Transparency and patterned drawing

256-color transparency

In paletted video modes, translucency and lighting are implemented with a 64k lookup table, which contains the result of combining any two colors c1 and c2. You must set up this table before you use any of the translucency or lighting routines. Depending on how you construct the table, a range of different effects are possible. For example, translucency can be implemented by using a color halfway between c1 and c2 as the result of the combination. Lighting is achieved by treating one of the colors as a light level (0-255) rather than a color, and setting up the table appropriately. A range of specialised effects are possible, for instance replacing any color with any other color and making individual source or destination colors completely solid or invisible. Color mapping tables can be precalculated with the colormap utility, or generated at runtime. Read chapter "Structures and types defined by Allegro" for an internal description of the COLOR_MAP structure.

Truecolor transparency

In truecolor video modes, translucency and lighting are implemented by a blender function of the form: 
   unsigned long (*BLENDER_FUNC)(unsigned long x, y, n);


For each pixel to be drawn, this routine is passed two color parameters x and y, decomposes them into their red, green and blue components, combines them according to some mathematical transformation involving the interpolation factor n, and then merges the result back into a single return color value, which will be used to draw the pixel onto the destination bitmap.

The parameter x represents the blending modifier color and the parameter y represents the base color to be modified. The interpolation factor n is in the range [0-255] and controls the solidity of the blending.

When a translucent drawing function is used, x is the color of the source, y is the color of the bitmap being drawn onto and n is the alpha level that was passed to the function that sets the blending mode (the RGB triplet that was passed to this function is not taken into account).

When a lit sprite drawing function is used, x is the color represented by the RGB triplet that was passed to the function that sets the blending mode (the alpha level that was passed to this function is not taken into account), y is the color of the sprite and n is the alpha level that was passed to the drawing function itself.

Since these routines may be used from various different color depths, there are three such callbacks, one for use with 15-bit 5.5.5 pixels, one for 16 bit 5.6.5 pixels, and one for 24-bit 8.8.8 pixels (this can be shared between the 24 and 32-bit code since the bit packing is the same).