Tips for Users

Following is a list of tips for users of PNG-supporting software:

Use the correct image format

If you have photographic images and their quality as JPEGs is acceptable, use JPEG! JPEG will almost always be smaller than PNG, especially for color images. Conversely, if you have images with just a few colors and/or sharp edges (such as text and simple graphics), JPEG is almost never the correct solution; use PNG or GIF instead. For binary transparency, also use PNG or GIF; for partial transparency or lossless RGB, use PNG or TIFF; for animations, use MNG or GIF.

Use the correct pixel depth

For example, don't convert a GIF (which, from a practical perspective, always has a depth of 8 bits or less) to a 24-bit PNG; that will automatically boost the file size by a factor of three. Similarly, if given the option, don't save a grayscale image as RGB; save it as grayscale or, at worst, as a palette-based PNG. Likewise, don't use a full alpha channel if single-color transparency (à la GIF) would suffice; it doubles the size of grayscale images and adds 33% to the size of RGB.

Corollary: Quantize and dither truecolor images to a palette if quality is acceptable

Likewise, quantize and dither RGBA or gray+alpha PNGs to a palette, if possible. This is something that only you, the user, can judge; no reasonable image application will ever quantize (which is a lossy transformation) unless instructed to do so by you. This is not an issue for GIF, which realistically supports only colormapped images (i.e., your choice of GIF as an output format amounts to an explicit instruction to quantize) nor is it an issue for JPEG, which supports only grayscale and truecolor. Only PNG supports colormapped, grayscale, and truecolor images, as well as alpha channels.

Use interlacing with care

Interlacing is a way to transmit the useful parts of an image more quickly, particularly on the Web, so that the end user can click on a hot-linked region before the image is fully downloaded, if she so chooses. But as I saw earlier, PNG's two-dimensional interlacing scheme can degrade compression by 15% in some cases, especially for small images. Since small images are transmitted over the network fairly quickly anyway, they usually do not need to be interlaced.

Use the correct tools

In the first six chapters, I discussed a number of PNG-supporting applications and noted their limitations wherever possible; use that as a guide when choosing your tools, assuming you have a choice. Even if your program generally compresses PNG images well, consider using an optimizer such as pngcrush on everything when you're done;[76] definitely do so if your program is not known for its compression performance. For converting GIFs to PNGs, the dedicated gif2png is the most capable solution, even given its permanently beta version number; it preserves both transparency and embedded text comments.

[76] It is one of my favorite tools, in case that wasn't already apparent. As of April 1999, there are still a few optimization tricks it doesn't do, but its author is addressing those even as this is written.

Don't include unnecessary information

A lengthy copyright message or other text can add 100 bytes or more, which is a lot for icons and other small images.