What is XPM? X PixMap Format Explained

How XPM Works

An XPM file, short for X PixMap, is the color successor to the monochrome XBM format, and it shares XBM's most distinctive trait: the file is valid C source code. An XPM stores an image as a C array of strings that a program can include and compile directly, so the picture travels as text that a compiler can read.

The first string in the array is a header that lists four numbers: the width, the height, the number of colors used, and the number of characters per pixel. Allowing more than one character per pixel is what lets XPM describe images with many distinct colors, since each color needs its own unique character code.

After the header comes one string for each color, mapping an ASCII character (or group of characters) to a color value. A color can be given as a named color, as a hexadecimal value, or as the special keyword None, which marks that character as fully transparent. This gives XPM a real, if simple, form of transparency that XBM lacks.

Finally, the array contains one string per row of the image. Each character in a row references one of the color entries defined above, so reading the image is a matter of looking up each character in the color table. Because every part of the file is ordinary C text, the whole pixmap can be compiled straight into a program.

History and Standardization

XPM appeared in the late 1980s within the ecosystem of the X Window System, as a direct answer to the limitations of the monochrome XBM format. As graphical Unix applications began using color, developers needed a way to describe color icons that was as easy to embed in source code as XBM had been.

The format became widely used for application icons and toolbar images across X11 and Linux desktop software, including many programs built on older toolkits and early versions of common widget libraries. Its ability to include an image simply by compiling a C array made it a natural fit for the source-driven world of Unix development.

Like XBM, XPM was never ratified by a formal international standards body. It grew as a convention of the X Window System and its toolkits, and its widespread support in Unix graphics libraries is what turned it into a stable de facto standard for embedded icons on those platforms.

XPM Compared to XBM, PNG, and SVG

Compared to its sibling XBM, XPM is a clear step up. XBM is limited to two colors and offers no transparency, while XPM supports full color and a working None transparency value. That made XPM the better choice whenever an icon needed more than a plain black-and-white silhouette.

Compared to PNG, XPM is inefficient. Because it stores the image as human-readable C text, an XPM file is verbose and much larger than an equivalent compressed binary PNG, which also offers deeper color and smooth alpha transparency rather than a single all-or-nothing transparent color.

Compared to SVG, which is vector-based and scales cleanly to any size, XPM is a fixed-resolution raster and does not scale without quality loss. For modern interface icons, PNG and SVG have almost entirely displaced it. XPM remains legacy but is still occasionally encountered in older Unix and Linux source trees, where its compile-in-place convenience once made it popular.