What are the differences when displaying a 1080p and 720p version of the same medium on a 720p screen?
Suppose I have 1280×720 pixels on my screen and the current resolution is also set to 1280×720. What are the differences between the 1080p version and the 720p version of the same medium? Is it obvious to an end user (a video enthusiast or non-enthusiast)?
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Theory
Inexperienced eyes may not notice the differences in the video. 1080p videos still need to be resized. However, it will not be exactly the same because compression and scaling are applied in a different order.
Suppose the original video was 1080p. In this case, the video was resized from 720p first and then compressed. On the other hand, the 1080p clip was initially compressed on the server side and then resized on the computer. The 1080p file will obviously be larger. (otherwise it offers a higher resolution, but in a lower quality, ruining the viewing experience and invalidating the highest resolution point of use 1)
Lossy compression usually causes visual artifacts that appear as square blocks with noticeable edges when video is paused, but are not visible when played at normal frame rates. The 1080p file will contain more square blocks (caused by compression) than the 720p video, but those blocks will be roughly the same size in both videos.
By doing simple calculations, we can calculate that 1080p video will contain 2.25 times more such blocks, so after resizing to 720p, those blocks will be 2.25 times smaller than actual 720p video. The smaller those blocks, the better the quality of the final video, so 1080p video will look better than 720p video, even on the 720p screen. 1080p upscaled video will appear slightly sharper than the current 720 clip.
Things get a bit more complicated if the source material is higher than 1080p. The 1080p clip is first resized to 1080p and compressed before playing, then resized once more during playback. The 720p clip is resized only once and then compressed. The intermediate scale step present in the case of 1080p video will degrade its quality slightly 2. Compression will make 720p even worse, so 1080p still wins.
One more thing: it is not only compressed video, but also audio. When people decide to use a higher 1 bit rate for video compression, they often do the same for audio. The 1080p version of the same video can offer better sound quality than 720p video.
1: Bitrate is the factor that decides how good compressed video is at the cost of file size. It is specified manually when the video is compressed. Specifies the amount of disk space that can be used for each frame (or unit of time) of compressed video. Higher bitrate = better quality and larger file. Using the same bitrate with the same frame rate will produce files of (roughly) the same size regardless of the video resolution, but the higher the resolution, the less disk space can be wasted on a single pixel, thus increasing the output resolution without increasing the bit rate can make compressed video worse than it would be with a lower output resolution.
