Color Keys and Bitmap Animation

The sample in previous example (DDEX3) shows a primitive method of placing bitmaps into an off-screen buffer before they are blitted to the back buffer. The sample in this example (DDEX4) uses the techniques described in the previous tutorials to load a background and a series of sprites into an off-screen surface. Then it uses the IDirectDrawSurface::BltFast method to copy portions of the off-screen surface to the back buffer, thereby generating a simple bitmap animation.

The bitmap file that DDEX4 uses, All.bmp, contains the background and 60 iterations of a rotating red donut with a black background. The DDEX4 sample contains new functions that set the color key for the rotating donut sprites. Then, the sample copies the appropriate sprite to the back buffer from the off-screen surface. The new functionality demonstrated in DDEX4 is shown in the following steps:

• Step 1: Setting the Color Key

• Step 2: Creating a Simple Animation

Step 1: Setting the Color Key. In addition to the other functions found in the doInit function of some of the other DirectDraw samples, the DDEX4 sample contains the code to set the color key for the sprites. Color keys are used for setting a color value that will be used for transparency. When the system contains a hardware blitter, all the pixels of a rectangle are blitted except the value that was set as the color key, thereby creating nonrectangular sprites on a surface. The code for setting the color key in DDEX4 is shown below:

// Set the color key for this bitmap (black).

DDSetColorKey(lpDDSOne, RGB(0,0,0));

return TRUE;

You can select the color key by setting the RGB values for the color you want in the call to the DDSetColorKey function. The RGB value for black is (0, 0, 0). The DDSetColorKey function calls the DDColorMatch function. (Both functions are in Ddutil.cpp.) The DDColorMatch function stores the current color value of the pixel at location (0, 0) on the bitmap located in the lpDDSOne surface. Then it takes the RGB values you supplied and sets the pixel at location (0, 0) to that color. Finally, it masks the value of the color with the number of bits per pixel that are available. After that is done, the original color is put back in location (0, 0), and the call returns to DDSetColorKey with the actual color key value. After it is returned, the color key value is placed in the dwColorSpaceLowValue member of the DDCOLORKEY structure. It is also copied to the dwColorSpaceHighValue member. The call to IDirectDrawSurface::SetColorKey then sets the color key.

You may have noticed the reference to CLR_INVALID in DDSetColorKey and DDColorMatch. If you pass CLR_INVALID as the color key in the DDSetColorKey call in DDEX4, the pixel in the upper-left corner (0, 0) of the bitmap will be used as the color key. As the DDEX4 bitmap is delivered, that does not mean much because the color of the pixel at (0, 0) is a shade of gray. If, however, you would like to see how to use the pixel at (0, 0) as the color key for the DDEX4 sample, open the All.bmp bitmap file in a drawing application and then change the single pixel at (0, 0) to black. Be sure to save the change (it's hard to see). Then change the DDEX4 line that calls DDSetColorKey to the following:

DSetColorKey(lpDDSOne, CLR_INVALID);

Recompile the DDEX4 sample, and ensure that the resource definition file is also recompiled so that the new bitmap is included. (To do this, you can simply add and then delete a space in the Ddex4.rc file.) The DDEX4 sample will then use the pixel at (0, 0), which is now set to black, as the color key.

Step 2: Creating a Simple Animation. The DDEX4 sample uses the updateFrame function to create a simple animation using the red donuts included in the All.bmp file. The animation consists of three red donuts positioned in a triangle and rotating at various speeds. This sample compares the Win32 GetTickCount function with the number of milliseconds since the last call to GetTickCount to determine whether to redraw any of the sprites. It subsequently uses the IDirectDrawSurface::BltFast method first to blit the background from the off-screen surface (lpDDSOne) to the back buffer, and then to blit the sprites to the back buffer using the color key that you set earlier to determine which pixels are transparent. After the sprites are blitted to the back buffer, DDEX4 calls the IDirectDrawSurface::Flip method to flip the back buffer and the primary surface.

Note that when you use IDirectDrawSurface::BltFast to blit the background from the off-screen surface, the dwTrans parameter that specifies the type of transfer is set to DDBLTFAST_NOCOLORKEY. This indicates that a normal blit will occur with no transparency bits. Later, when the red donuts are blitted to the back buffer, the dwTrans parameter is set to DDBLTFAST_SRCCOLORKEY. This indicates that a blit will occur with the color key for transparency as it is defined, in this case, in the lpDDSOne buffer.

In this sample, the entire background is redrawn each time through the updateFrame function. One way of optimizing this sample would be to redraw only that portion of the background that changes while rotating the red donuts. Because the location and size of the rectangles that make up the donut sprites never change, you should be able to easily modify the DDEX4 sample with this optimization.