Convert video with 2 pass encoding for best quality/size

A 2-pass encoding (or two-pass encoding) is a video compression technique that processes your video twice to achieve better quality for a given file size.

Here's how it works:

First Pass: The encoder analyzes the entire video without actually creating a final output file. It collects data about scene complexity, motion, and other characteristics throughout the video. During this phase, it's just gathering information and creating statistical data about the video.

Second Pass: The encoder uses the analysis data from the first pass to make smarter decisions about how to allocate bits. It can assign more bits to complex scenes that need them and fewer bits to simpler scenes, resulting in more consistent quality throughout.

The benefits of 2-pass encoding include:

• More efficient use of the target bitrate
• More consistent quality across the entire video
• Better handling of complex scenes
• Better overall quality-to-size ratio

The downside is that encoding takes approximately twice as long as single-pass encoding. However, for final distribution versions of videos (especially when file size constraints are important), the quality improvement is often worth the extra processing time.

Let's take as example:

ffmpeg -i input.mp4 -c:v libx264 -preset medium -b:v 1500k -pass 1 -an -f mp4 /dev/null && \
ffmpeg -i input.mp4 -c:v libx264 -preset medium -b:v 1500k -pass 2 -c:a aac -b:a 128k -movflags +faststart output.mp4

This command performs a 2-pass encoding of a video file for optimal quality-to-size ratio. The first command runs the first pass of the encoding process, while the second command runs the second pass and creates the final output file.

First pass:

• -i input.mp4 Specifies the input video file
• -c:v libx264 Uses H.264 video codec
• -preset medium Balances encoding speed and compression efficiency
• -b:v 1500k Sets target video bitrate to 1500 kbps
• -pass 1 Indicates this is the first pass (analysis phase)
• -an Disables audio processing (not needed for first pass)
• -f mp4 Forces MP4 format output
• /dev/null Discards the actual output (only statistics are needed)

Second pass:

• Uses the same input and video settings
• -pass 2 Indicates this is the second pass (encoding with knowledge from first pass)
• -c:a aac Encodes audio using AAC codec
• -b:a 128k Sets audio bitrate to 128 kbps
• -movflags +faststart Optimizes for web streaming

The && operator ensures the second command only runs if the first succeeds. This method produces better quality at the same file size compared to single-pass encoding.