What is XBM? X BitMap Format Explained

How XBM Works

An XBM file, short for X BitMap, has an unusual defining trait: the file is itself valid C source code. Rather than storing binary image data, an XBM stores the image as C declarations that a program can include and compile directly. This is what sets XBM apart from almost every other image format.

The layout is straightforward. The file begins with two #define lines that set the image width and height in pixels. These are followed by a declaration of a static array of bytes that holds the actual bitmap. Each bit in the array corresponds to one pixel, so eight pixels are packed into every byte, with rows arranged in order across the image.

The image is strictly 1-bit monochrome: a bit is either set or clear, giving a foreground pixel or a background pixel, and nothing in between. There is no color, no grayscale, and no alpha channel. The two colors are decided at display time by the program that draws the bitmap, not stored in the file itself.

Because the whole image is expressed as ordinary C text, an XBM could be dropped straight into a program's source with an #include and compiled into the binary. That made it a convenient way to embed small graphics such as icons and cursors directly into an application without loading an external file at runtime.

History and Standardization

XBM originated in the X Window System (X11) during the 1980s. X was the windowing system that underpinned graphical Unix workstations, and it needed a simple, portable way to describe the small monochrome bitmaps used throughout the interface.

Within X11, XBM images served as icons, mouse cursors, stipple and tiling patterns, and the classic gray root-window background that many early X users will remember. Storing these as compilable C made them easy to bundle into applications and into X itself, which suited the source-driven culture of Unix development at the time.

The format was later understood by some early web browsers, which could display XBM images inline, though that support was dropped as richer formats became standard. XBM was never formalized by an international standards body; it lived as a convention of the X Window System and its toolkits, which is why it remains tied to legacy X11 assets.

XBM Compared to XPM and Modern Formats

The closest relative to XBM is XPM, the X PixMap format, which extended the same idea of storing an image as compilable C source but added color and a form of transparency. Where XBM is limited to two colors, XPM can represent full color icons, which is why XPM largely replaced XBM for anything beyond simple monochrome shapes.

Compared to modern raster formats like PNG or GIF, XBM is inefficient. Because the image is stored as human-readable text with C syntax, the file is considerably larger than an equivalent packed binary image, and it must be parsed as code rather than read as data. It also carries none of the color depth or compression those formats provide.

XBM's strengths were simplicity and the ability to compile an image straight into a program, both of which mattered in early X11. Today it is obsolete for general use, restricted to 1-bit monochrome, and kept alive mainly for legacy X11 icons, cursors, and patterns found in older Unix and Linux source trees.