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Block
For almost all image file editing techniques the image is subdivided into 8 x 8 pixel
blocks (image points). This should be noted if you would like to used user-defined
image resolutions (width/height), and they should always be a multiple of 8.
Chroma format
The color value of each image point consists of the color values for the primary colors
red, green, and blue (RGB), and for traditional and technical reasons it is transformed
into one brightness value (Y = 0.299*R = 0.587*G + 0.114*B) and two color
difference values (U = R - Y, V = G -Y).
The Y value alone produces the black and white picture. These signal components
allow brightness and color information to be handled separately. The first data
reduction occurs when single rows comprising a picture are read. Because the human
eye has a lower color resolution than a brightness resolution, the color components
are recorded only for every other point of a row (4:2:2) for each four pixels grouped
(4:1:0), i.e. color signal under-reading.
4:2:2 This corresponds to the established TV standard. One piece of color information
is transmitted per row for two pixels which corresponds to a 2/3 compression of the
output data.
4:1:0 This is the color coding used for DVDs and most other consumer video
applications. For each 4 pixels grouped together on two rows, one unit of color
information is saved. This corresponds to a output data compression of 1/2.
Field
A half-image, i.e. two halves which combine to produce a frame (see de-interlacing
(view page 359)).
Frame
A frame is a single image from a video sequence which also called a full image. PAL
video, for example, contains 25 frames per second, NTSC 29.97 frames.
Video recordings, with the exception of computer animations and still frames, don't
contain full images. Instead, they have double numbers of half-images (fields) which
are transmitted in an interlaced state. However, we still refer to frames, since many
predecessors of MPEG compression are based on such frames. Video editing
literature usually refers to frames.