDR as is normally used in still photography is not the same thing as HDR when used to describe 4K TVs. The reason we have more content containing the expression 'HDR' is probably because it's been around still photography for about 165 years and is a relatively new term with regard to video screens.
Michael C
CC BY-SA 3.0
2017-06-13T11:06:26.243Z
It doesn't mean much of anything. The extra bit depth can be used for finer gradations between the same 'darkest' and 'brightest' emissions that can be simultaneously displayed *or* the extra bit depth can be used to increase the difference between the 'darkest' and 'brightest' emissions that can be simultaneously displayed. But neither choice is anywhere remotely near the range that can be contained in a 32-bit floating point light map that is what 'HDR' has come to mean in the context of digital still imaging.
Michael C
<hdr>
How does HDR relate to color space and bit depth?
Vikram
# How does HDR relate to color space and bit depth?

(Asked by: username_0 on 2017-06-13)

I'm not a photographer, but I'm struggling to understand HDR technology as it relates to TVs and monitors, so I thought this would be a good place to ask.

First, I got a good understanding of the terms "color space" and "bit depth" by reading this [excellent answer](https://photo.stackexchange.com/a/50551/63891).

How does HDR technology relate to these terms? Is HDR simply a combination of a 10-bit color depth and really wide color space, or is there something more to it?

In the marketing language, certain areas of the screen (such as the rising sun) are said to be "brighter" than other areas of the screen on a HDR display. What does "brighter" actually mean here — does it just mean "farther to the edge of the color space"?

I currently have a DELL U2410 monitor, which, as I understand, has a 12-bit internal processor and an 8-bit panel with FRC dithering, and supports an Adobe RGB color space. Even though this is not true 10-bit, will it produce a result similar to HDR if I use a device which can output HDR? I currently set the color space to "sRGB" since Adobe RGB looks too oversaturated. Could this be because my graphics card does not support a 10-bit output?

EDIT: I am talking about "HDR" specifically as it relates to how the term is used in modern 4K TVs.

**Tags:** hdr

> **Comments:**
> **username_1**: "HDR" means a lot of different things to a lot of different people. In terms of bit depth, HDR usually refers to a 32-bit floating point light map that isn't viewable on any monitor in existence. To view an HDR image it must be tone mapped (or otherwise interpolated) to squeeze those 32 bits into an 8 or 10 bit capable output device.  
> **username_1**: The problem with bit depth is that it has absolutely nothing to do with the size of a color space. You could have a very high bit depth in a very narrow color space where the steps between each gradation are very fine. You could also have a fairly low bit depth in a very wide color space where the steps between each gradation are much coarser.  
> **username_1**: "Could this be because my graphics card does not support a 10-bit output?" It's probably much more likely your monitor's saturation is just turned up too high. Most screens come from the factory way too bright, way too saturated, and way too cool (high color temperature). Have you calibrated and profiled your system with a colorimeter?  
> **username_1**: Related: [What's the difference between “Fake HDR” and real, bracketed exposure HDR?](https://photo.stackexchange.com/q/80591/15871)  
> **username_1**: Related: [What's the point of capturing 14 bit images and editing on 8 bit monitors?](https://photo.stackexchange.com/q/72116/15871)  
> **username_1**: HDR as is normally used in still photography is not the same thing as HDR when used to describe 4K TVs. The reason we have more content containing the expression 'HDR' is probably because it's been around still photography for about 165 years and is a relatively new term with regard to video screens.  
> **username_1**: It doesn't mean much of anything. The extra bit depth can be used for finer gradations between the same 'darkest' and 'brightest' emissions that can be simultaneously displayed *or* the extra bit depth can be used to increase the difference between the 'darkest' and 'brightest' emissions that can be simultaneously displayed. But neither choice is anywhere remotely near the range that can be contained in a 32-bit floating point light map that is what 'HDR' has come to mean in the context of digital still imaging.  

---

## 5 answers

### Answer by username_6 on 2017-06-13. Score: 7 | Accepted answer

TV technology and video encoding is in principle outside the scope of this site.
Still, as photographers we are presumably interested in new display standards that promise both more dynamic range and wider color gamut. HDR computer monitors [are coming](http://www.pcworld.com/article/3154466/displays/jaw-droppingly-gorgeous-hdr-explodes-onto-pc-monitors-at-ces-2017.html), so it probably won't be long before photographers too will be interested in HDR displays.

HDR by itself just means High Dynamic Range. Dynamic range is the brightness ratio between the brightest and the darkest parts. "High" is relative to what the sensor can capture or to what a standard display can show.

[HDR video](https://en.wikipedia.org/wiki/High-dynamic-range_video) is one of several standards for video encoding. They have names like HDR10, Dolby Vision, and Hybrid Log-Gamma. They all support higher bit depth (10-12 bits), wider color gamut ([Rec. 2020](https://en.wikipedia.org/wiki/Rec._2020) and/or [DCI-P3](https://en.wikipedia.org/wiki/DCI-P3)), higher dynamic range, and 4k resolution. They are not compatible with each other, but some devices support more than one HDR standard.

["HDR TV"](http://www.trustedreviews.com/opinions/hdr-tv-high-dynamic-television-explained) doesn't necessarily mean much, except that it will accept a HDR video signal and display an image. It does *not* necessarily mean that it can *display* the wider gamut and higher dynamic range, just that it can read the signals and show something, even if what it shows is plain sRGB with a mundane dynamic range.

["UHD Premium"](https://www.cnet.com/news/what-is-uhd-alliance-premium-certified/) is a certification standard for displays that can actually *show* HDR. Requirements include:

* minimum 10 bit color
* minimum 90% of the DCI-P3 color gamut
* use a HDR transfer function *(The transfer function for sRGB is called 'gamma', it translates bit value to brightness value. HDR uses a different function so it can cover a larger dynamic range with a small increase in the number of bits)*
* either: Minimum 1,000 nits brightness and max 0.05 nits black level, contrast ratio 20,000:1 or better (for LCD displays)
* or: Minimum 540 nits brightness and max 0.0005 nits black level, contrast ratio 1,080,000:1 or better (for OLED displays, they can't go as bright but have better black levels)

Standard displays are 200-400 nits and rarely go beyond 1,000:1 contrast ratio. So a "Premium" HDR display has significantly higher peak brightness as well as an order of magnitude better contrast.

Brighter highlights, higher contrast ratios, and a different "bits to brightness" encoding/decoding all combine to enable the "high dynamic range" part of a HDR TV.

Color space and bit depth is only related to HDR in the sense that they are part of the same standards. Yes, you need more bits to represent a higher dynamic range without banding. But the larger color space is a different decision, it's more a matter of including enough improvements to make a new video standard worth the trouble. (4k is also part of the standard, and 4k is not related to HDR either.)

"The sun looks brighter on a HDR display" means that a) the display emits more light in the brightest parts of the scene, and b) it may look even brighter because the rest of the scene can be dimmer without losing detail.

So it's a real improvement; the wider color gamut and higher dynamic range can contribute to more vivid photos and video.

The caveat is that TVs/monitors may be sold as "HDR compatible" without any actual HDR capability in the display. The content needs to be made specifically for HDR to make use of the new capabilities. And for computer monitors, you need a graphics card that supports HDR output.


---

### Answer by username_2 on 2017-06-13. Score: 1

HDR doesn't relate to color space at all, as it stands for "High *Dynamic* Range", ie. brightness, not color, contrast.

Traditional TV signals had a rather low contrast ratio, as old TV sets had a contrast of only [30:1](http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html#contrast_ratio). Modern displays can produce a lot more, 1000:1 or more with dynamic backlight (local dimming).

"HDR" displays are just a marketing term for displays with a very high contrast ratio.

But using this high contrast will require a higher bit depth signal or you'll have only a few steps spread out over the whole range which may introduce a kind of banding just like in the color domain.

You can't squeeze 1000:1 into an 8 bit brightness signal, so you need a higher bit depth. 10 bit can encode 1024 steps, which still isn't that high really, but better than 256.

Calling those displays "HDR" is, from a photography perspective, still laughable, as HDR photos are encoded in 32 bit floating point values with near infinite steps, but the dynamic range *is* higher than that of a traditional CRT TV or the standard NTSC/PAL signal. (Note that you need an "HDR" signal source to actually enjoy that capacity.)

> **Comments:**
> **username_1**: @username_0 With two dimensional color gamut maps both the *x* and *y* coordinates specify a particular hue (color). Brightness is not represented at all. If brightness were included the map would need a third *z* axis perpendicular to the plane formed by the *x* and *y* axes.Keep in mind that most color gamut maps you see aren't the *actual* colors. If that were the case you couldn't view the edges of an Adobe RGB color map on an sRGB device, much less a printed piece of paper. You wouldn't be able to see anything outside the gamut lines even on an Adobe RGB monitor.  

---

### Answer by username_3 on 2017-06-13. Score: 1

HDR in the modern TV sense refers to one of several different HDR specifications for color. To understand what High Dynamic Range means, we first need to understand what dynamic range is.

Dynamic Range is the difference between the highest and lowest values in a signal or display. A high dynamic range TV generally means that there is a large amount of brightness levels between the darkest and lightest possible portions of the display that can be shown at the same time.

The HDR specifications specify more than just the max and min brightness levels that must be able to be displayed though. They also specify that a relatively high bit depth must be possible. This allows for a video to have a greater number of levels of brightness within it, which also means that more subtle variations in color can be created and thus better use of the increased range of brightness that can be produced. The higher the dynamic range, the more change in color between each value at a particular bit depth, but you can have fairly low dynamic range displays or fairly high dynamic range displays that still have a high bit depth.

There is also requirements for contrast ratio. Contrast ratio is similar to dynamic range, but not exactly the same thing. Contrast ratio is the relationship, in absolute light level, between the darkest and brightest values the display can simultaneously display. It is dependent both on how bright the display can get and how well it can block light. The contrast ratio determines how well a bright and dark portion of a scene can co-exist at the same time. Static contrast ratio is the meaningful value as it is what the display can really do. Dynamic doesn't mean a whole lot because it depends on being able to do adjustments that aren't at a pixel level and don't really produce an ideal image quality.


---

### Answer by username_4 on 2024-01-29. Score: 0

HDR is high dynamic range. The older technology is SDR - standard dynamic range. When we talk about colorspaces, there are also colorspaces specifically designed for HDR like ICtCp. ICtCp is derived from IPT (a colorspace for SDR) by using a different function for lightness.

Standard colorspaces are usually poorly defined for highlights and dark tones. This means highlights would be most likely encoded as a small region around pure white (i.e. loose precision/data/information there). Similar with shadows.

So, HDR in the colorspace terms is: a different lightness function (HLG - hybrid log or PQ function) + better defined color information for highlights and shadows.

Using HDR colorspace is only relevant for editing/storing when you deal with SDR content (that is why you can recover shadows/highlights when editing RAW files). When you deal with HDR content - you would have to use a colorspace for HDR content.

A lot of research papers clearly say that SDR colorspaces (i.e. CIECAM02, Lab) were only tested for illuminant around 100 nits. Higher illuminant or extreme changes in lightness and SDR colorspaces enter an "uncharted" territory. They could work fine, but less optimal as colorspaces defined for HDR.


---

### Answer by username_5 on 2017-07-24. Score: 0

To comment two well-known statements here:

"You could also have a fairly low bit depth in a very wide color space where the steps between each gradation are much coarser."

"Color space and bit depth is only related to HDR in the sense that they are part of the same standards. Yes, you need more bits to represent a higher dynamic range without banding."

Both don't take dithering into account. In the absence of dither, the statements are correct but with dither applied, in theory, a smooth gradient would be even possible with only one bit, although it's extremely noisy when applying spatial dithering.

If one accepts a lower SNR, there won't be banding regardless of the bit depth.

If it is true that bit depth and colour space are not related (which makes sense as any colour space could exist in a purely analog domain as well without any bits to consider at all), then the logical conclusion would be: theoretical perfect dithering techniques assumed, a given bit depth - analog to audio - only limits the achievable signal to noise ratio and thereas the dynamic range as the noise will mask the lower end, however not the possible amount of colors, not any banding and no brightest whites or whatever.

> **Comments:**
> **username_3**: Sure, but then you are sacrificing resolution for color and it only works to a point because if you don't have enough pixels to dither over, it becomes noticeable to the eye.  Also,. to the best of my knowledge, most displays aren't applying dithering on the fly to compensate for something like this, they are just displaying pixel values provided, so dithering doesn't make much of a practical difference in relation to things like the various HDR tv specs.  
> **username_5**: @username_3:  thanks for your reply. You are right that many practical implementations lead to effects, which the oversimplified and - from the signal theory's point of view incorrect - assumptions suggest.  However, the fact alone that practical implementations are so often so bad, doesn't make the oversimplified assumptions correct vice-versa, either.  I can't prove it yet either, but I doubt that dithering in fact reduces resolution as I think most people confuse resolution and signal-to-noise ratio and these are two different parameters determining the quality.  
> **username_5**: One can beautifully test this on one's own by using the renderer "madVR" under Windows (recommended in conjunction with the player MPC, but it should work with others, too) which supports several great implementations of dither.  Using "randomised dither", one can reduce the bit depth to even only 1-bit per color channel (leading to 8 distinct colors only in total) and still have all the colors and no banding.  
> **username_5**: The image is very noisy (just like a bad analog TV signal back in the days) and certain detail is lost in the noise of course, but the nominal frequency response of the image (resolution) seems to remain unaffected which can be indirectly proven by the absence of introduced aliasing which would be to be expected if the real resolution frequency-wise would be lowered by the dithering.  In the audio domain, it's also exactly the same. There dither definitely does NOT reduce or limit the frequency response in any way, but depending on the bit depth it will raise the overall noise floor.  
> **username_5**: Video resolution equals audio frequency response.  Hence from my understanding, it is correct to talk about a higher dynamic range and lower noise floor with a higher bit depth (one bit giving about 6dB of additional SNR), but not about "finer resolution of colors" or even a higher color space as all these assumptions are wrong in the audio domain and a video signal could always also be interpreted as a high frequency audio signal too so all parameters logically have to have a 1:1 expression (which they seem to have in practise too).  
> **username_5**: Hence, we might not necessarily need more data, but a better implementation in practise when it comes to video applications.  
> **username_5**: Sorry, forgot another important remark. You wrote "Also, to the best of my knowledge, most displays aren't applying dithering on the fly to compensate for something like this". Actually, that isn't the display's task anyway just like it wouldn't be a D/A-converter for audio to apply dither either. The dither is supposed to be part of the image so to say. The only exception is that if some render device accepts a higher bit depth and has to reduce it "on the fly" due to its own limitations. Otherwise, the dither is part of the image so the pixels are displayed 1:1 at the given color depth.  

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