compressPNG

Path: build/compressPNG | Language: Bash | Lines: ~79

Lossy and lossless PNG compression using pngnq and optipng with intelligent replacement.

---

Overview

compressPNG optimizes PNG files through a two-stage process: first lossy color quantization with pngnq, then lossless compression with optipng. Like compressGIF, it only replaces the original when savings exceed 10%, avoiding unnecessary file churn.

The script processes files in parallel and handles both individual files and directories.

---

Usage

# Compress specific files
./compressPNG image1.png image2.png

# Compress all image files in current directory
./compressPNG

# Process entire directory recursively
./compressPNG /path/to/images/

# Mixed files and directories
./compressPNG file.png directory/

Arguments:

• File paths and/or directory paths (optional)
• If no arguments, searches current directory for image files

---

How It Works

Optimization Pipeline

Input PNG
    ↓
Copy to temp file
    ↓
pngnq -v -s1 (lossy quantization → *-nq8.png)
    ↓
optipng -o9 (lossless compression)
    ↓
Compare sizes
    ↓
┌─────────────────────────┐
│ Size reduction >= 10%?  │
└─────────────────────────┘
    ↓ Yes           ↓ No
Replace original    Keep original
    ↓               (delete temp)
Done

Two-Stage Compression

1. pngnq (lossy): Reduces PNG to 8-bit palette (256 colors max)

   • -v: Verbose output
   • -s1: Sampling rate for color selection
   • Creates file with -nq8.png suffix

2. optipng (lossless): Maximum compression of the quantized image

   • -o9: Highest optimization level (slowest but smallest)

---

Key Functions

pngOne()

Core optimization function for individual files:

pngOne() {
    if [[ "$1" =~ .*\.png ]]; then
        orig_size=$(stat --printf="%s" "$1")
        temp_file=$(mktemp --suffix=.png)
        cp "$1" "$temp_file"

        # WARNING: pngnq on Ubuntu is buggy, will delete on overwrite
        nice --adjustment=19 pngnq -v -s1 "$temp_file"
        optimized_file="${temp_file%.*}"-nq8.png

        if [ -f "$optimized_file" ]; then
            nice --adjustment=19 optipng -o9 "$optimized_file"
            new_size=$(stat --printf="%s" "$optimized_file")

            # Calculate percentage difference
            diff=$(echo "scale=2; 100*(($orig_size - $new_size)/$orig_size)" | bc)

            # Replace only if 10%+ smaller
            if (( $(echo "$diff >= 10" | bc --mathlib) )); then
                mv "$optimized_file" "$1"
            else
                green "Not at least 10% smaller ($orig_size → $new_size). Keeping original."
            fi
        fi

        rm --force -- "$temp_file" "$optimized_file"
    else
        green "File $1 is not a PNG. Skipping."
    fi
}

Processing Logic

for ITEM in "${args[@]}"; do
    if [ -f "$ITEM" ]; then
        # Single file: process in background
        pngOne "$ITEM" &
    elif [ -d "$ITEM" ]; then
        # Directory: find all PNGs, process in parallel
        find "$ITEM" -type f -name "*.png" | sort | parallel --jobs "$N" -- pngOne
    fi
done

---

Configuration

Variable	Default	Purpose
N	14	Max parallel jobs for directory processing

---

Dependencies

• pngnq: PNG color quantizer (lossy)
• optipng: PNG optimizer (lossless)
• parallel: GNU parallel for batch processing
• bc: Calculator for percentage math

Install on Debian/Ubuntu:

apt-get install pngnq optipng parallel

---

Important Notes

pngnq Bug Warning

# WARNING: can't overwrite on Ubuntu as pngnq is buggy
# and will simply delete the file!

The script works around this by:

1. Copying to a temp file first
2. Running pngnq on the temp file (creates new -nq8.png)
3. Moving result back to original if optimization is worthwhile

Nice Priority

Both compression tools run with lowest CPU priority:

nice --adjustment=19 pngnq ...
nice --adjustment=19 optipng ...

This prevents the compression from starving other processes.

File Type Check

Only processes .png files, skips others with a message:

if [[ "$1" =~ .*\.png ]]; then
    # process
else
    green "File $1 is not a PNG. Skipping."
fi

---

Comparison with compressGIF

Feature	compressPNG	compressGIF
Compression	Lossy + Lossless	Lossless only
Tools	pngnq + optipng	gifsicle
Threshold	10%	10%
Directory support	Yes (recursive)	No (flat only)
Parallelism	Background + parallel	parallel only

---

See Also

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