Markdown

WBMP

WBMP is the Acronym for Wireless Application Protocol Bitmap Format

Early mobile browsers had to show pictures on tiny screens, and the wireless link was slow. This monochrome raster stores each pixel as black or white, which kept logos, buttons, and simple diagrams small enough to download. Pages that used it were written for the Wireless Application Protocol (WAP).

Current phones do not require the format. It belongs to the first wave of wireless browsing, when a handset’s memory was scarce and every extra byte added delay.

Black and White Pixels

The picture is a grid with no gray and no color. A stored 0 is white and a stored 1 is black. Rows run from left to right. The high bit of each byte is the leftmost pixel in that group. When the width is not a multiple of eight, the row is padded to the next byte so the following row starts on a fresh boundary.

Type Field and Header

The file starts with a short header and then the pixel rows. The type value is 0, the monochrome encoding in that era’s wireless bitmap spec. Other type numbers were left for later and were not what early browsers drew. Width and height follow as variable-length integers, so a small icon does not spend extra bytes on its own size.

  • Type: Set to 0, which selects uncompressed black and white pixels.
  • Fixed header: Records whether extra header fields follow. A plain file leaves that flag unset.
  • Width and height: The pixel size of the grid, stored so small numbers stay short.
  • Image data: Uncompressed rows of single-bit pixels, padded to a whole byte at the end of each row.
PartWhat It Holds
Type0, the monochrome encoding in that era’s wireless bitmap spec.
Fixed headerA flag for extra fields, unset on a plain file.
Width and heightPixel counts, stored as variable-length integers.
Image dataOne bit per pixel, white as 0 and black as 1, with each row padded to a byte.

Early Handset Limits

A handset of that period could draw the file inline: a logo, a navigation mark, or a diagram made of stark shapes. The same limits explain why the files stayed small and why they could not hold a photograph. Designers kept the artwork simple so it still read at a few dozen pixels wide. Later phones and browsers moved to richer picture files. This format is a record of early wireless pages, not a requirement for a phone today.