As 4K replaces HD in our homes, manufacturers are unveiling some interesting marketing jargon, like "Ultra HD upscaling" (UHD). But upscaling isn't some unique feature---it just allows 4K TVs to work with lower resolution video formats, like 1080p and 720p.

All TVs Have Upscaling

Upscaling means low-resolution content will fill your entire TV screen. Without it, a low-resolution video takes up less than half of the screen space. This is a typical feature on all TVs. Even 1080p TVs had it---they could upscale 720p content and display it in full-screen mode on a 1080p screen.

UHD upscaling is what makes your 4K TV work like any other. It can take lower-resolution content and display it on the entire 4K screen.

Upscaled 1080p content on a 4K screen often looks better than 1080p content on a normal 1080p screen. But upscaling isn't magic---you won't get the sharp image you would from true, native 4K content. Here's how it works.

Resolution Exists at a Physical and Visual Level

Before getting into upscaling, we need to understand the concept of image resolution. At a glance, it's a relatively simple concept. An image or video with a high resolution looks "better" than an image or video with a low resolution.

However, we tend to forget some key aspects, namely, the difference between physical resolution and optical resolution. These aspects work together to create a good image, and they're the basis for understanding upscaling. We're also going to cover pixel density---but don't worry---we'll keep things short and sweet.

• Physical Resolution: On a TV spec sheet, the physical resolution is simply referred to as "resolution." It's the number of pixels on a display. A 4K TV has more pixels than a 1080p TV, and a 4K image is four times the size of a 1080p image. All 4K displays, regardless of their size, contain the same number of pixels. While TVs with a high physical resolution can use their extra pixels to offer additional detail, it doesn't always work out that way. Physical resolution is at the mercy of optical resolution.
• Optical Resolution: This is why your old disposable camera photos look better than your pretentious friend's fancy digital camera photos. When a photo looks sharp and has a clear dynamic range, it has a high optical resolution. TVs sometimes squander their high physical resolution by displaying video with a crappy optical resolution. This leads to blurry images and contrast. Sometimes, this is a result of upscaling, but we'll get back to that in a minute.
• Pixel Density: The number of pixels per inch on a display. All 4K displays contain the same amount of pixels, but on smaller 4K displays, the pixels are closer to each other, so they have a high pixel density. A 4K iPhone, for example, has a higher pixel density than a 70-inch 4K TV. We're mentioning this to reinforce the idea that screen size isn't the same thing as physical resolution, and that a screen's pixel density doesn't define its physical resolution.

Now that we're all brushed up on the difference between physical and optical resolution, it's time to get into upscaling.

Upscaling Makes an Image "Bigger"

Every TV contains a mess of interpolation algorithms, which are used to upscale low-resolution images. These algorithms effectively add pixels to an image to increase their resolution. But why would you need to increase an image's resolution?

Remember, physical resolution is defined by the number of pixels on a display. It has nothing to do with the actual size of your TV. A 1080p TV screen is comprised of only 2,073,600 pixels, while a 4K screen has 8,294,400. If you show a 1080p video on a 4K TV without upscaling, the video will take up only a quarter of the screen.

For a 1080p image to fit a 4K display, it needs to gain 6 million pixels through the upscaling process (at which point, it will become a 4K image). However, upscaling relies on a process called interpolation, which is really just a glorified guessing game.

Upscaling Reduces Optical Resolution

There are several ways to interpolate an image. The most basic is called "nearest neighbor" interpolation. To perform this process, an algorithm adds a mesh of "blank" pixels to an image, and then guesses which color value each blank pixel should be by looking at its four neighboring pixels.

For example, a blank pixel surrounded by white pixels will turn out white; whereas a blank pixel surrounded by white and blue pixels might come out light blue. It's a straightforward process, but it leaves a lot of digital artifacts, blur, and rugged outlines in an image. In other words, interpolated images have a poor optical resolution.

Compare these two images. The one on the left is unedited, and the one on the right is the victim of the nearest neighbor interpolation process. The image on the right looks terrible, even though it's the same physical resolution as the one on the left. This happens on a small scale every time your 4K TV uses nearest neighbor interpolation to upscale an image.

"Wait a minute," you might be saying. "My new 4K TV doesn't look anything like this!" Well, that's because it doesn't rely entirely on nearest neighbor interpolation---it uses a mix of methods to upscale images.

Upscaling Tries to Tackle Optical Resolution, Too

Okay, so nearest neighbor interpolation is flawed. It's a brute-force method for increasing an image's resolution that doesn't take optical resolution into account. That's why TVs use two other forms of interpolation alongside nearest neighbor interpolation. These are called bicubic (smoothing) interpolation and bilinear (sharpening) interpolation.

With bicubic (smoothing) interpolation, each pixel added to an image looks to its 16 neighboring pixels to take on a color. This results in an image that's decidedly "soft." On the other hand, bilinear (sharpening) interpolation only looks to its nearest two neighbors and produces a "sharp" image. By mixing these methods---and applying some filters for contrast and color---your TV can generate an image that has no noticeable loss in optical quality.

Of course, interpolation is still a guessing game. Even with proper interpolation, some video can take on "ghosting" after being upscaled---especially if your cheap TV sucks at upscaling. These artifacts also become more apparent when super-low-quality images (720p and lower) are upscaled to 4K resolution, or when images are upscaled on insanely large TVs with a low pixel density.

The above image isn't an example of upscaling from a TV. Instead, it's an example of the upscaling done for the Buffy The Vampire Slayer HD DVD release (taken from a video essay by Passion of The Nerd). It's a good (albeit extreme) example of how poor interpolation can ruin an image. No, Nicholas Brendon isn't wearing some waxy vampire makeup, that's just what happened to his face during the upscaling process.

While all TVs offer upscaling, some might have better upscaling algorithms than others, resulting in a better picture.

Upscaling Is Necessary and Rarely Noticeable

Even with all of its faults, upscaling is a good thing. It's a process that usually goes off without a hitch and enables you to watch a variety of video formats on the same TV. Is it perfect? Of course not. That's why some film and video game purists prefer to enjoy old art on its intended medium: old-ass TVs. But, as of right now, upscaling isn't something to get too excited about. Nor is it something to get too upset about.

It's worth mentioning that 8K, 10K, and 16K video formats are already supported by some of the hardware we use every day. If upscaling technology can't catch up with these high-resolution formats, there's a chance it will result in a much greater loss in quality than what we're used to.

Since manufacturers and streaming services are still dragging their feet toward 4K, though, maybe we shouldn't worry about 8K just yet.