The PNG file format

PNG or Portable Network Graphics is a file format that was designed to replace GIF. Not only is GIF a technically limited file format but LZW, the compression algorithm it uses, is owned by Unisys who are more than happy to charge for the privilege of using it. PNG is patent-free and offers enough features to also make it a valid alternative to the TIFF file format in some cases. The file format is meant to store bitmap data.

PNG was developed around 1995 by an Internet working group led by Thomas Boutell. Its popularity got a big boost when W3C, the organization that defines web standards, started promoting its use in 1996. Major graphic arts applications such as Photoshop and InDesign fully support PNG, although the file format isn’t that popular in prepress because it doesn’t support CMYK. I often use InDesign to create presentations and for this type of application, PNG can be very useful.

There is a ‘sister file format’ called MNG which is meant for video applications.

File format specs

Color spaces

PNG supports the following image types:

• Line-art – pure black-and-white, essentially 1-bit grayscale
• Grayscale – with up to 65536 shades of gray (16-bit) are supported although often 256 shades are used.
• indexed color – from 1-bit to 8-bit (also called palette-based color or pseudocolor)
• RGB – up to 48-bit although 24-bit (16 million colors) is most popular.

Compression

PNG’s compression is fully lossless. No image information is lost by compressing the image.

Alpha channels

Alpha channels are comparable to Photoshop masks. It is a way of making sure that part of the image is transparent so that a colored background underneath a PNG image with alpha channel remains visible.

Gamma correction

Images created on Macs have a tendency to look too dark on a PC screen. The reverse is also true: PC images look too light on a Mac. This is due to a difference in gamma (image brightness) between both systems. A PNG image can contain the gamma value used by the authoring system so that applications can compensate for this if needed. A full-blown color management system is superior to a simple algorithm such as gamma curves. PNG can support this through extensions, but its use is not widespread (yet).

Interlacing

Interlacing is a web-specific feature. It is a mechanism that makes images appear faster on-screen by first displaying a low-res version of the image and gradually showing the full version. This feature cannot be used by prepress software.

Limitations

PNG files cannot contain ICC profiles (a mechanism that describes what the color space or gamut of the image is). Metadata (who made this image, what is it about, who holds the copyright,…) also aren’t supported. Even though the image resolution of a PNG file can be stored within the pHYs chunk (in pixels per meter), some design applications (like Adobe InDesign CS3) don’t support this properly and seem to assume PNG images use 72 dpi. This makes PNG unsuitable for use in print publishing.

Specsheet

Name: PNG
Developer: independent workgroup
Release date: 1995 for the original file format, 2003/2004 for the second version
Type of data: bitmap only
Number of colors: 2 to 65536 per channel
Color spaces: line-art, grayscale, indexed color, RGB
Compression algorithms: lossless (5 types of filter supported)
Ideal use: web publishing
Extension on PC-platform: .PNG
Macintosh file type: PNGf
WWW mime type: image/png
Special features: support for alpha channels, gamma correction, and interlacing
Remarks: A good reference site about PNG can be found here. PNG should be pronounced ‘ping’, not ‘pee and gee’.