Hey Everyone, I have created my own window style where I'm responsible for painting the window's background with WM_ERASEBKGND. The problem is that my background is 280KB at the smallest because its a linear gradient bitmap. To say the least this is slowing the window's painting and causing filckers, etc. Is there a way to "chop up" the bitmap so I can paint only the necessary areas like in WM_PAINT? I'd use the MSIMG32 gradient functions but I still want the app to work on Windows 95.
Posted on 2001-02-21 14:05:00 by Xtreme
Xtreme, Device Contexts are pretty cheap these days, under Windows 3.1 there used to be 5 of them, and once you has used them up then you were stuck, Windows NT has no limit on them and they are only 800 bytes each, so you can get quite a few into your 2Gb address space. Windows 95 has a limit of 64K for Device contexts, at 800 bytes each you can still get a hell of alot of them. I don't know about win98 but I assume the worst case of 64K. Anyway on to the problem in hand. As device contexts cheap these days, use them! On startup you app should create a background DC, and a gradient DC, put you gradient bitmap into it, when you need to draw the screen do the following:- BitBlt your GradientDC into your MemoryDC Draw whatever you app needs onto the Memory DC BitBlt the MemoryDC onto the ScreenDC. Now, for WM_ERASEBKGND return non-zero, to say you have already wiped the screen, a WM_PAINT always comes in next, so you'll wipe it anyway. Note, you can keep the MemoryDC and GradientDC all the time, but then you get a WM_SIZE message, you should re-calculate you bitmap and put it back into the GradientDC. If you want to make it more efficient, then when you get WM_PAINT call BeginPaint, passing it a PAINTSTRUCT, withing PAINTSTRUCT there is a rectangle (rcPaint) which is the area you need to re-draw. With a little calulation you can BitBlt less of the DC's across. However with modern GFX cards, I don't think you will really notice the difference... umbongo
Posted on 2001-02-22 05:15:00 by umbongo
Thanks. I was already doing what you suggested with BitBlt but I hate the fact that every time notepad or calc moves in front of my window a big, clunky 280KB bitmap gets painted across my entire client area. But if I figure out my linear gradient proc maybe things will get better! Xtreme
Posted on 2001-02-22 16:00:00 by Xtreme
BeginPaint returns a PAINTSTRUCT with the invalid region inside... this is the only part you need paint, actually, all you can paint, the rest is clipped. Things might speed up a tad if you use param this to limit your blit or re-rainbow-paint.
Posted on 2001-02-23 01:44:00 by Ernie
Hey Ernie, Yeah, I use the Ps struct to limit painting in the WM_PAINT handler but WM_ERASEBKGND passes no such information. Thats unfortunate because "rcPaint" would really help... Xtreme
Posted on 2001-02-24 15:24:00 by Xtreme
Nothing says you HAVE to re-draw the background during WM_ERASEBK. Just eat that message and do it all in WM_PAINT (where you have an invalid rect) How to eat a massage:

        ; so don't do anything (how ZEN!)
    .ELSEIF uMsg == {Something else}
Posted on 2001-02-24 18:51:00 by Ernie