Blitting and sprites

As far as Allegro is concerned, a bitmap and a sprite are the same thing, but to many people the two words imply slightly different things. The function draw_sprite() is called so rather than draw_bitmap() partly because it indicates that it uses a masked drawing mode (if it existed, you could expect draw_bitmap() to be a simple block copy), and partly for historical reasons. In Allegro 1.0 there were actually different structures for sprites and bitmaps, each with their own set of abilities. Allegro 2.0 merged these into a single more flexible structure, but retained some names like draw_sprite().

In wider (non-Allegro) terms, the two words can mean quite different things. Generally you can say that sprites are a subset of bitmaps, but even that isn't true in 100% of cases.

BITMAP: a widely accepted term that will be understood by anyone even remotely connected with computer graphics. It simply means an image built up from a grid of pixels, ie. just about any picture that you are likely to come across on a computer (vector graphics formats are the exception, but those must be rendered into a bitmap format before they can be displayed by most hardware). A more accurate term but slightly rarer term with the same meaning is "pixmap" (pixel-map).

SPRITE: a particular usage of bitmapped images, restricted to video games (other types of programmer probably won't be familiar with this term). Originally on machines like the C64, sprites were a hardware feature that allowed a number of small bitmap images to be loaded into special registers, and they could then be superimposed over the main graphics display and moved around just by modifying the position register. They were used for the moving objects (player and enemy characters), and enabled the C64 to do much more impressive things than would have been possible if all the drawing had to be done directly by the puny CPU.

Later on, a lot of old C64 programmers upgraded to machines like the Atari ST, which didn't have any special sprite hardware, but they carried on referring to their main moving objects as sprites (the routine to draw such a thing would obviously be called draw_sprite()). A sprite is really just a bitmap graphic which is drawn onto the screen, but when you call it a sprite rather than a bitmap, this suggests it is a gameplay element that can move freely around the world rather than being a static part of the environment, and that it will be drawn in a masked overlay mode rather than as a solid rectangle (there is also a strong implication that a sprite will be animated by cycling through a number of frames, but that isn't always the case).

In recent years some people have started using "sprite" to refer to any character graphics, even if they are not in fact drawn as 2d bitmaps, eg. "this game uses 3d polygonal player sprites". This is a confusing misuse of the word (Doom uses sprites, Quake does not), but it does happen.

The origin of the term "blit" is also rather interesting. This was originally BitBlt, an abbreviation of BITmap BLock Transfer, which was a function designed (possibly) by the people at Xerox who did so much of the pioneering work on graphics display systems, and subsequently copied by virtually everybody doing computer graphics (the Microsoft Windows GDI still provides a BitBlt function with identical functionality to the original). This routine was a workhorse for all sorts of drawing operations, basically copying bitmap graphics from one place to another, but including a number of different ROP modes (Raster OPerations) for doing things like XOR, inverting pixels, etc. A whole family of related words grew up around the BitBlt function, but "blt" is impossible to speak (try saying "bltter" or "bltting" :-) so people added the vowel to make it easier to pronounce.

Thusly, the act of calling the BitBlt function came to be known as "doing a blit". The obvious next step was to rename the function itself to blit(), which generally took place at the same time as people decided to simplify the original, removing the different ROP modes on the grounds that they aren't needed for games coding and don't work well with anything higher than monochrome images in any case. This leaves us with a function called blit(), which is an abbreviation for "block transfer". A strong case could be made for calling this blot() instead, but somehow that just doesn't sound the same!

Anyway, all the routines in this chapter are affected by the clipping rectangle of the destination bitmap.

blit - Copies a rectangular area from one bitmap to another.
stretch_blit - Scales a rectangular area from one bitmap to another.
masked_blit - Copies a rectangle skipping pixels with the mask color.
masked_stretch_blit - Scales a rectangular area skipping pixels with the mask color.
draw_sprite - Draws a copy of the sprite onto the destination bitmap.
stretch_sprite - Stretches a sprite to the destination bitmap.
draw_sprite_v_flip
draw_sprite_h_flip
draw_sprite_vh_flip - Draws the sprite transformed to the destination bitmap.
draw_trans_sprite - Draws a sprite blending it with the destination.
draw_lit_sprite - Draws a sprite tinted with a specific color.
draw_gouraud_sprite - Draws a sprite with gouraud shading.
draw_character_ex - Draws non transparent pixels of the sprite with a color.
rotate_sprite - Rotates a sprite.
rotate_sprite_v_flip - Rotates and flips a sprite.
rotate_scaled_sprite - Rotates and stretches a sprite.
rotate_scaled_sprite_v_flip - Rotates, stretches and flips a sprite.
pivot_sprite - Rotates a sprite around a specified point.
pivot_sprite_v_flip - Rotates and flips a sprite around a specified point.
pivot_scaled_sprite - Rotates and stretches a sprite around a specified point.
pivot_scaled_sprite_v_flip - Rotates, stretches and flips a sprite around a specified point.

Allegro Manual

API

Blitting and sprites