video-comparer

SKILL.md

Video Comparer

Overview

Compare two videos and generate an interactive HTML report analyzing compression results. The script extracts video metadata, calculates quality metrics (PSNR, SSIM), and creates frame-by-frame visual comparisons with three viewing modes: slider, side-by-side, and grid.

When to Use This Skill

Use this skill when:

Comparing original and compressed videos

Analyzing video compression quality and efficiency

Evaluating codec performance or bitrate reduction impact

Users mention "compare videos", "video quality", "compression analysis", or "before/after compression"

Core Usage

Basic Command
python3 scripts/compare.py original.mp4 compressed.mp4

Generates comparison.html with:

Video parameters (codec, resolution, bitrate, duration, file size)

Quality metrics (PSNR, SSIM, size/bitrate reduction percentages)

Frame-by-frame comparison (default: frames at 5s intervals)

Command Options

# Custom output file
python3 scripts/compare.py original.mp4 compressed.mp4 -o report.html

# Custom frame interval (larger = fewer frames, faster processing)
python3 scripts/compare.py original.mp4 compressed.mp4 --interval 10

# Batch comparison
for original in originals/*.mp4; do
    compressed="compressed/$(basename "$original")"
    output="reports/$(basename "$original" .mp4).html"
    python3 scripts/compare.py "$original" "$compressed" -o "$output"
done

Requirements

System Dependencies

FFmpeg and FFprobe (required for video analysis and frame extraction):

# macOS
brew install ffmpeg

# Ubuntu/Debian
sudo apt update && sudo apt install ffmpeg

# Windows
# Download from https://ffmpeg.org/download.html
# Or use: winget install ffmpeg

Python 3.8+ (uses type hints, f-strings, pathlib)

Video Specifications

Supported formats: .mp4 (recommended), .mov, .avi, .mkv, .webm

File size limit: 500MB per video (configurable)

Processing time: ~1-2 minutes for typical videos; varies by duration and frame interval

Script Behavior

Automatic Validation

The script automatically validates:

FFmpeg/FFprobe installation and availability

File existence, extensions, and size limits

Path security (prevents directory traversal)

Clear error messages with resolution guidance appear when validation fails.

Quality Metrics

The script calculates two standard quality metrics:

PSNR (Peak Signal-to-Noise Ratio): Pixel-level similarity measurement (20-50 dB scale, higher is better)

SSIM (Structural Similarity Index): Perceptual similarity measurement (0.0-1.0 scale, higher is better)

For detailed interpretation scales and quality thresholds, consult references/video_metrics.md.

Frame Extraction

The script extracts frames at specified intervals (default: 5 seconds), scales them to consistent height (800px) for comparison, and embeds them as base64 data URLs in self-contained HTML. Temporary files are automatically cleaned after processing.

Output Report

The generated HTML report includes:

Slider Mode: Drag to reveal original vs compressed (default)

Side-by-Side Mode: Simultaneous display for direct comparison

Grid Mode: Compact 2-column layout

Zoom Controls: 50%-200% magnification

Self-contained format (no server required, works offline)

Important Implementation Details

Security

The script implements:

Path validation (absolute paths, prevents directory traversal)

Command injection prevention (no shell=True, validated arguments)

Resource limits (file size, timeouts)

Custom exceptions: ValidationError, FFmpegError, VideoComparisonError

Common Error Scenarios

"FFmpeg not found": Install FFmpeg via platform package manager (see Requirements section)

"File too large": Compress videos before comparison, or adjust MAX_FILE_SIZE_MB in scripts/compare.py

"Operation timed out": Increase FFMPEG_TIMEOUT constant or use larger --interval value (processes fewer frames)

"Frame count mismatch": Videos have different durations/frame rates; script auto-truncates to minimum frame count and shows warning

Configuration

The script includes adjustable constants for file size limits, timeouts, frame dimensions, and extraction intervals. To customize behavior, edit the constants at the top of scripts/compare.py. For detailed configuration options and their impacts, consult references/configuration.md.

Reference Materials

Consult these files for detailed information:

references/video_metrics.md: Quality metrics interpretation (PSNR/SSIM scales, compression targets, bitrate guidelines)

references/ffmpeg_commands.md: FFmpeg command reference (metadata extraction, frame extraction, troubleshooting)

references/configuration.md: Script configuration options and adjustable constants

assets/template.html: HTML report template for customizing viewing modes and styling