Why does a computer run a game poorly, but is able to show a video of it perfectly fine
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Why is it easier to read a book than to write one?
In a video all the work of making it look like that has already been done—the computer just had to read the instructions that tell it what the video should look like. With a game it has to figure out for itself what has to go in the screen, which is a lot harder. The final result—what appears on the screen—may be the same but the path to get there is very different.
(This is also why cutscenes sometimes look a lot better than the rest of the game—they’ve been rendered ahead of time and are just videos that are being played back)
This is a frequently asked question both here and on the other question-asking subreddits, I am surprised you have not found the answer already.
In either case the computer needs resources such as memory/RAM and processor/CPU to do the tasks it is being asked. The main difference is that, since a game does not have linear progression, the computer must store all the possible progression paths in memory, then render them as they are chosen. Compared to a video, which is linear (no decisions are made during the video that could alter what would be shown) so there’s only 1 path to process.
You know how long it took Michelangelo to paint the ceiling of the Sistine Chapel? 4 years. You know how long it took me to take a picture of it with my phone?
That’s the difference. Computer graphics means a computer has to take the numbers that describe the virtual world (positions of all the characters and objects, their textures, lighting, etc.) and translate them to a picture on your screen. And then it has to do it again for the next frame, and again, and again. That takes a lot of computation, and your computer has to do it in real time, because it (mostly) doesn’t know ahead of time what the game world will be like. If your computer can’t do this fast enough (let’s say at least 30 times per second), then the game becomes unplayable. Not only do the images start to look stuttery rather than a smooth video, but also there starts to be a lag between when you give an input to the game and seeing the result of that input.
A video of the same game has been rendered ahead of time. Some computer has already done the job of taking the game data and making it into pictures. So all your device now has to do is show these picture on the screen, and that’s a much easier process.
The same reason movies take months or years to make, but you can watch them in just a couple hours.
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