screenshot-compression

SKILL.md

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: pngquant and jpegoptim must 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 pngquant and jpegoptim are 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 .jpeg files are processed. Other formats are silently skipped.

The script never installs dependencies. If tools are missing, it prints install instructions and exits.