Tips for Programmers

Following is a list of tips for programmers:

Use the correct pixel depth

Count colors! Or at least do so when the compression setting is ``best'' and you don't know that the image is grayscale--it doesn't take that long, and computers are good at that sort of thing. If there are 256 or fewer colors, write a colormapped image; doing so will translate to a factor-of-three savings in the PNG file size relative to an RGB image.

Use the correct pixel depth II

If the image is colormapped, don't assume that the pixels must be 8 bits deep. If there are only one or two colors, write a 1-bit image. If there are three or four colors, write a 2-bit image. If there are between 5 and 16 colors, write a 4-bit image. (These are the only useful cases for PNG.) The compression engine cannot compensate for bloated pixels! Choosing the correct depth for a palette-based image will reduce the file size by a factor of anywhere from two to eight relative to an 8-bit image.

Use grayscale if possible

If you know the image is gray, see if it can be written more compactly as a grayscale PNG than as a colormapped PNG--this is automatically true if there are more than 16 shades of gray. Doing so will save up to 780 bytes by eliminating the palette. But don't assume that 16 or fewer shades automatically means the image can be written as 4-bit (or smaller) grayscale. Grayscale necessarily implies that the shades are evenly distributed from black to white. If, for example, the 16 shades are bunched up in one part of the gray spectrum, the image must be written as 8-bit grayscale or 4-bit palette-based. For larger images, the palette-based approach is almost certainly better; for small ones it depends, but the 8-bit grayscale case may end up being smaller. Try both, if possible; it's very fast for small images.

Set the compression and filtering options intelligently

For programs that use libpng (discussed at length in Part III, "Programming with PNG"), this is not a serious issue; it will automatically do the right thing if left to itself. But if you are writing custom PNG code, follow the guidelines in the PNG specification for matching filter strategies with image types. In particular, use filter type None for colormapped images and for grayscale images less than 8 bits deep. Use adaptive filtering (the ``minimum sum of absolute differences'' heuristic) for all other cases.

Truncate the palette

Unlike GIF, PNG's palette size is determined by the chunk size, so there is no need to include 256 entries if only 173 are used in the image. At 3 bytes per entry, wasted slots can make a big difference in icons and other small images.

Truncate the transparency chunk

It is extremely rare for every palette entry to be partially or fully transparent. If there are any opaque entries--in particular, if all but one are opaque--reorder the palette so that the opaque entries are at the end. The transparency entries corresponding to these opaque colors can then be omitted. The absolute worst possible approach is to put the single transparent entry at the end of the palette! Those 255 extra bytes are a lot for a file that would otherwise be 500 (or even 150) bytes long.

Do transparency intelligently

Understand how PNG's alpha channels and tRNS chunk work. If the alpha mask is binary (that is, either fully transparent or fully opaque), see if the transparent parts correspond to a single color or gray shade; if so, eliminate the alpha channel from the PNG file and use the tRNS chunk (``cheap transparency'') instead. Alternatively, see if the total number of color+alpha combinations is 256 or fewer; if so, write a colormapped image with a tRNS chunk. If the user requests that an RGBA image be converted to indexed color, do so intelligently. The combination of PNG's PLTE and tRNS chunks amounts to a palette whose entries are RGBA values. The exact same algorithms that quantize and dither a 24-bit RGB image down to an 8-bit palette-based image can be used to quantize and dither a 32-bit RGBA or 16-bit grayscale+alpha image down to an 8-bit RGBA palette. In particular, you cannot treat color values and transparency values as if they are separate, unrelated entities; attempting to partition the palette into a ``color part'' and a ``transparent part'' makes no more sense than attempting to partition a standard RGB palette into red, green, and blue parts. If you do cheap transparency poorly, the user will be forced to use a full alpha channel, quadrupling her file size. For grayscale, an alpha channel ``merely'' doubles the size. Note that the icicle image in Figure C-1 in the color insert is actually colormapped. Aside from the garish background--which was actually generated by the viewing application--the full-resolution half looks pretty darned good, doesn't it?

Don't include unnecessary chunks in small images

Gamma information (or the sRGB chunk) is always good, but a full ICC profile may quadruple the size of a small image file. Consider not including a Software text chunk or tIME chunk, or do so only for images larger than, say, 100 × 100 pixels. Include dots-per-inch information (pHYs chunk) only if it is actually relevant to the image; but the user may be the only one who can make that call.

Offer the user reasonable options

Don't overwhelm him with unnecessary detail about filters or other technical jargon. For example, offer a simple checkbox to turn on interlacing. Offer a simple dial or even just two or three choices for compression level--fastest, typical, and best, perhaps. Even though it will make the file bigger, offer to include at least a few text annotations--Author, Title, Description, and/or Copyright, for example. On the other hand, offer to omit certain optional information, such as that described in the previous item.

Warn the user about data loss

If a region is completely transparent, don't zero out the underlying color pixels in order to improve compression unless you've notified the user in some way. Make sure she understands that quantization and dithering are lossy transformations, but don't make this an overly scary issue.