With four times the resolution, the picture may lose to the other. This may sound counterintuitive, but the issue is not how many pixels there are on the screen, but how much information is lost before those pixels reach the TV. Engadget reported that in order to avoid buffering, streaming services must first compress the video before transmitting it through the network. The TV or streaming device is responsible for "decoding" after receiving it, but decoding does not mean restoration. The series of processes of compression, transmission, and decoding will sacrifice details, including fine film grains, shadow gradients, material textures, and complex motion information. This is especially obvious in complex scenes such as smoke, rain scenes, large numbers of people moving, or dark shadows. In comparison, 1080p Blu-ray discs can retain these details more completely, and the presentation effect is sometimes more natural.
The report used canvas and paint as a metaphor for the relationship between resolution and bit rate: resolution determines the size of the canvas, and bit rate is the amount of paint that can cover the canvas. The canvas of 4K streaming is much larger, but if there is not enough data to fill in the color changes and dynamic details of the millions of pixels in each frame, the picture will still be distorted. Netflix officially recommends that 4K streaming requires at least a 15Mbps network connection, but this is only a recommended value and does not mean that every moment of the picture can be completely transmitted at the highest bit rate. Major streaming platforms adopt an "adaptive bit rate" mechanism that will instantly switch between different quality versions of videos based on network conditions.
In comparison, the work of Blu-ray disc is much simpler. It only needs to read data from the local disc and does not need to adjust according to the network conditions. Standard Blu-ray can provide 1080p picture bandwidth up to 40Mbps, Ultra HD Blu-ray supports up to 108Mbps on a 66GB disc, and 128Mbps on a 100GB disc. These numbers are the upper limit of the format specifications, and not every film can use them, but they are enough to explain why the disc is better at retaining details. There are also differences in encoding efficiency: modern streaming encoding technologies such as HEVC can do more with less data, which means a stable 1080p encoding can often outperform a 4K stream struggling with a limited bit rate because it has to deal with a much smaller number of pixels.
