DEFINITION January 2022 – Web


No time machine

The word ‘intra’ comes up a lot in video compression, referring not to a particular compression technique, but an absence of one. It had long been realised that much of the picture content in any two video frames is similar, and a lot of codecs make use of that, shuffling areas of one picture around to make another. It’s a method called inter-frame compression. Intra-frame avoids that, requiring less work to compress and decompress – letting software move faster. ProRes and DNxHD are both intra-frame codecs.

want computers to decode many streams simultaneously. It is technologically related to JPEG still images, and thus the Motion JPEG moving image format, as well as the original MiniDV, DVCPro and DNxHD formats. All of these (in addition to AVC, in part) use a technique called discrete cosine transform, where the picture is broken up into blocks and the pixel values of those blocks are expressed as a waveform – a wiggly line. That line can then be approximated as a sum of various sine waves.

BIG RED The Atomos Ninja Cast (above) uses ProRes and AVC-based tech. The Red Helium 8K (below) has been going strong since 2016

“There’s no avoiding the complexity, but solving problems is easier when everyone’s on the same page with the terms”

A Russian doll of data

behind the file format don’t need to know anything about the codec. The picture itself will be encoded in a colour space. Most of us are familiar with the idea that colour images are made up of red, green and blue channels; a colour space defines which red, green and blue. Mistakes mean colours won’t be reproduced properly. Often, a colour space will be accompanied by a gamma setting, which defines a relationship between the numbers in a file and the brightness displayed. Get this wrong, and the picture will look darker or brighter than it should. Colour space or gamma encoding can be compressed with any of several codecs. There’s no avoiding the complexity, but solving problems is easier when everyone’s on the same page with the terms.

Terms like ‘file format’, ‘colour space’, ‘gamma’ and ‘codec’ are often mixed up. Anything about a file could be said to be part of its ‘format’; the term ‘container’ is perhaps better, and can refer to QuickTime movies, MXF files or file-per- frame. The file format describes a general layout of data. Many formats can represent video in a variety of frame rates and resolutions, defining where the data for each frame begins and ends, how fast those frames should be displayed, how many audio channels exist and other administrative tasks. Images are compressed using a codec, short for coder/decoder, which defines a compression scheme. The people behind the codec don’t need to know anything about the file format; and those

49. JANUARY 2022

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