How to Configure Two-Pass Encoding Using libvpx

This article provides a straightforward guide on how to configure and run two-pass video encoding using the libvpx encoder (specifically for VP9 and VP8 formats) via FFmpeg. You will learn the exact command-line syntax required for both the analysis and encoding passes, as well as the key parameters needed to achieve optimal video quality and file size.

Understanding Two-Pass Encoding

Two-pass encoding optimizes video quality by analyzing the video content before compressing it: * Pass 1: The encoder analyzes the video input to identify motion, scene cuts, and complexity. It writes this analysis data to a log file (usually named ffmpeg2pass-0.log) without generating a playable video file. * Pass 2: The encoder uses the log file from the first pass to allocate the target bitrate efficiently, giving more data to complex action scenes and less to static scenes.

Step-by-Step Configuration

To perform two-pass encoding with libvpx-vp9 in FFmpeg, you must run two separate commands in sequence.

Step 1: Run the First Pass

The first pass analyzes the video. Audio processing is disabled to save time, and the output video is discarded because we only need the generated log file.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -pass 1 -an -f null -

• -c:v libvpx-vp9: Selects the VP9 encoder (use libvpx if you are encoding to the older VP8 format).
• -b:v 2M: Sets the target video bitrate (e.g., 2 Megabits per second). This must match in both passes.
• -pass 1: Instructs the encoder to perform the analysis pass.
• -an: Disables audio to speed up the process.
• -f null -: Discards the video output since we only need the log file.

Step 2: Run the Second Pass

The second pass reads the log file created in Step 1, processes the audio, and outputs the final compressed WebM file.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -pass 2 -c:a libopus -b:a 128k output.webm

• -pass 2: Instructs the encoder to use the log file from the first pass to write the final video.
• -c:a libopus: Encodes the audio using the Opus codec, which is standard for WebM.
• -b:a 128k: Sets the audio bitrate to 128 kbps.

Best Practices for libvpx Two-Pass Encoding

• Keep Settings Identical: Video parameters such as bitrate (-b:v), resolution, framerate, and keyframe intervals must be identical in both the first and second-pass commands.
• Use the Speed Parameter: You can use the -row-mt 1 flag to enable row-based multi-threading for faster encoding. Additionally, setting -speed 4 for the first pass and -speed 1 or -speed 2 for the second pass provides an optimal balance between speed and quality.
• Constrained Quality Mode: For even better results, you can use Constrained Quality (CQ) mode by adding -crf (e.g., -crf 31) and setting a maximum bitrate limit with -maxrate alongside your target -b:v parameter.