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Two-pass encoding is a controlled bitrate strategy that allows VP9 to balance compression efficiency and visual consistency across different segments of a video. It is particularly important for on-demand encoding , where encoding speed can be traded for higher efficiency and stable output size. Understanding Two-Pass Encoding VP9’s two-pass process separates encoding into two stages: First Pass (Analysis) : The encoder analyzes the entire video to collect motion vectors, scene complexity, and bit distribution statistics. Second Pass (Encoding) : The encoder uses that data to allocate bitrate more accurately, giving more bits to complex scenes and fewer to static areas. Unlike single-pass encoding, where bitrate allocation is based only on real-time frame analysis, two-pass encoding produces predictable file sizes and consistent perceptual quality across the entire sequence. When to Use VP9 Two-Pass Encoding Two-pass encoding is used when you need precise control over file size or bitrate and can afford longer processing times. It is best suited for: VOD Workflows : Encoding pre-recorded videos for streaming platforms, where consistency between renditions is essential. Adaptive Bitrate Packaging : Preparing multiple VP9 renditions (e.g., 1080p, 720p, 480p) that must align perfectly for DASH or HLS manifests. Bandwidth-constrained Delivery : When each output must stay under a strict bitrate cap (e.g., 2500 kbps for 1080p). Archival or High-Quality Exports : Ensuring optimal quality retention at fixed storage limits. Two-pass is not ideal for live streaming or latency-sensitive pipelines, as the process requires two full encoding passes over the input. How Two-Pass VP9 Encoding Works During the first pass , the encoder writes a statistics log file instead of generating output. This file stores frame complexity metrics such as motion intensity and quantizer decisions. During the second pass , the encoder reads the log file and encodes using that data, applying bitrate distribution according to target settings. This allows VP9 to allocate bits efficiently — high-motion frames get more bitrate, while static segments use less — without exceeding the total bitrate budget. Example: Two-Pass VP9 Encoding with FFmpeg First Pass (Analysis) ffmpeg -i input.mp4 \ -c:v libvpx-vp9 -b:v 2500k -pass 1 -an -f null /dev/null This pass scans the entire video and generates ffmpeg2pass-0.log, which contains bitrate and complexity data. Second Pass (Encoding) ffmpeg -i input.mp4 \ -c:v libvpx-vp9 -b:v 2500k -pass 2 -pix_fmt yuv420p -auto-alt-ref 1 -lag-in-frames 25 output_vp9.webm This pass reads the statistics file and encodes the video using optimal bit allocation. Explanation : -b:v 2500k : Specifies target bitrate. -auto-alt-ref 1 : Enables alternate reference frames for better temporal compression. -lag-in-frames 25 : Allows lookahead across frames for improved rate control. -pass 2 : Uses analysis data from the first pass to guide compression. Performance Considerations Two-pass encoding requires twice the processing time , but it produces more predictable results. Encoding time scales linearly with content duration and scene complexity. For long-form or high-resolution content, parallelization through tile-based encoding (-tile-columns) and row-multithreading (-row-mt 1) can reduce total processing time. For Example : ffmpeg -i input.mp4 \ -c:v libvpx-vp9 -b:v 2000k -row-mt 1 -tile-columns 4 -pass 2 output_vp9.webm This distributes encoding work across multiple CPU cores to speed up the second pass without reducing accuracy.