PNG
PNG
Portable Network Graphics (PNG)
A comprehensive guide to the lossless image format
Introduction to PNG
The Portable Network Graphics (PNG) format is a raster-graphics file format that supports lossless data compression. PNG was developed as an improved, non-patented replacement for Graphics Interchange Format (GIF), and is the most widely used lossless image compression format on the Internet.
History
PNG was created in 1995 after the GIF format became patent-encumbered. The first version of the PNG specification was released in 1996 and became an ISO standard in 2004.
Purpose
PNG was designed for transferring images on the Internet, not for professional-quality print graphics. It excels at displaying high-quality digital images with transparency.
Popularity
PNG is one of the most popular image formats on the web, supported by all major web browsers and image editing software. It's particularly favored for logos and graphics requiring transparency.
Technical Specifications
PNG files consist of a series of chunks that contain various types of information about the image. The format supports several types of images:
Color Types
- Grayscale: 1, 2, 4, 8, or 16 bits per pixel
- Truecolor (RGB): 8 or 16 bits per channel (24 or 48 bits per pixel)
- Indexed-color: 1, 2, 4, or 8 bits per pixel (palette-based)
- Grayscale with alpha: 8 or 16 bits per pixel, plus 8 or 16 bits for alpha
- Truecolor with alpha (RGBA): 8 or 16 bits per channel (32 or 64 bits per pixel)
Compression
PNG uses a two-stage compression process:
- Filtering: Each scanline is filtered using one of five algorithms to optimize the data for compression
- Deflate compression: The filtered data is compressed using the DEFLATE algorithm (the same used in ZIP files)
Chunks
PNG files contain mandatory and optional chunks that store different types of information:
| Chunk Type | Description | Critical? |
|---|---|---|
| IHDR | Image header (width, height, bit depth, etc.) | Yes |
| PLTE | Palette for indexed-color images | For indexed-color |
| IDAT | Image data (compressed) | Yes |
| IEND | Image trailer (marks end of file) | Yes |
| tEXt | Textual information (keywords and text) | No |
| gAMA | Gamma correction information | No |
Key Features of PNG
Lossless Compression
PNG uses a lossless compression algorithm, meaning the image quality doesn't degrade when the file is saved. This makes it ideal for images that require precise reproduction.
Transparency Support
PNG supports alpha channel transparency, allowing for smooth edges and partial transparency effects. This is superior to GIF's binary transparency.
Wide Color Range
PNG supports 24-bit RGB color (16.7 million colors) and 48-bit color for specialized applications, far exceeding GIF's 256-color limitation.
Gamma Correction
PNG stores gamma information, allowing images to be displayed with correct brightness across different platforms and display devices.
Interlacing
PNG supports Adam7 interlacing, which allows progressive display of images as they download, similar to GIF's interlacing but more sophisticated.
Error Detection
PNG includes robust error detection through CRC checksums, helping to identify corrupted files and ensuring data integrity.
PNG vs Other Image Formats
| Feature | PNG | JPEG | GIF | WebP |
|---|---|---|---|---|
| Compression | Lossless | Lossy (can be lossless) | Lossless | Lossy or Lossless |
| Transparency | Alpha channel | No | Binary (on/off) | Alpha channel |
| Color Depth | Up to 48-bit | 24-bit | 8-bit (256 colors) | 24-bit or 32-bit |
| Animation | No (APNG is separate) | No | Yes | Yes |
| Best For | Graphics, logos, screenshots | Photographs | Simple animations | Web images (modern) |
When to Use PNG
- Images requiring transparency (logos, icons)
- Images with text or line art
- Screenshots and digital captures
- Images requiring multiple edits (no generation loss)
- When exact reproduction is needed
When Not to Use PNG
- Photographic images (JPEG or WebP is better)
- When small file size is critical for many images
- Simple graphics with few colors (GIF might be smaller)
- Animations (use APNG, GIF, or WebP)
Interactive PNG Demo
Explore how PNG handles transparency and compare it with other formats:
Observations:
- PNG supports smooth transparency (alpha channel)
- JPEG doesn't support any transparency
- GIF only supports on/off transparency (no partial opacity)
Advanced PNG Features
APNG (Animated PNG)
Animated Portable Network Graphics (APNG) is an extension to PNG that adds support for animated images. It works similarly to animated GIFs but supports 24-bit images and 8-bit transparency.
PNG Optimization
Several techniques can optimize PNG files for smaller size:
- Color reduction: Convert truecolor images to palette-based when possible
- Filter selection: Choose the optimal filter for each scanline
- Compression level: Higher compression levels (though slower to encode)
- Remove metadata: Strip unnecessary chunks (comments, etc.)
PNG Crushing Tools
Popular tools for optimizing PNG files:
- PNGCRUSH
- OptiPNG
- PNGQuant (for converting to 8-bit palette with minimal quality loss)
- ZopfliPNG (extremely slow but excellent compression)
- Online tools like TinyPNG (which often use PNGQuant)
Browser and Software Support
Web Browser Support
PNG is supported by all modern web browsers, including:
- Chrome
- Firefox
- Safari
- Edge
- Opera
- Internet Explorer (since IE6 with some limitations)
Image Editing Software
All major image editors support PNG, including:
- Adobe Photoshop
- GIMP
- Paint.NET
- CorelDRAW
- Affinity Photo
Programming Libraries
Most programming languages have libraries for PNG manipulation:
- JavaScript: Canvas API, libpng.js
- Python: Pillow, PyPNG
- Java: ImageIO
- C/C++: libpng
- PHP: GD, Imagick
Future of PNG
While newer formats like WebP and AVIF offer better compression ratios, PNG remains relevant because:
- It's universally supported across all platforms and browsers
- Its lossless nature is essential for certain applications
- It's simple and well-understood
- The patent-free, open standard ensures wide adoption
Future developments may include:
- Wider adoption of APNG for animations
- Improved compression algorithms while maintaining compatibility
- Better tools for optimizing PNG files
- Continued use in applications where lossless compression is required