How VCR# Uses FFmpeg

Overview

VCR# renders every frame in-process. The from-scratch VT500 engine parses the shell's output into a cell grid, and RasterRenderer (built on SixLabors.ImageSharp and SixLabors.Fonts, running entirely on the CPU) draws each grid snapshot directly to a bitmap. There is no browser, no screenshot capture, and no compositing of separate text and cursor layers — each frame is rasterized once, fully composited.

FFmpeg only enters the picture when an Output target is an animated raster video. VideoWriter drives FFmpeg through the FFMpegCore .NET wrapper, which shells out to the ffmpeg binary on your PATH. FFMpegCore builds the argument list; VCR# does not assemble hand-written command lines.

What uses FFmpeg, and what doesn't

For the GIF/MP4/WebM formats, VCR# renders the composited PNG frames in-process, then hands them to FFmpeg only for the final encode. The frame directory itself uses the same PNG sequence plus a frames.txt concat-demuxer manifest, but it performs no encode — the files are written and left as-is.

GIF

GIF encoding uses FFmpeg's palettegen/paletteuse filters to build and apply a color palette in a single pass. The palette size honors MaxColors, and the loop behavior honors Loop/LoopCount.

Output demo.gif
Set MaxColors 128
Set Loop false        # play once and hold on the last frame
Type "echo hello"
Enter

MP4

MP4 encoding uses the libx264 H.264 encoder with the yuv420p pixel format for broad player and browser compatibility. MP4 has no alpha channel, so TransparentBackground has no effect on .mp4 output.

WebM

WebM encoding uses the libvpx-vp9 VP9 encoder. When Set TransparentBackground true is set, VCR# uses the yuva420p pixel format, preserving an alpha channel so the recording can sit over any page background.

Output demo.webm
Set TransparentBackground true
Type "echo transparent"
Enter

Settings that affect encoding

The renderer fixes most encoding parameters in code, but a handful of tape settings change the frames VCR# produces or the FFmpeg pipeline it drives:

For the full list of settings and their defaults, see the configuration options reference.

Error handling

FFmpeg encoding failures surface as exceptions from FFMpegCore, which VCR# rethrows with a Failed to render <FORMAT> output: <message> prefix. VCR# does not branch on specific FFmpeg exit codes. If .gif, .mp4, or .webm output fails with an FFmpeg error, confirm that ffmpeg is on your PATH and runnable.

External documentation

• FFmpeg official documentation
• FFMpegCore
• H.264 encoding guide
• VP9 encoding guide
• Palette generation filter