1998 comment: I wrote this back when Allegro 3.0 was just going into beta, for the benefit of people who were used to the old 256 color functions and just wanted to know what was different about the new truecolor modes. If the wording sometimes seems a little strange, that is why: it was written to explain some new features to experienced users, rather than as a tutorial for beginners. But it can probably serve in either role, which is why I've left it up here.
Well, here it is. Lots of new graphics code, millions of additional colors, and the end to those pesky eight bit paletted images. There aren't actually very many new functions, though: it is more a matter of the old routines working in slightly different ways. Details can of course be found in the main library documentation, but I thought this guide might be useful as a quick-start for people who are already familar with the 256 color API, and simply need to know what is different in a truecolor mode. So, how do you go about using a truecolor resolution?
Call set_color_depth() before set_gfx_mode().
Call set_color_depth() at the start of your program (before set_gfx_mode(), and before you read in any bitmap data from the disk).
Use the makecol() function (or makecol15(), makecol16(), etc, if you know for certain what color depth you will be using), to convert color values from red, green, and blue intensities into the format being used by the video hardware, and then pass the resulting integer color to any of the drawing functions.
Use magenta (maximum red and blue, zero green) instead of zero as a marker for masked pixels.
As well as the standard 8 bit paletted mode, Allegro now supports 15 and 16 bit hicolor and 24 and 32 bit truecolor resolutions. In the 15 bit modes, there is 5 bits each of red, green, and blue, and one unused bit to pad out the size of each pixel. In a 16 bit mode there are 5 bits of red and blue and 6 bits of green. In 24 bit modes each pixel is three bytes, one each of red, green, and blue (this tends to slow things down quite a bit due to bad data alignments!), and 32 bit modes are the same as 24 bit ones but with an extra spare byte per pixel to keep things nicely aligned.
Which modes you can use will depend on your graphics card and VESA driver. Most cards support both 15 and 16 bit resolutions, but if at all possible I would advise you to support both (it's not hard...) in case one is not available. Some cards support both 24 and 32 bit truecolor, in which case it is a choice between 24 (saves memory) or 32 (faster), but many older cards don't have any 32 bit mode and many newer ones don't support 24 bits. As to what screen sizes you can use, most cards will support all the standard SVGA resolutions, and with UniVBE you may also get some lowres truecolor modes like 320x200, 320x240, etc. On a card with one meg of video RAM you will be able to use 640x480 hicolor modes, and also 800x600 if you have a linear framebuffer. With two meg of video RAM you will be able to use 640x480 hicolor with two pages, 800x600 hicolor (two pages if you have a linear framebuffer), and 1024x768 hicolor, plus 640x480 truecolor (also 800x600 truecolor if you have a linear framebuffer). With four meg of video RAM just about anything is possible :-)
Allegro maintains a global color depth setting, which can be changed by calling set_color_depth(newdepth). This does nothing in itself, but it affects all subsequent calls to set_gfx_mode(), create_bitmap(), and any of the bitmap or datafile loading commands, so typically you will use it once at the start of your program to specify which color depth you would like to work in.
All truecolor pixel values are passed to the drawing functions as regular ints, just as in eight bit modes. How the individual color bits are layed out within these pixels can vary depending on the resolution, though, so you should avoid hardcoding any bit patterns into your program (other than zero as black and all ones as white). Instead, use the makecol() function to pack red, green, and blue values (range 0-255) into the correct format for the current color depth, or the faster versions makecol15(), makecol16(), etc, if you know for sure what color depth you are using. Use getr(), getg(), and getb(), or the faster getr15(), getr16(), etc, to extract the individual color components from a pixel value. All of these functions depend on knowing how the color bits are layed out (RGB or BGR format), which isn't detected until after you call set_gfx_mode(), so you should always select the video mode before using these routines.
Bitmap objects can be read in from datafiles (any color depth), .PCX files (8 or 24 bit color), .BMP files (8 or 24 bit color), .LBM files (8 bit color), and TGA files (8, 15, or 24 bit color). By default the data will be converted into the current color depth format, but you can request it to be left in the original form by calling the set_color_conversion() function. The image loader routines need to know what pixel format your display hardware is using, so you must set the video mode before reading in any graphics (if you load the images while still in text mode, your program will be prone to obscure bugs with colors being displayed wrongly on certain graphics cards). If you use any compiled datafiles containing truecolor graphics, you should call the function fixup_datafile() after you set the screen mode, to make sure the data is in the correct format.
For marking transparent pixels in sprites, zero cannot be used in truecolor modes, because unlike a 256 color paletted image it is required for storing black pixels. Instead, Allegro uses bright pink (maximum red and blue, zero green). You can obtain this value by calling makecol(0xFF, 0, 0xFF), or using the constants for specific color depths, MASK_COLOR_8, MASK_COLOR_15, MASK_COLOR_16, etc, or if you want to know the transparent color for a specific bitmap, use bitmap_mask_color(bmp).
In general, Allegro is designed to be used in only one color depth at a time, so you will call set_color_depth() once and then store all your bitmaps in the same format. If you want to mix several different pixel formats, you can use create_bitmap_ex() in place of create_bitmap(), and call bitmap_color_depth() to query the format of a specific image. Most of the graphics routines require all their input parameters to be in the same format (eg. you cannot stretch a 15 bit source bitmap onto a 24 bit destination), but there are three exceptions: blit() can copy between bitmaps of any format, converting the data as required, draw_sprite() can draw 256 color source images onto destinations of any format, and draw_character() _always_ uses a 256 color source bitmap, whatever the format of the destination. Expanding a 256 color source onto a truecolor destination is fairly fast (obviously you must set the correct palette before doing this conversion!). Converting between different truecolor formats is slightly slower, and reducing truecolor images to a 256 color destination is very slow (it can be sped up significantly if you set up the global rgb_map table before doing the conversion).
There are no hardware palettes in truecolor modes, so the palette fading functions no longer work. You can still call set_palette(), though, to specifiy what palette should be used when converting bitmaps between 256 and truecolor formats. There is a global array called palette_color which lists the values of every entry in the palette, in the correct format for the current video mode. So if you are in a truecolor mode and want to draw a line using color #3 from the current palette, pass palette_color as the color parameter to the line() function.
Translucency works totally differently in truecolor modes: the color_map table is no longer used. Instead, you must call set_blender_mode() or set_trans_blender() before drawing any lit or translucent objects.
If you want to access the contents of a truecolor bitmap directly, you can use the bmp->line table or the bmp_write_line() function just as you would in a 256 color mode. In a 15 or 16 bit mode you must cast the line pointer to an (unsigned short *) before you dereference it, or use the _farpokew() function, and in 32 bit modes you must cast the pointer to an (unsigned long *) or use _farpokel(). In 24 bit modes a pixel consists of three bytes, which makes accessing it rather more of a pain: you could use three consecutive writes into the 8 bit line data, or cast and use one write to a short and another to the single extra byte.
The _putpixel() and _getpixel() inline functions only work in 256 color modes, and the 3d polygon drawing routines are also unsupported in truecolor resolutions. All the other graphics functions will work in any mode.
If you want to use the GUI routines in a truecolor mode, you must set the global variables gui_fg_color, gui_mg_color, and gui_bg_color to sensible values (as returned by the makecol() function, typically black, grey, and white respectively).
I hope somebody will find this document useful (because otherwise I just wasted my entire morning writing it :-) Send any comments, questions, problems, etc, to email@example.com.