Another project with problems.
I'm trying to learn how to move things around the screen with the mouse, using GDI.
Previously I was moving child controls around, for my DialogGen on-screen dialog editing program, so I pretty much know how to do that (although there are still problems with that scheme).
This time I'm moving other things around. Mainly bitmaps at this point.
I'm able to move them pretty well, but there are problems. Mainly flicker. Actually pretty horrible flicker.
(Use the attached program and move the NPN transistor around and see what happens.)
For the time being I'd like to keep this on a conceptual level and proceed without posting any code. (I'll be happy to do that later.) This isn't so much a problem with coding as it is with design, and with understanding how the OS works with graphics and what its limitations are.
What I'm doing is this:
- Creating a client window (using CreateWindowEx())
- Creating a memory DC from that window to use for drawing into which I'm calling AltDC:
- Using CreateCompatibleDC() to get a memory DC
- Using CreateCompatibleBitmap() to get a bitmap the size of the client window's display DC
- Selecting that bitmap into AltDC so the memory DC is the same size as the display DC
OK, now that I have a memory DC, I'm doing this:
- When the program starts, I draw all my "objects" on the display DC, through WM_PAINT. This includes the blue rectangle, the big text and the bitmap. (So far AltDC is just blank, set to the background color.)
- When the user clicks on the bitmap (determined through hit-testing), I draw the selection box (dotted line) around the bitmap; the bitmap is now considered "selected".
- If the user holds down the left button and moves the mouse, here's where things get interesting:
- I go through the list of "objects" and I draw all of them except the bitmap to AltDC; this is my "clean slate" to use later
- I draw the selection box around the bitmap to the display DC
- I draw the bitmap to the display DC, at the current position
- As the user continues moving the mouse, things get really interesting:
- For each move, I first restore the image underneath the bitmap, copying just the bitmap's rectangle from AltDC (which has the "clean slate" before I drew the bitmap)
- I then draw the selection box and the bitmap at their new positions on the display DC
As you can see this works, but at a very high price. If you move the mouse at any speed above s-l-o-w, it flickers badly.
Any ideas here? I'm pretty sure I'm doing what I'm doing correctly, so far as coding goes. In other words, I don't think this is a matter of bugs. More a case of poor use of resources.
Am I doing too much moving of stuff between DCs? is that what's causing the flicker?
I was hoping that by only copying a small part of
AltDC to the display for each mouse move that I'd be avoiding too much processing.
Should I be doing some kind of double buffering? (Not even sure how that works, to be honest.)
Any better way to do what I'm trying to do here?
For starters, you have a gdi leak...
Also, double buffering would indeed help.
untitled.PNG
ALso that white rectangle with border appears sometimes when moving the transistor around.
Quote from: zedd on April 25, 2025, 12:04:17 PMFor starters, you have a gdi leak...
No doubt. Let's not worry about that for the moment. That's not the problem.
QuoteAlso, double buffering would indeed help.
I'm going to read up on it next thing.
Well, a quick read showed ... basically the code I have now.
Question: Does using double buffering mean that I draw
everything to a separate buffer (my
AltDC) and then copy that entire thing over to the display DC? Because I'm not doing that exactly.
This is how I would do it... and so far works for me.
If you have the client area already from GetClientRect, you can use that here instead of ps.rcPaint...
.elseif uMsg == WM_ERASEBKGND ; adding this helps greatly with flicker issues
mov eax, 1
ret
.elseif uMsg == WM_PAINT
invoke BeginPaint, hWin, addr ps
mov hDC, eax ; window client area DC
invoke CreateCompatibleDC, hDC
mov memDC, eax ; memory DC
invoke CreateCompatibleBitmap, hDC, ps.rcPaint.right, ps.rcPaint.bottom
mov hBmp, eax ; compatible bitmap handle
invoke SelectObject, memDC, hBmp
mov hBmp_old, eax
;; ###########################################################
;; in this example do all of your drawing here to memory DC (memDC) not to window DC (hDC)
;; ###########################################################
;; bitblt memory Dc to window DC
invoke BitBlt, hDC, 0, 0, ps.rcPaint.right, ps.rcPaint.bottom, memDC, 0, 0, SRCCOPY
invoke SelectObject, memDC, hBmp_old
invoke DeleteObject, hBmp
invoke DeleteDC, memDC
invoke EndPaint, hWin, addr ps
Quote from: NoCforMe on April 25, 2025, 12:08:50 PMQuestion: Does using double buffering mean that I draw everything to a separate buffer (my AltDC) and then copy that entire thing over to the display DC?
Yes, everything.
I get artifacts when the bitmap is dragged past the top or left
Untitled.png
Quote from: sinsi on April 25, 2025, 12:27:49 PMQuote from: NoCforMe on April 25, 2025, 12:08:50 PMQuestion: Does using double buffering mean that I draw everything to a separate buffer (my AltDC) and then copy that entire thing over to the display DC?
Yes, everything.
I get artifacts when the bitmap is dragged past the top or left
Yes, shoulda warned you 'bout that. There's no code to check for boundary conditions (yet). Makes a mess.
So maybe I should concentrate on "everything"?
I will also add that however you are selecting the transistor and moving it around, that part indeed is working nicely. Congrats. :thumbsup:
I had tried something similar years ago, but with piss poor results. :tongue:
I think I could do something similar now, if needed, though -with better results. :smiley:
Quote from: NoCforMe on April 25, 2025, 12:46:40 PMSo maybe I should concentrate on "everything"?
Work on getting the double buffering working first. Once done, then you can do the boundary checks, and handle the resource issues (source of gdi leaks), imo.