Once a video signal is digital, it requires a large amount of storage space and
transmission bandwidth. To reduce the amount of data, several strategies are employed
that compress the information without negatively affecting the quality of the image.
Some methods are lossless, meaning that no data is lost, but most are lossy, meaning
that information is thrown away that can’t be retrieved.
Some simple methods of data compression are:
Â Throw away pixels at regular intervals: This essentially scales the image, or makes it
Â Average several pixel values together (subsampling): This involves taking several
adjacent pixel values and averaging them together, resulting in a single rectangular
pixel that approximates the value of several. For more information, see “
” on page 384.
Â Throw away color channel information at regular intervals: This results in color sample
ratios like 4:2:2 and 4:1:1. Ideally, throwing away this color information is not
noticeable to the viewer, but it may be a problem if you are trying to do detailed color
correction or chroma keying that requires a lot of color information to start with.