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compressGIF

Path: build/compressGIF | Language: Bash | Lines: ~65

Lossless GIF compression script using gifsicle with intelligent file replacement.

Overview​

compressGIF optimizes GIF files using the gifsicle tool, but only replaces the original when the savings are worthwhile (>10% reduction). This threshold prevents unnecessary file churn—gifsicle always modifies file metadata even when no real optimization occurs, which would create spurious git changes.

The script processes files in parallel using GNU parallel for efficient batch optimization.

Usage​

# Compress specific files
./compressGIF image1.gif image2.gif

# Compress auto-detected images in current directory (jpg/jpeg/png/webp/avif)
./compressGIF

# Process from a list
find . -name "*.gif" | xargs ./compressGIF

Arguments:

  • File paths to compress (optional)
  • If no arguments, searches current directory for *.jpg, *.jpeg, *.png, *.webp, *.avif (note: does not auto-discover .gif)

How It Works​

Optimization Process​

Input GIF
    ↓
Create temp file
    ↓
gifsicle --colors=256 --optimize=3
    ↓
Compare sizes
    ↓
┌─────────────────────────┐
│ Size reduction >= 10%?  │
└─────────────────────────┘
    ↓ Yes           ↓ No
Replace original    Delete temp
    ↓               (keep original)
Done

The 10% Threshold​

The script calculates:

size_delta = original_size - optimized_size
min_reduction = original_size * 0.1

Only if size_delta >= min_reduction does it replace the original file.

Why this matters:

  • gifsicle always changes metadata, even with no real optimization
  • Without the threshold, every GIF would show as modified in git
  • The 10% threshold ensures only meaningful optimizations are kept

Key Functions​

optimize_gif()​

Core optimization function applied to each file:

optimize_gif() {
    local gif="$1"

    # Skip if file doesn't exist
    if [ ! -f "$gif" ]; then return; fi

    # Create temp file and optimize
    temp_gif="$(mktemp /tmp/XXXXXX.gif)"
    gifsicle --colors=256 --optimize=3 "$gif" > "$temp_gif"

    # Compare sizes
    original_size="$(stat --printf="%s" "$gif")"
    optimized_size="$(stat --printf="%s" "$temp_gif")"

    # Replace only if >10% savings
    size_delta="$((original_size - optimized_size))"
    min_reduction="$(echo "scale=0; $original_size * 0.1 / 1" | bc)"

    if [ "$size_delta" -ge "$min_reduction" ]; then
        mv "$temp_gif" "$gif"
        echo "Optimized $gif"
    else
        rm "$temp_gif"
    fi
}

get_image_files()​

Auto-discovers image files when no arguments provided (jpg/jpeg/png/webp/avif only):

find . -maxdepth 1 -type f \
    \( -iname "*.jpg" -o -iname "*.jpeg" -o -iname "*.png" \
       -o -iname "*.webp" -o -iname "*.avif" \) \
    | sort --version-sort

Gifsicle Options​

OptionPurpose
--colors=256Reduce to 256 colors (GIF maximum)
--optimize=3Maximum optimization level

The -O3 optimization includes:

  • Cross-frame optimization
  • LZW compression tuning
  • Transparency optimization

Dependencies​

  • gifsicle: GIF manipulation tool
  • parallel: GNU parallel for batch processing
  • bc: Calculator for percentage math
  • stat: File size information

Install on Debian/Ubuntu:

apt-get install gifsicle parallel bc

Helper Functions (Inlined)​

The script inlines common helpers for portability:

red()   { echo -e "\e[41m$@\e[0m"; }   # Red background text
green() { echo -e "\e[32m$@\e[0m"; }   # Green text

Error Handling​

  • Exits immediately on error (set -e)
  • Checks for required commands before processing
  • Gracefully handles non-existent files in optimize_gif()
  • Reports when no arguments are available

Known Issue​

The script comment references a gifsicle behavior:

NOTE: this also avoids the issue where gifsicle always changes the file metadata by clobbering the original, even when no real change was made (which is a WONTFIX by the maintainer: https://github.com/kohler/gifsicle/issues/201)

This is why the threshold-based replacement is essential—without it, every GIF would appear modified in version control.

See Also​

  • compressPNG - Similar script for PNG compression
  • sync.sh - Main build script that may call image optimizers
  • Image.hs - Haskell image processing utilities