Discrete Cosine Transformation, also called DCT, is used to compress digital images by rounding the values used to express 8x8 blocks of Pixels into a smaller number of values that can be grouped together to avoid redundant bits. This type of lossy encoding is used by every lossy video Compression format.
The inverse of the compression function, also known as iDCT (inverse DCT) is used to re-create the original images from those compressed using DCT.
Additional Reading
The Discrete Cosine Transform: Theory and Application
Image Compression Using the Discrete Cosine Transform
Discrete Cosine Transform
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