In image and video compression CODECs, the quantisation operation is usually made up of two parts: a forward quantiser FQ in the encoder and an ‘inverse quantiser’ or (IQ) in the decoder (in fact quantization is not reversible and so a more accurate term is ‘scaler’ or ‘rescaler’). A critical parameter is the step size QP between successive re-scaled values. If the step size is large, the range of quantised values is small and can therefore be efficiently represented
(highly compressed) during transmission, but the re-scaled values are a crude approximation to the original signal. If the step size is small, the re-scaled values match the original signal more closely but the larger range of quantised values reduces compression efficiency.
Quantisation may be used to reduce the precision of image data after applying a transform such as the DCT or wavelet transform removing remove insignificant values such as near-zero DCT or wavelet coefficients. The forward quantiser in an image or video encoder is designed to map insignificant coefficient values to zero whilst retaining a reduced number of significant, nonzero coefficients. The output of a forward quantiser is typically a ‘sparse’ array of quantised coefficients, mainly containing zeros.