Description of PNG File Formats

The .PNG (Portable / Public Network Graphic) file extension is used by a wide variety of programs and applications, and .PNG files can be opened under any operating system. Dozens of applications support the .PNG file format, including the Adobe Creative Suite, Microsoft Office Suite, Corel Paint Shop Pro, IrfanView, Spore (a computer game); all major web browsers now can interpret and display .PNG files correctly.

Why .PNG?

The .PNG (pronounced "ping") format was developed to replace the outdated .GIF format (Graphics Interchange File). Like .GIFs, .PNG files are bitmapped images that use a compression method similar to .GIF files, but with no patents or licenses to worry about. The lossless compression algorithm used for .PNG files is far more efficient than for .GIFs, and is royalty-free. (See the official home page of PNG; link in Resources).

Transparency is another big plus for .PNG files: the .GIF format does not support a graduated transparent background for images (it's either opaque or transparent). The features in .PNG files make for a long list, but essentially, when the World Wide Web (WWW) consortium approved .PNG files as a standard, .PNG files became widely supported, and the format is still evolving and improving.

Basic .PNG

The basic .PNG format is used for bitmapped (raster) images, with indexed color, transparency, and lossless compression. One thing that sets .PNG files above .JPEGs and .TIFFs is an alpha channel--1 byte of extra data for every pixel. Having this extra data byte allows files to represent 16.7 million colors, and 256 levels of transparency. The 8-bit transparency channel permits you to have colors in the image that graduate from fully transparent to fully opaque. By contrast, .GIF files only allow on/off transparency.

.PNG8, .PNG24, .PNG32...

You may see files with an extension of .PNG8, or .PNG24, or .PNG32. These are called, respectively, 8-bit (or 24-bit, or 32-bit) PNG. In .PNG8 files, images can have 1-,2-,4-, or 8-bit pixels. An 8-bit image is capable of having up to 256 colors.

If you need graduated transparency in an image, saving it in .PNG32 format gives you 24-bit color (16.7 million colors), plus an 8-bit alpha channel for variable transparency. If you don't need the variable transparency, use the .PNG24 format, which gives you millions of colors, but no transparency.

What Doesn't .PNG Do?

There is one thing .PNG files can't handle. They can't be animated, as .GIFs can be. So there's yet another flavor of .PNG: the .MNG format (Multiple-Image Network Graphic), which does everything the .PNG format can, and it supports image animations (http://www.libpng.org/pub/mng/).

Tidbit

Macintosh computers running OS X 10.4 and later save screen shots as .PNG files.

References

• The Official Home Page for PNG
• File-Extensions.org

Resources

• FileInfo.com
• The Official Home Page for PNG
• Comparisons of .GIF, .TIFF, .JPEG Formats