Bitmap objects



Once you have selected a graphics mode, you can draw things onto the display via the 'screen' bitmap. All the Allegro graphics routines draw onto BITMAP structures, which are areas of memory containing rectangular images, stored as packed byte arrays (eight bits per pixel). You can create and manipulate bitmaps in system RAM, or you can write to the special 'screen' bitmap which represents the video memory in your graphics card.

For example, to draw a pixel onto the screen you would write: 

   putpixel(screen, x, y, color);
Or to implement a double-buffered system: 

   BITMAP *bmp = create_bitmap(320, 200);    // make a bitmap in system RAM
   clear(bmp);                               // zero the memory bitmap
   putpixel(bmp, x, y, color);               // draw onto the memory bitmap
   blit(bmp, screen, 0, 0, 0, 0, 320, 200);  // copy it to the screen
See below for information on how to get direct access to the image memory in a bitmap.

Allegro supports several different types of bitmaps:

- The screen bitmap, which represents the hardware video memory. Ultimately you have to draw onto this in order for your image to be visible.

- Memory bitmaps, which are located in system RAM and can be used to store graphics or as temporary drawing spaces for double buffered systems. These can be obtained by calling create_bitmap(), load_pcx(), or by loading a grabber datafile.

- Sub-bitmaps. These share image memory with a parent bitmap (which can be the screen, a memory bitmap, or another sub-bitmap), so drawing onto them will also change their parent. They can be of any size and located anywhere within the parent bitmap, and can have their own clipping rectangles, so they are a useful way of dividing a bitmap into several smaller units, eg. splitting a large virtual screen into two sections for page flipping (see examples/ex9.c).

- Video memory bitmaps. These are created by the create_video_bitmap() function, and are usually implemented as sub-bitmaps of the screen object.

extern BITMAP *screen;
Global pointer to a bitmap, sized VIRTUAL_W x VIRTUAL_H. This is created by set_gfx_mode(), and represents the hardware video memory. Only a part of this bitmap will actually be visible, sized SCREEN_W x SCREEN_H. Normally this is the top left corner of the larger virtual screen, so you can ignore the extra invisible virtual size of the bitmap if you aren't interested in hardware scrolling or page flipping. To move the visible window to other parts of the screen bitmap, call scroll_screen(). Initially the clipping rectangle will be limited to the physical screen size, so if you want to draw onto a larger virtual screen space outside this rectangle, you will need to adjust the clipping.

BITMAP *create_bitmap(int width, int height);
Creates a memory bitmap sized width by height, and returns a pointer to it. The bitmap will have clipping turned on, and the clipping rectangle set to the full size of the bitmap. The image memory will not be cleared, so it will probably contain garbage: you should clear the bitmap before using it. This routine always uses the global pixel format, as specified by calling set_color_depth().

BITMAP *create_bitmap_ex(int color_depth, int width, int height);
Creates a bitmap in a specific color depth (8, 15, 16, 24 or 32 bits per pixel).

BITMAP *create_sub_bitmap(BITMAP *parent, int x, y, width, height);
Creates a sub-bitmap, ie. a bitmap sharing drawing memory with a pre-existing bitmap, but possibly with a different size and clipping settings. When creating a sub-bitmap of the mode-X screen, the x position must be a multiple of four. The sub-bitmap width and height can extend beyond the right and bottom edges of the parent (they will be clipped), but the origin point must lie within the parent region.

BITMAP *create_video_bitmap(int width, int height);
Allocates a video memory bitmap of the specified size, returning a pointer to it on success or NULL on failure (ie. if you have run out of vram). This can be used to allocate offscreen video memory for storing source graphics ready for a hardware accelerated blitting operation, or to create multiple video memory pages which can then be displayed by calling show_video_bitmap(). Video memory bitmaps are usually allocated from the same space as the screen bitmap, so they may overlap with it: it is not therefore a good idea to use the global screen at the same time as any surfaces returned by this function.

void destroy_bitmap(BITMAP *bitmap);
Destroys a memory bitmap, sub-bitmap, or video memory bitmap when you are finished with it.

int bitmap_color_depth(BITMAP *bmp);
Returns the color depth of the specified bitmap (8, 15, 16, 24, or 32).

int bitmap_mask_color(BITMAP *bmp);
Returns the mask color for the specified bitmap (the value which is skipped when drawing sprites). For 256 color bitmaps this is zero, and for truecolor bitmaps it is bright pink (maximum red and blue, zero green).

int is_same_bitmap(BITMAP *bmp1, BITMAP *bmp2);
Returns TRUE if the two bitmaps describe the same drawing surface, ie. the pointers are equal, one is a sub-bitmap of the other, or they are both sub-bitmaps of a common parent.

int is_linear_bitmap(BITMAP *bmp);
Returns TRUE if bmp is a linear bitmap, ie. a memory bitmap, mode 13h screen, or SVGA screen. Linear bitmaps can be used with the _putpixel(), _getpixel(), bmp_write_line(), and bmp_read_line() functions.

int is_planar_bitmap(BITMAP *bmp);
Returns TRUE if bmp is a planar (mode-X or Xtended mode) screen bitmap.

int is_memory_bitmap(BITMAP *bmp);
Returns TRUE if bmp is a memory bitmap, ie. it was created by calling create_bitmap() or loaded from a grabber datafile or image file. Memory bitmaps can be accessed directly via the line pointers in the bitmap structure, eg. bmp->line[y][x] = color.

int is_screen_bitmap(BITMAP *bmp);
Returns TRUE if bmp is the screen bitmap, a video memory bitmap, or a sub-bitmap of either.

int is_sub_bitmap(BITMAP *bmp);
Returns TRUE if bmp is a sub-bitmap.

void set_clip(BITMAP *bitmap, int x1, int y1, int x2, int y2);
Each bitmap has an associated clipping rectangle, which is the area of the image that it is ok to draw on. Nothing will be drawn to positions outside this space. Pass the two opposite corners of the clipping rectangle: these are inclusive, eg. set_clip(bitmap, 16, 16, 32, 32) will allow drawing to (16, 16) and (32, 32), but not to (15, 15) and (33, 33). If x1, y1, x2, and y2 are all zero, clipping will be turned off, which may slightly speed up some drawing operations (usually a negligible difference, although every little helps) but will result in your program dying a horrible death if you try to draw beyond the edges of the bitmap.



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