Bitmaps in Display Drivers

Certain devices, such as the 16-color VGA display, can more rapidly perform bit-block transfers from nonstandard bitmaps. To support this, a driver can hook DrvCreateDeviceBitmap which allows the driver to create a bitmap that a driver manages completely. When a driver creates such a bitmap, the driver can store it in any format. The driver examines the passed parameters and provides a bitmap with at least as many bits-per-pixel as requested. The contents of the bitmap are undefined after creation. If the application requests a device-managed bitmap, GDI calls the driver for drawing functions after DrvCreateDeviceBitmap returns control. If the driver returns FALSE, the driver-managed bitmap is not created, so GDI can handle drawing operations on an engine-managed surface.

The DrvSaveScreenBits function is also related to bit-block transfers in display drivers. Some display drivers can move data to or from off-screen device memory more rapidly than an area can be redrawn or copied from a DIB. These drivers can hook DrvSaveScreenBits, which lets the driver be called to save or restore a specified rectangle of a displayed image more quickly when a menu or dialog box appears.

Note   For bit-block transfer calls, GDI (not the driver) handles pointer exclusion and clip region locking.

Drivers that implement device bitmaps in off-screen memory can significantly improve system performance. Off-screen device bitmaps improve system performance by:

• Using accelerator hardware in place of GDI to draw.

• Improving the speed of bitmap-to-screen bit-block transfers.

• Reducing demands on main memory (a bitmap stored in off-screen memory isn't taking up space in main memory).

• Leveraging hardware to perform operations that support OpenGL, such as mask bit-block transfers and double-buffering.

Drivers should implement device bitmaps in off-screen memory through DrvCreateDeviceBitmap.