Because bitmaps can be used in so many different ways, the bitmap structure
is quite complicated, and it contains a lot of data. In many situations,
though, you will find yourself storing images that are only ever copied to
the screen, rather than being drawn onto or used as filling patterns, etc.
If this is the case you may be better off storing your images in RLE_SPRITE
(read chapter "Structures and types defined by Allegro" for an internal
description of the RLE_SPRITE structure) or COMPILED_SPRITE (see next
chapter) structures rather than regular bitmaps.
RLE sprites store the image in a simple run-length encoded format, where
repeated zero pixels are replaced by a single length count, and strings of
non-zero pixels are preceded by a counter giving the length of the solid
run. RLE sprites are usually much smaller than normal bitmaps, both because
of the run length compression, and because they avoid most of the overhead
of the bitmap structure. They are often also faster than normal bitmaps,
because rather than having to compare every single pixel with zero to
determine whether it should be drawn, it is possible to skip over a whole
run of zeros with a single add, or to copy a long run of non-zero pixels
with fast string instructions.
Every silver lining has a cloud, though, and in the case of RLE sprites it
is a lack of flexibility. You can't draw onto them, and you can't flip them,
rotate them, or stretch them. In fact the only thing you can do with them is
to blast them onto a bitmap with the draw_rle_sprite() function, which is
equivalent to using draw_sprite() with a regular bitmap. You can convert
bitmaps into RLE sprites at runtime, or you can create RLE sprite structures
in grabber datafiles by making a new object of type 'RLE sprite'.