Quenching a Thirst for Sprites

With Windows 3.1, the Graphical Device Interface (GDI) could not produce smooth, interactive sprites. You built images in the GDI and blted ("blitted") them from conventional memory to display memory, but this process could not support complex sprites. Enter DirectDraw, the graphical architecture of Windows 95 that lets developers bypass the GDI and access video hardware through a hardware abstraction layer (HAL). You first create a DirectDraw object for your application (using the DirectDrawCreate API) and then configure a primary and secondary surface of video memory with the CreateSurfa ce method. You also need to build your palette with the CreatePalette call, but since DirectDraw can, in effect, take over the Windows environment, you don't have to share the 256 palette colors with other applications. You apply the full palette to the primary video surface and forget about it.

Developers must create a series of bit-mapped sprites to match the different poses of a character. To create a sprite, you reserve the video surface for it, specify the palette entry that represents your transparent color, then fill in the bits to draw your character.

DirectDraw performs warp-speed blts by working entirely in video memory. While your program is displaying one frame of graphics, the next frame is being rendered on the secondary surface. The secondary surface is then flipped onto the primary surface. The blts are occurring from video RAM to video RAM, bypassing the CPU and conventional memory. For even faster performance you can use the BltFast method, but you give up some features such as automatic clipping. Your sprites will now run and jump and mutilate legions of m utant space demons. Ah, sweet justice.

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How Direct Draw Draws

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DirectDraw speeds blts by working in video memory. After an image is rendered in the secondary DirectDraw surface, it is "flipped" onto the primary surface and onto the screen.