How can I clear TCanvas? Or any other solution

Question

I'm using this TCanvas to draw cursors for my mice

Canv := TCanvas.Create;
Canv.Handle := GetWindowDC(0);

.... For every mice event do the following

Bitmap:=TBitmap.Create;
CursorInfo.cbSize := sizeof(CursorInfo);
GetCursorInfo(CursorInfo);

Bitmap.Width := 32;
Bitmap.Height := 32;
Bitmap.Transparent:=true;

DrawIconEx(Bitmap.Canvas.Handle, 0,0, CursorInfo.hCursor, 32,32, 0,0, DI_NORMAL) ;

Bitmap.Canvas.Brush.Color := RGB(250,250,250);
Bitmap.Canvas.FloodFill(31,0, clWhite, fsSurface);
Bitmap.Canvas.FloodFill(0,0, clWhite, fsSurface);

currentX:=getcurrentxpos;
currentY:=getcurrentypos;

Canv.Draw(currentX,currentY,Bitmap);
Bitmap.Free;

The problem is instead of just showing the individual cursors, it makes mouse trails. Can I clear the whole Canvas evertime a mouse moves? (doesn't sound like a good idea though). Maybe I could clear my previous Canv.Draw by doing the reverse of that code (if it is possible)? Any suggestions as to how I can show the cursors?

EDIT: tried inserting another Canv.Draw(currentX,currentY,Bitmap); just after setting the bitmap width and height...and now the problem is I have a white trail (rather than a mouse trail), much cleaner but still no good.

Answer 1

You're drawing on the DESKTOP, and that's something you should never do, because it's unreliable. As I understand it, you're hoping to find a way to paint your mouse cursor on the desktop, and when the mice moves again, "undo" your last paint and repaint the mice at the new coordinates. Imagine this: You move the mice somewhere over a Memo box, move your hands to the keyboard, type something and then move the mouse again. The image under the mice changed!

What you can do: Create a mouse-cursor-shaped form, there are known techniques to do that. Make your pseudo-cursor stay on top (you'll get into a bit of problem with that too, because Windows no longer likes stay-on-top things). This is not going to be easy, but it's manageable and it's PLAYING BY THE RULES.

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A little code review on what you've got so far, because I spotted what I think it's a mistake, and you should know about. Fixing this is not enough to fix your problem, you need to stop drawing onto the desktop:

Don't free the Bitmap that holds the transparent cursor image, keep if for the life of the application: You'll save both RAM and CPU. This is critical in something that needs to react to the movement of a mice.

Answer 2

I will show you one way, which is different from what I previously suggest you.
You store your canvas using Canvas.CopyRect in some temporary canvas. when your mouse is first hover on your canvas.
Then, when your mouse is moved, first copy the temporary canvas to your destination canvas then draw your cursor.

Answer 3

Have a reference image (which is what your canvas should looks like without the mouse cursor painted on it), copy it to a temporary bitmap and then draw your cursor on that temp bitmap. Finally, draw your image on the canvas.

It also will allow you to have more complex temporary overlays very easily: you just have to draw them on top of the reference bitmap according to their Z-order. Since they aren't touching the background, you don't have to worry about clearing the ghosts.