The quantization parameter (QP) is the codec setting that controls how coarse quantization is - effectively the size of the step by which each DCT coefficient is divided before rounding. A higher QP means a larger step, so more high-frequency coefficients round to zero, fewer bits are spent, and the picture collapses faster toward flat blocks. A lower QP keeps the step small, preserves more detail, and costs more bits. QP is therefore the central dial trading file size against visible quality, and it sits directly upstream of every block-transform artifact: raise it too far and blocking opens on the grid, ringing grows around edges, texture blurs, and smooth regions band. Rate control adjusts QP frame by frame, and even region by region, to hit a target bitrate. The catch is that the same QP behaves very differently on easy versus hard content, so a fixed QP does not guarantee a fixed perceptual quality - which is why metrics and per-content tuning matter rather than trusting one number.
