In today’s post we examine PNG, a format that was specifically designed for the internet age. PNG also solves some problems around the inclusion of text and graphical elements in image files.
The Portable Network Graphics (PNG) format was specifically designed as an image format for use on the web. It supports a broad range of image object characteristics, including multiple color models, the use of alpha channels, transparency, and high bit depth images. PNG uses lossless compression. In response to the uncertainty of patent status of Compuserve Gif and TIFF’s LZW compression scheme, PNG was created using features that are not covered by any patents so it could be used freely. It is supported by almost all operating systems and web browsers and is therefore a good choice for many on-screen uses.
PNG is most appropriate for simple images that do not include processing instructions, alternate versions or multiple image components, with the exception of alpha channels. It’s a good format for archiving screenshots, logos and other text-heavy images.
Format details
Interoperability
PNG is widely supported by both operating systems and applications making it a good choice for interoperability. However, because PNG supports color models and bit depths that are not universally supported, it’s possible that a PNG will not open in a particular application. (This would be most common for PNG files that include 16-bit per channel color information.)
Supported color models
PNG can support grayscale, indexed color and RGB color. PNG supports bit depth from one bit (black or white) up to 16 bits per channel. Indexed color makes PNG efficient for saving graphics like logos and illustrations. (Transparency is important here, too, since most logos are not simple rectangles.)
PNG does support embedded color profiles, but this support does not seem to be as common as profile support in TIFF and JPEG.
Compression
PNG compression is lossless which maximizes image quality. PNG compression does a much better job of preserving and rendering hard lines like you find in type, logos or drawings. Since PNG does a better job with this type of image, it’s an ideal choice for logos and screenshots that may contain type.
Support for additional components
PNG supports alpha channels, which are probably most useful for adding transparency to an image, but have other uses as well. While PNG does not natively support EXIF metadata written by cameras, it is possible to add this metadata to PNG files. There is also an animated variant of PNG which was designed as a replacement for the animated GIF, but it is not widely supported.
Best uses for PNG files
PDF, web and other electronic publication
PNG is a good choice for electronic publication, particularly when the original is a screenshot, logo or other type-heavy image. It’s also fine for high-quality photographic images, but does not offer the same file size savings as JPEG for photos.
Capture logos, screenshots and converted vector graphics
PNG is the native format for many screenshot applications. It offers high quality, transparency, and some file-size saving over TIFF.
Library originals for read-only image applications
When the original image is captured as a PNG, that’s a perfectly acceptable format to use for read-only library storage (as long as the library application supports color profiles for PNG).
In the next post, we will look at TIFF, the most extensible image format.