Bitmap Devices and Contexts

Bitmap graphics context

CBitmapContext specializes a GDI context (CGraphicsContext) for bitmap graphics. It provides extra functionality for:

• clearing and copying rectangular areas

• bitmap block transfer

• setting pen color to a grey level

• setting the shadow mode

• resetting the GC settings to their default values

The Window Server client-side API provides one implementation, CWindowGc, used for screen drawing. Another implementation, CFbsBitGc, is used for drawing to in-memory bitmaps.

Bitmap graphics device

CBitmapDevice specializes a graphics device (CGraphicsDevice) for bitmap graphics.

The Window Server client-side API provides one implementation, CWsScreenDevice, for screen drawing. Another implementation, CFbsBitmapDevice, is used for drawing to in-memory bitmaps. A third, CFbsScreenDevice, is used (rarely) to access the screen directly, without the mediation of the window server.

Screen driver information

The screen width and height must be a multiple of four. In certain cases where the screen mode would result in a scanline (the scanline runs in the current horizontal direction of the screen) not being divisible by a whole word (32 bits) the screen width must also be a multiple of 16. This restriction will also apply to the height if there is a screen-size mode with a rotation of 90 or 270 (or both)

For example, the screen mode EGray4 is 2 bits per pixel, therefore one word would be 16 pixels, so in order for the scanline to be a whole number of words the width must be a multiple of 16.

The vertical height is unrestricted unless there is at least one screen-size mode with a rotation of 90 or 270 (or both).