HOW COMPRESSION WORKS
‐Fundamentally, compression works by summarizing a sequence of images and sounds as efficiently as possible. Video that has been compressed doesn’t provide an identical representation of the source file back to the
‐Instead, the encoded files describe the important details of the content as tersely as possible while still providing a reproduction that, to the human sensory system, provides the experience of the original as accurately as possible.
‐A matched pair of a compressor and a decompressor, better known as a codec, performs the compression.
ÆA compressor is part of the encoding process, reducing the amount of data required to store the video.
ÆA decompressor works on the receiving end, decoding the compressed data so that it can be presented to the viewer.
‐It’s important that the pair be matched because the decoder (decompressor) needs to understand the encoder’s (compressor’s) summary of the data.
TYPES OF COMPRESSION
Video codecs can use spatial compression, temporal compression, or a combination of both.
Spatial Compression
ÆIntraframe
ÆAffects only a single frame at a time
ÆMakes random access to any point in the video and editing easy
ÆFinds redundancy within a frame (eg. similar colours or areas that look like other areas) and encodes these similar areas by reference, rather than by a literal description for each and every pixel
ÆBased on Discrete Cosine Transformation (DCT) technique which are good with smooth gradations, but struggle with random details or sharp edges at lower bit rates Æ can cause visible degredation of the image = artifacts, ‐‐ such as ringing (halo or smudge) and blocking (smooth edges become blocky)