Best answer: HDR stands for High Dynamic Range and is used to describe anything that can process a wider range of input or output than a standard device of the same type. It's commonly used when talking about cameras and displays.
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What is HDR?
You'll see HDR being used as a descriptor in a lot of different products — cameras, televisions, microphones, and even industrial and medical equipment. In the generic sense, HDR means a thing has a wider range of acceptable input or output when compared to standard equipment. What we're going to focus on is HDR as it applies to camera and displays.
HDR Cameras and Photos
We all have used a crummy smartphone camera and tried to take a picture without enough light. It ends up being a dark, yellow mess that's not a great way to capture the moment on any level. We've seen the same when we use the flash on a smartphone camera to try and fix it, then everything turns a weird ethereal bluish-white. This is because of the dynamic range of the camera.
The range of light that the camera hardware can focus on and capture is fixed. HDR fixes that.
A camera can only process a certain amount of light that's brighter than a threshold, and a certain amount that's darker. You can move the threshold, but you can't expand it to see more of one side without losing some of the other. If parts of the image are brighter than the allowable range, they are washed out and white. If parts are darker, they are black. If you make an adjustment to see more of the brighter light, more things turn black. If you do the opposite to see more dark things, more things wash out. The range of light that the camera hardware can focus on and capture is fixed.
HDR settings aim to fix this and give you a picture that has more bright things correctly exposed and more dark things correctly exposed at the same time. This is done by taking more than one picture when you press the button. Photos are taken (usually three or five, which is called bracketing) in quick succession and each has the exposure set to a different level.
The software that turns the image data into a picture analyzes each and stitches a single photo together with the bits and pieces that are exposed properly. The picture that's set to see more dark areas has those spots combined with the normally-exposed picture, and the same is done for the brightly lit areas. The software picks the image with the best detail when it sees a trouble spot and tries to make a photo that looks good.
It mostly works. Even on an automatic setting where the image is analyzed and HDR is applied if needed, most of the time HDR can help turn an average photo into a better photo. It's not magic — it can't fix a bad photo and it doesn't help when you're taking one that has great lighting throughout. It depends on the people making your camera being able to program it to do a good job guessing what's right or looks good. Some are better than others, but it's always a lot better than any HDR post-processing filter.
HDR photos have nothing to do with HDR display technology. Both are used when the product can do its thing with a wider range — in this case, the thing is to show you a picture.
HDR photos have nothing to do with HDR display technology.
HDR displays do three things to make the image they show look better than a normal display. They have a better contrast ratio that lets them produce brighter whites and darker blacks at the same time, with both ends being sharp and well defined. They have a higher degree of color accuracy so the colors look closer to what your eye sees if you were looking at things in real life (or a producer's vision of real life). And they have a Wide Color Gamut (WCG) so more colors can be displayed on the screen itself. A TV that doesn't have a WCG display won't reproduce the actual color of a stop sign. You'll notice this the next time you see it. WCG lets it create that particular red and a lot more colors.
These three things are combined into one selling point called HDR. You might see other extras like 3D color management or a special rendering engine like Dolby, but HDR itself needs those essentials to be reproduced. It really does look better if you see it beside a display of the same content on a standard range screen.
The content matters, too. A TV or phone can't create all these new colors and put them where it thinks they should be. HDR content is required, and if you're watching standard videos no monkeying around with the image to try and upscale it to HDR is happening. This is possible, and upscaling to HDR algorithms are used to do things like detect blemishes on manufactured parts or bruises and soft spots on fruits and veggies. But not on your TV or monitor. HDR content carries metadata along with the signal that tells the display exactly how to display the content. The display itself just creates the right color for each pixel according to the metadata and the picture we see is brighter and more lifelike.
The important part is to make sure everything is HDR-capable. For example, you need a Chromecast Ultra, a 4K HDR TV, a 4K HDR source and enough bandwidth to stream the video and associated metadata to get a good 4K HDR picture from Netflix. If one piece of the chain is broken, you get a standard HD picture instead.
There's a lot more to know about HDR, like how HDR media is created or how a camera processor knows what looks good when faced with three photos to choose from. But these basics are all you need to know the next time someone starts talking about HDR so you can follow along.
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