Skill: Screenshot Compression
Compress screenshot images (PNG/JPEG) in place while keeping the original format. Uses pngquant for PNG and jpegoptim for JPEG — both are highly effective for screenshot content (UI elements, text, flat colors).
Prerequisites:
pngquantandjpegoptimmust be installed on the system. The script will not install them automatically — it checks for their presence and prints install instructions if missing.
When to Use
The user has screenshot files that are too large and wants to reduce file size without changing format. Common scenarios:
- Preparing images for GitHub READMEs, blog posts, or documentation
- Reducing image sizes before committing to a repository
- Batch compressing a directory of screenshots
Why Keep Original Format (Not WebP)
WebP has better compression, but poor compatibility in some contexts:
| Context | WebP Support |
|---|---|
| Browsers (Chrome/Firefox/Safari/Edge) | Yes |
| GitHub Issues/PRs | Yes |
| WeChat editor | No |
| Word / PowerPoint | No |
| Some forums/blog backends | Varies |
Keeping PNG/JPEG ensures the compressed images work everywhere.
Default Workflow
python /path/to/skills/screenshot-compression/scripts/compress_screenshots.py <files-or-directories>
The script will:
- Check that
pngquantandjpegoptimare installed — if not, print install instructions and exit - Auto-detect file format by extension
- Compress each file in place (overwrites the original)
- Print per-file and total compression summary
Dependency Check
The script requires two system tools. If either is missing, it exits with install instructions instead of proceeding. Do not install them on behalf of the user — just relay the error message so the user can install them.
Install commands:
# macOS
brew install pngquant jpegoptim
# Ubuntu / Debian
sudo apt install pngquant jpegoptim
# CentOS / RHEL
sudo yum install pngquant jpegoptim
Script Options
| Flag | Default | Description |
|---|---|---|
paths (positional) | required | Image files or directories to compress |
-r, --recursive | off | Recursively process directories |
--png-quality | 80-95 | pngquant quality range (min-max, 0-100) |
--jpeg-quality | 85 | jpegoptim max quality (0-100) |
Examples
# Compress a single file
python .../compress_screenshots.py screenshot.png
# Compress all images in a directory
python .../compress_screenshots.py ./images/
# Recursive directory scan
python .../compress_screenshots.py ./docs/ --recursive
# High quality for code screenshots
python .../compress_screenshots.py *.png --png-quality 90-100
# Aggressive compression for thumbnails
python .../compress_screenshots.py *.jpg --jpeg-quality 70
Quality Tuning Guide
| Scenario | --png-quality | --jpeg-quality |
|---|---|---|
| General screenshots (docs, web pages) | 80-95 | 85 |
| Code screenshots (need sharp text) | 90-100 | 90 |
| Thumbnails / previews (size priority) | 60-80 | 70 |
How It Works
PNG (pngquant)
- Quantizes 24-bit true color (16M colors) down to an 8-bit palette (256 colors)
- Uses Floyd-Steinberg dithering for smooth gradients
- Screenshots are ideal candidates — UI colors are typically well under 256 unique values
- Typical reduction: 60-80%
JPEG (jpegoptim)
- Re-encodes at the specified quality level
- Strips metadata (EXIF, ICC profiles, thumbnails) via
--strip-all - Optimizes Huffman tables
- Typical reduction: 20-50%
Important Notes
- Files are compressed in place — the original is overwritten. Back up files first if needed.
- Only
.png,.jpg, and.jpegfiles are processed. Other formats are silently skipped. - The script never installs dependencies. If tools are missing, it prints install instructions and exits.