void glBitmap(GLsizeiwidth, GLsizei height,
GLfloat xbo, GLfloat ybo,
GLfloat xbi, GLfloat ybi,
const GLubyte *bitmap);
Draws the bitmap specified by bitmap, which is a pointer to the
bitmap image. The origin of the bitmap is placed at the most recently defined
current raster position. If the current raster position is invalid, nothing
is drawn, and the raster position remains invalid. The width and
height arguments indicate the width and height, in pixels, of the
bitmap. The width need not be a multiple of 8, although the data is stored
in unsigned characters of 8 bits each. (In the F example, it wouldn't matter
if there were garbage bits in the data beyond the tenth bit; since glBitmap()
was called with a width of 10, only 10 bits of the row are rendered.) Use
xbo and ybo to define the origin of the bitmap (positive
values move the origin up and to the right; negative values move it down
and to the left); xbi and ybi indicate the x and y
increments that are added to the raster position after the bitmap is rasterized.
Allowing the origin of the bitmap to be placed arbitrarily makes it
easy for characters to extend below the origin (typically used for characters
with descenders, such as g, j, and y), or to extend beyond the left of
the origin (used for various swash characters, which have extended flourishes,
or for characters in fonts that lean to the left).
After the bitmap is drawn, the current raster position is advanced by xbi and ybi in the x- and y-directions, respectively. For standard Latin fonts, ybi is typically 0.0 and xbi is positive (since successive characters are drawn from left to right). For Hebrew, where characters go from right to left, the xbi values would typically be negative. Fonts that draw successive characters vertically in columns would use zero for xbi and nonzero values for ybi. In the Figure each time the F is drawn, the current raster position advances by 12 pixels, allowing a 2-pixel space between successive characters.
Since xbo, ybo, xbi, and ybi are floating-point values, characters need not be an integral number of pixels wide. Actual characters are drawn on exact pixel boundaries, but the current raster position is kept in floating point so that each character is drawn as close as possible to where it belongs. For example, if the code in the F example was modified so that xbi is 11.5 instead of 12, and if more characters were drawn, the space between letters would alternate between one and two pixels, giving the best approximation to the requested 1.5-pixel space. Note that bitmaps can't be used for rotatable fonts because the bitmap is always drawn aligned to the x and y framebuffer axes.