The reason that I want to have a bigger back surface is this: I will use a tile based system. This way I have to place all 32x32 (or 40x40) pixel sprites each frame (one layer). But why the bigger surface you may ask.
Well I want to do some scrolling. When you place the sprites on the screen there are 'half' sprites shown on the edges of the screen (when the screen scrolls). I don't want to put pixels out of the screen boundaries. Then I want to flip the backbuffer to the primary surface (/w hardware acceleration).

As for your problem with using a bigger back buffer, I do not believe a back buffer can have different dimensions than the front. However, let me say that for smooth scrolling (which is what you seem to want to do), you should have a larger offscreen surface where you blit your tiles and stuff, then from that you should blit to the back buffer, then flip. Either that or blit partial tiles to the borders of the back buffer, then normal blit the rest. Either way should get you the effect you want.

- Splat

Niels: Keeping that pointer to video memory after Unlock()ing it is a "Bad Thing." Just think, the video card could decide to optimize its memory and rearrange the blocks of memory. Or you could be accessing memory that was being Blit()ed to and get unexpected results. Etc. I realize that dealing with debugging with DirectDraw is difficult when using your own blitting routines, so maybe you should only use Unlock()ed pointers in debug mode and turn them off when you want to compile a release version of your game.

- Splat

Regards

Starwynd

However, you're still missing out on a much bigger speed increase. If you threw away your big surface approach, and DID clip the edges, then you'd be able to keep all the surfaces in vid mem, use hardware blitting to copy tiles to the back page (which will be much faster than drawing to your system surface), and then just flip video pages instead of copying your sys surface to the video card, which is generally VERY costly.

Rock

Now the next question:
I have a 640x480 screen (let's keep it simple: 256 colors). Can I make the backbuffer bigger, let's see 680,520? Then it would look like this:
_____________
| _______ |
| | | |
| |_____| |
|___________|

The big rectangle is the backbuffer and the little one is the screen. Can you use blit to copy only the part that's in the screen to the primary surface?