Why we see these frames (frames as in lines) moving vertically in pc monitors or tvs or other screens when they are recorded through a camera but not when see them with our eyes?
In: Physics
The screens are making us see moving images by blinking different still images really quickly, like a flip book. The camera records video by taking lots of still pictures, also very quickly. When the speed of one lines up incorrectly with the speed of the other, you see those frame lines. What you are seeing is the camera taking a picture of the exact instant the monitor is redrawing the next picture on the screen.
I’m guessing you’re talking about CRT monitors (i.e., tube screens, not flat screens). These screens create their image by two dots of light being shot inside a cathode ray tube onto the back of the screen in very fast alternating horizontal lines, hundreds of lines per “frame” which is the series of still images that when combined (around 30 frames per second) create the illusion of motion that your eyes perceive.
The vertical rolling has to do with the frame refresh cycle of the tv/monitor/screen not being in synch with the camera. So even if they are both set at a frame rate of 30 (ish) fps, they’re not starting at the same time and the camera is recording an incomplete scan of the interlaced lines that create the image on the tv/monitor/screen. The recording is basically showing you the part that is too fast for your eyes to perceive.
There is a process that lets you synch the frame rate of a screen and the camera, but if you have multiple screens you’re recording, it’s a mess.
Modern digital screens don’t create their image this way and are usually less of an issue to capture properly.
Both the camera and the monitor/tv capture or display pictures very rapidly – so rapidly that your eye can’t tell that they’re actually still images.
When you record a TV picture, your camera is recording at a certain speed, and the tv is displaying at a slightly different speed, and even if they were perfectly the same speed, the time each one starts is going to be slightly different.
So what happens is you’re getting a new set of still pictures taken of an old set of still pictures that are constantly being changed at different rates. Result? Gaps. Blanks. Kind of like looking at the world through a spinning fan – you can see what’s there, but there are constant interruptions.
Latest Answers