Understanding Device Contexts

Question

As a relative newcomer to MFC, I see Device Contexts (DCs) a lot. I vaguely understand that it's something to do with drawing, but the specifics are not very well explained anywhere that I can find. What does creating a "compatible Device Context" mean, and why is it important? What does SelectObject do, and how must I make a DC compatible first?

Answer 1

A Device Context is just a place that drawing occurs, so if you have two different DC's, you're drawing in two different places. Kind of like a file handle.

Device Contexts can refer to real-estate on screen, or to bitmaps that just reside in memory, and probably other places, too, those are just the two I can think of at the moment.

Compatible contexts are ones that have the same underlying pixel organization, by which is meant number of bits per pixel, bytes per pixel, color organization and so forth. Memory bitmap device contexts can have any organization you want, but your screen contexts are going to be related (eventually) to buffers on your graphics card, which will (depending on mode, etc) have a very specific pixel organization.

Having compatible contexts means its efficient to transfer image data between them, because little or no translation of the data is required. At the other extreme, if you have a 256 color palette, 8 bit map and you try to blit it to a screen that has 8 bits each of RGBA per pixel, every last pixel will require significant massaging as it is copied and so copying incompatible bitmaps is very much slower. According to the Win32 SDK documentation, at least BitBlt() and StretchBlt() "convert the source color format to match the destination format", so it can be done.

Investigate CreateCompatibleDC() and CreateCompatibleBitmap() as starting points for how to create drawing contexts that are compatible with already existing ones.

SelectObject() controls which resources are currently active within the device context. A context has a current pen, brush, font, and bitmap. These make a lot of the other GDI calls simpler by allowing you to specifiy fewer parameters. For instance, you don't have to specify the font when you use TextOut(), but if you want to change the font, that's where SelectObject() comes in. If you feed SelectObject() a handle to a font, the return value is a handle to the font that was in effect, and subsequent operations use the new font. Behavior is the same for the other kinds of resources, pens, brushes, etc.