How To InvalidateRect With GDIPlus - delphi

Question

All of the GDIPlus demo code I can find draws without invalidation. So how do you invalidate a rectangle in GDIPlus API when drawing with MouseMove with TImage on a TScrollbox?
function NormalizeRect ( R: TRect ): TRect;
begin
// This routine normalizes a rectangle. It makes sure that the Left,Top
// coords are always above and to the left of the Bottom,Right coords.
with R do
begin
if Left > Right then
if Top > Bottom then
Result := Rect ( Right, Bottom, Left, Top )
else
Result := Rect ( Right, Top, Left, Bottom )
else if Top > Bottom then
Result := Rect ( Left, Bottom, Right, Top )
else
Result := Rect ( Left, Top, Right, Bottom );
end;
end;
procedure TFormMain.Image1MouseDown ( Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer );
begin
if Line1.Down then
begin
GPPointStart := MakePoint ( X, Y );
end;
end;
procedure TFormMain.Image1MouseMove ( Sender: TObject; Shift: TShiftState; X, Y: Integer );
var
graphics: TGPGraphics;
pen: TGPPen;
SolidBrush: TGPSolidBrush;
rgbTriple: windows.RGBTRIPLE;
iRect: TRect;
begin
if Line1.Down then
begin
if ssLeft in Shift then
begin
iRect := NormalizeRect ( Rect ( X, Y, Image1.Picture.Bitmap.Width, Image1.Picture.Bitmap.Height ) );
InvalidateRect ( ScrollBox1.Handle, #iRect, TRUE );
graphics := TGPGraphics.Create ( Image1.Picture.Bitmap.Canvas.Handle );
graphics.Flush ( FlushIntentionFlush );
GPPointEnd := MakePoint ( X, Y );
rgbTriple := ColorToRGBTriple ( ColorBox1.Selected );
pen := TGPPen.Create ( MakeColor ( StrToInt ( Alpha1.Text ), rgbTriple.rgbtRed, rgbTriple.rgbtGreen, rgbTriple.rgbtBlue )
);
pen.SetWidth ( StrToInt ( Size1.Text ) );
graphics.DrawLine ( pen, GPPointStart.X, GPPointStart.Y, GPPointEnd.X, GPPointEnd.Y );
graphics.Free;
Image1.Refresh;
end;
end;
end;
This is what it looks like:
Using GDIPlus Library from http://www.progdigy.com with Delphi 2010.

Answer 1

The InvalidateRect command has nothing to do with GDI+. It's a command that tells the OS that a certain portion of a window is invalid and should be repainted. When the OS next decides to repaint that window, the program can ask the OS how much of the window needs painting.
Your code is calling InvalidateRect, and then it's painting over that same portion of the window. The window is still invalidated, though, so the OS will ask your program to repaint that area later, when you next process a wm_Paint message.
I don't know why you would expect your image to look any different, and it has nothing to do with invalidating the scroll box. It looks like you clicked on the character's eye, and the dragged the mouse down and the the right, clockwise.
At each mouse movement, you draw a new line from the original pint to the current mouse position. You draw the line directly on the currently displayed bitmap, and then you ask the image control to redraw itself. It obeys and draws the bitmap — that bitmap that you just added another line to.
I suspect what you intended to happen was for each mouse movement to result in one black line to appear over an otherwise unsullied image. InvalidateRect won't help with that. You need to redraw the original image over the previous line position, and the draw the new line. InvalidateRect does not help you "undo" a previous graphic operation. It just tells the OS that a certain portion of a window should be repainted sometime. It doesn't say what colors those invalidated pixels should be repainted with. That's what wm_Paint is for.