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.
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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
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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.
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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
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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) |
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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 |
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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%
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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.
