Screen ratios, frame rate, video forats, compressionPresentation Transcript
The aspect ratio of either an image or screen describes the proportional relationshipbetween its width an heightIt is commonly expressed as two numbers hat are separated by a colon as in the two wesee at the top 1:1 and 4:3.For an X:Y aspect ratio, no matter how big or small the image is if the width is dividedinto x units of equal length and the height is measured in y units. It is easier to visualiseif the units are equate to whole numbers rather than decimals.
In still camera photography the most common aspect ratios are 4:3, 3:2 and 16:9. 16:9 is mostcommonly found in consumer cameras.With Television, DVD and Blu-ray Disc, converting formats of unequal ratios is achieved byenlarging the original image to fill the receiving formats display area and cut of any parts ofthe picture that arent wanted or needed.As you can see in this image, they usually get rid of the unwanted parts by justsimply cropping or zooming in.
Letterboxing: This is when you add horizontal mattes to retain the original formatsaspect ratio.Pillarboxing: When you add vertical mattes to retain the original formats aspect ratio.By stretching an image to fill the receiving formats ratio or by scaling different factors in bothdirections or even possibly scaling by a different factor in the centre and at the edges.
Pixel aspect ratiosThis describes the relationship between the width and height of a single pixel.Different pixel aspect ratios are the reason why two video images that have identicalframe sizes can look like different sizes on screen.If the width is bigger than the height then the pixel is non square and the aspect ratiois greater than 1.0Then if the width is smaller than the height then the pixel is non square and the aspectratio is less than 1.0.
This is also known as frame frequency, it is the rate at which imaging devices producesunique consecutive images called frames.The term applies well to film, video cameras, computer, graphics ad motion capturesystems.Frame rate is most often expressed in frames per second and it is also expressed inprogressive scan monitors as hertz.The human visual system can process 10 to 12 separate images per second, perceivingthem individually. Early silent films had a frame rate from 14 to 24 fps which was enoughfor the sense of motion.The persistence of vision was a commonly-accepted although somewhat controversialtheory which states at the human eye always retains images for a fraction of a second, thismeans that everything that we see is a subtle blend of what is happening now and whathappened a faction of a second ago.This is how many frames per second there are when recording or playing video. Animationworks by recording each frame individually and then playing them back at a frame rate.Video camera in Europe use 25 frames per second.USA ad Japan use either 29.97 or 30fps.
A video format defines the way that video is recorded and stored.It normally specifies:Codec/compressorFrame rateFrame sizeFrame aspect ratioPixel aspect ratioScanning methodThere are different layers of video transmission and storage, each with its own set offormats to chose from. A physical link can carry certain “display standards” which specify aparticular refresh date, display resolution and colour space.
Most video formats are compressed during encoding so the terms codec andcompressor are usually used for the same thing.Codec is shot for a coder-decoder and describes the method that data is encoded into afile and decoded when the file is played back.Transcoding is the process of converting from one codec to another codec. They can belossless which means that they don’t throw away any of their data. Lossless codecs arehigher quality than lossy codecs but they produce larger file sizes.Some examples of common codecs: