Originally Posted by x265_Project
RD = Rate Distortion Optimization level
At the core of x265 (and most video encoders) is an algorithm that figures out the best way to encode each block of video. This algorithm weighs the distortion of each candidate "mode" against the bits used (the rate). To do this rate distortion optimization as accurately as possible, the encoder has to accurately calculate the bits that each candidate mode would use. In other words, it has to fully encode the candidate mode, calculating the residual error after prediction (the difference between the inter or intra-predicted block and the source pixels), performing a discrete cosine transform on the residual error, quantizing the residual error, and entropy coding the predicted block plus the residual error. This method is used for RD level 5. For lower RD levels, x265 weighs the bits used by each candidate mode, but it doesn't go through all of the above steps (it will skip entropy coding, for example). Lower levels are roughly accurate when it comes to choosing the best mode, but not as accurate as the highest rd level. Note that there are a number of decisions in x265 that can be rd optimized. More decisions are rd optimized at higher rd level, to a higher degree of precision.
RECT = allow x265 to evaluate and code rectangular blocks (CUs). Each CTU (usually a 64x64 pixel block, but this depends on your --ctu setting) is analyzed to find the optimal way to encode the pixels in the CTU, partitioning the CTU into smaller CUs. For example, a 64x64 pixel CTU might be encoded as four 32x32 CUs, or sixteen 16x16 pixel CUs. HEVC supports rectangular CUs. For example, a 32x32 pixel block could be encoded as two 16x32 CUs, or two 32x16 CUs. Rectangular CUs allow the partitioning to more ideally match the video content, but of course the more possibilities the encoder wants to evaluate, the longer it takes to evaluate all of the possibilities.
AMP = allow x265 to evaluate asymmetric partitions. Asymmetric partitions are rectangular CUs that have a ratio of 1:4, 3:4, 4:1 or 4:3. For example, a 32x32 pixel block can be encoded as a 8x32 pixel block plus a 24x32 pixel block. Again, these additional possibilities can allow for a more accurate fit, but more possibilities = more time needed to evaluate all possibilities.
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