4 Apr 2009

Quantization in an H.264 encoder is controlled by a quantization parameter, QP, that ranges from 0 to 51. QP is an index used to derive a scaling matrix. It is possible to calculate the equivalent quantizer step size (Qstep) for each value of QP (see my tutorial on H.264 4x4 transform). As QP increases, Qstep increases; in fact, Qstep doubles for every increase of 6 in QP. The logarithmic relationship can be seen in this graph of QP (x-axis, linear) vs. Qstep (y-axis, logarithmic).

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