a) Used in JPEG and the MPEG, H.261 and H.263 video compression algorithms, DCT techniques allow images to be represented in the frequency rather than time domain.

Images can be represented in the frequency domain using less information than in the time domain.

b) A mathematical transform that can be perfectly undone and which is useful in image compression.

c) Many encoders perform a DCT on an eight-by-eight block of image data as the first step in the image compression process. The DCT converts the video data from the time domain into the frequency domain. The DCT takes each block, which is a 64-point discrete signal, and breaks it into 64 basis signals. The output of the operation is a set of 64 basis-signal amplitudes, called DCT coefficients. These coefficients are unique for each input signal. The DCT provides a basis for compression because most of the coefficients for a block will be zero (or close to zero) and do not need to be encoded.