Tuesday, October 16, 2012

Understanding HD - Scan Lines

What are Scan Lines?

What is ‘p’ and ‘i’ we often see when frame rates are mentioned like 25p or 60i, well, the answer lies with Scan Lines.


The video you see is created by a series of horizontal lines as they are scanned across the screen starting the top. When is image creations happen at one go all the way to the bottom, it’s called Progressive
Scanning (p).

With other types of video, the scan lines first draw the even-numbered lines until they get to the bottom and then again resume from top, this time around with the odd-numbered lines, it’s called Interlaced scanning (i).

Each time they pass through either the odd or even numbered lines is called a Field. So, Interlaced scanning has two fields. The order in which the fields are drawn can change, depending on how the video is recorded. So, a 25 fps interlaced video is updated 50 times, 2 frames each. Remember? Hence the term – 50 i
25 Frames per second = 50 Interlaced Fields per second

Which one is better?

Since interlaced video uses two fields, so when shooting anything in motion, interlaced fares better. But, with all household electronics or laptop or LCD monitors in the market – Progressive is definitely the way to go. Moreover, when you are converting your video in terms of frame rates and resizing, for Progressive as the conversion software doesn’t have to worry about fields. Progressive have no fields.

Watch this video tutorial explaining about Interlaced Vs Progressive – it will become clearer.


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