Why Do Dark Videos Look So Bad?

If you follow YouTubers closely, you'll see that one of the main difficulties they face is light... or rather, its lack. This also extends to animated films and even TV series: There's something not quite right with dark scenes. Blocks, color bands, a combination of both... in the age of high definition and ultra-fast streaming, these playback flaws shouldn't exist, but they live comfortably among us. Why? Tom Scott explains in great detail.

Let's say you start watching a movie via streaming on your HD TV, and in a particularly dark scene you discover horrible bands and blocks. The first suspicions point to an unexpected drop in streaming quality, or excessive compression in the original material. From a technical standpoint, both are valid possibilities, but it's an unwanted effect that requires deeper analysis.

That brings us to Tom Scott's channel, who taught us the most important problems when developing an application, and explored the challenge of driving cars in third person using drones. As a YouTuber with over two million subscribers, Tom knows these nasty bands very well, and the first reason behind their existence is... that there aren't enough colors:

The 16.7 Million Color Problem

In most of digital video, the number of available colors reaches 16.7 million. That number is tied to how a screen works. When playing video on a computer, TV, or mobile device, the digital signal is converted into instructions for the screen, which modifies the brightness of millions of red, green, and blue lights, up to 60 times per second.

That sounds like a very complete solution, and we must admit it has served us well, but it carries a limitation: those red, green, and blue lights can't be adjusted to any brightness. In theory, the most basic signal would work with one bit of depth, that is, "on-1" or "off-0" for each combination of red, green, and blue. 2 by 2 by 2... eight colors. If we move to two bits per color, the new limit is 64 colors. But on a modern screen, we need a total of eight bits per pixel, delivering the current 16,777,216 colors (256 x 256 x 256).

Why Dark Scenes Look Worse

While there are applications that can take advantage of 10 or even 12 bits per pixel, the 16.7 million threshold is considered "adequate"... until it isn't. Today it's normal to play YouTube videos 1,920 pixels wide, but if you decide to place a green background and only have 256 shades of that color available, banding every seven or eight pixels is inevitable, and if the scene gets darker, the effect is even worse. There aren't that many "dark colors" available.

Tom also explains why banding is less evident in bright scenes (it's still there, and the absolute change from one color to another remains intact, but the relative difference is minimal), what the main "tricks" are to minimize its impact (starting with the classic "dithering"), and of course, the effects of the aforementioned compression. A "pure" unprocessed HD video demands about one gigabit per second of streaming data. And it would look great... but it's expensive and inefficient. That's why all video services use lossy compression, "lossy", giving up fine details in favor of smaller size and better efficiency during streaming.

The Price of Streaming

Now, this remains a balancing game, and the compression algorithm prioritizes its quality-preservation resources on the parts most interesting to the viewer, but if what remains is very dark, banding and blocks will be there.