Fun fact - HISTV actually has two-pass encoding! Though, with enough system RAM you can actually look ahead far enough that you can get the benefits of two-pass in just a single pass. I have a bit about this in the README.md:

Precision mode

One checkbox for the best quality the app can produce. It picks the smartest encoding strategy based on how much RAM your system has:

Your RAM	What it does
16GB or more	Looks 250 frames ahead to plan bitrate (single pass)
8-16GB	Looks 120 frames ahead to plan bitrate (single pass)
Under 8GB	Scans the whole file first, then encodes (two passes)

Two-pass only happens when precision mode is on AND the system has less than 8GB RAM AND the file would be CRF-encoded. Reason being, Precision Mode normally uses CRF with extended lookahead (120-250 frames depending on RAM). Lookahead buffers live in memory. On low-RAM systems that buffer would be too large, so the app falls back to two-pass instead and stores the analysis run in a tempfile on disk. To break down each one:

• Pass 1: Runs ffmpeg with -pass 1 writing to a null output. ffmpeg analyses the entire file and writes a statistics log (the passlog file) describing the complexity of every scene. No actual video is produced - this pass is pure analysis.
• Pass 2: Runs ffmpeg with -pass 2 using the statistics from pass 1. The encoder now knows what’s coming and can distribute bits intelligently across the whole file - spending more on complex scenes, less on simple ones - without needing a large lookahead buffer in RAM. After both passes complete, the passlog temp files are cleaned up.

I went with this architecture to mitigate the biggest problem with two-pass encoding, when there’s enough system RAM to store the lookahead frames: the speed. The quality of single-pass with a 250-frame lookahead is equal to two-pass; 120-frame lookahead is just as fast, and still close enough in quality that it doesn’t really make a difference.