Resolutions can be a tricky idea to get a handle on. In today's High Definition (HD) world, we usually see 720p or 1080i plastered everywhere. We also can see on various popular video sharing websites that this idea is a foundation of viewing videos. Well, what does that all mean? To start, the number you see represents horizontal lines of resolution...in other words, how many horizontal (left to right) lines of the picture are drawn on the screen from top to bottom, or in the case of online videos, how many are drawn in the viewing window. Of course, the higher the number, the more lines are drawn on the screen.
The letter, i vs. p, explains how the lines are drawn, interlaced or progressive. With interlaced (i), every other line is drawn on the first pass or cycle and then the other remaining lines are drawn in the second cycle (ie. lines 1, 3, 5, 7, etc. are drawn on the first cycle and lines 2, 4, 6, 8, etc. are drawn on the second cycle) and then repeated. Progressive (p) draws all the lines sequentially (ie. lines 1, 2, 3, 4, etc. are drawn in order.) As you may have figured out, progressive is better than interlaced, however with a higher frequency screen, say 120Hz*, you will likely not notice the difference due to the fact our brains visual acuity is around 30Hz.
The above graphic represents the various common resolutions you can find in the wild. This is to give you a visual representation of how one looks compared to another. If for instance you have a computer monitor with the resolution of 1920 x 1080 and watch a video that is 800 x 600, the box you will see on your screen will be the size of the lime-green box above compared to the fuchsia box. When you expand that small window to fit your screen, the computer translates the pixels (scales them) and makes them larger and that is why things can look blocky. In the reverse, if you have an 800 x 600 screen but are watching a 1920 x 1080 video, initially the video will be larger than your screen and you will only see a portion of it until you scale it down. In this instance, you will likely see some aliasing when you scale the picture down, which is a fancy way of saying lines in the video, like the outline of objects, will have a saw-tooth, jaged effect. To get the best picture possible you would ideally have your screen and the media you are watching to be the same resolution.
The idea here isn't to go into so much detail where your eyes glaze over, but to give you a better understanding of the complexities associated with video scaling and a little insight into what those numbers actually mean. :-)
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*Hz - Hertz, cycles-per-second
Jimmy Andrews has been working with computers for nearly 30 years, semi-professionally for over 20 years and professionally for over 15. His experience ranges up and down the IT scale, from the grunt work of fixing and installing PCs, to managing and administering hundreds of servers and Enterprise technologies, up to the Business side of IT where decisions are made at a 50,000 foot level.