Look-ahead improves the video encoder’s rate control accuracy by enabling the encoder to buffer the specified number of frames, estimate their complexity and allocate the bits appropriately among these frames proportional to their complexity. This also dynamically allocates B and P frames. Look-ahead
When determining the QP to use for encoding a frame, it is beneficial if NVENC knows the overall complexity of the frame to distribute the available bit budget in the most optimal manner. In some situations, multi-pass encoding may also help catch larger motion between frames. For this purpose, NVENC supports the following types of multi-pass frame encoding modes:
- 1-pass per frame encoding (NV_ENC_MULTI_PASS_DISABLED)
- 2-passes per frame, with first pass in quarter resolution and second pass in full resolution (NV_ENC_TWO_PASS_QUARTER_RESOLUTION)
- 2-passes per frame, with both passes in full resolution (NV_ENC_TWO_PASS_FULL_RESOLUION).
In 1-pass rate control modes, NVENC estimates the required QP for the macroblock and immediately encodes the macroblock. In 2-pass rate control modes, NVENC estimates the complexity of the frame to be encoded and determines bit distribution across the frame in the first pass. In the second pass, NVENC encodes macroblocks in the frame using the distribution determined in the first pass. As a result, with 2-pass rate control modes, NVENC can distribute the bits more optimally within the frame and can reach closer to the target bitrate, especially for CBR encoding. Note, however, that everything else being the same,
performance of 2-pass rate control mode is lower than that of 1-pass rate control mode. The client application (you) should choose an appropriate multi-pass rate control mode after evaluating various modes, as each of the modes has its own advantages and disadvantages. NV_ENC_TWO_PASS_FULL_RESOLUION generates better statistics for the second pass, whereas NV_ENC_TWO_PASS_QUARTER_RESOLUTION results in larger motion vectors being caught and fed as hints to second pass.
