TImage problems
lazarus / 39-Graphics

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Ross · 2011-05-18 14:52

I am saving the bitmap of a TImage to a MemoryStream as a JPegImage
before allowing the user to modify the image.  An UNDO option will
load the previous JPegImage in from the saved MemoryStream and assign
it to the TImage.Picture.Bitmap.  This seems to work except that after
the previous Image is loaded the Pen mode and color don't work as
expected.
I have attached a small test pas unit that shows this behavior.
What am I missing?  Any help is appreciated.

Ross

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circular · 2011-05-18 19:28

It doesn't work here either.

I've managed to make a real xor rectangle like this :
QuotePen.Mode := pmXor;
      Pen.Color := clWhite;  
But then drawing the final rectangle do not work as expected.

This way it should draw black rectangle filled with blue :
Quoteprocedure TForm1.Image1MouseDown(Sender: TObject; Button:
TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if Button = mbLeft then
  begin
    SaveBitmap(Image1.Picture.Bitmap);
    DLeft := X;
    DTop := Y;
    DRight := X;
    DBottom := Y;

    with Image1.Picture.Bitmap.Canvas do
    begin
      Pen.Style := psSolid;
      Pen.Mode := pmXor;
      Pen.Color := clWhite;
      Brush.Style := bsClear;
    end;
  end;
end;


procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState;
X,
  Y: Integer);
begin
  if ssLeft in Shift then
    with Image1.Picture.Bitmap.Canvas do
    begin
      Rectangle(DLeft, DTop, DRight, DBottom);
      DRight := X;
      DBottom := Y;
      Rectangle(DLeft, DTop, DRight, DBottom);
    end;
end;


procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if Button = mbLeft then
    with Image1.Picture.Bitmap.Canvas do
    begin
      Rectangle(DLeft, DTop, DRight, DBottom);
      Pen.Mode := pmCopy;
      Pen.Color := clBlack;
      Brush.Style := bsSolid;
      brush.Color := clBlue;
      Rectangle(DLeft, DTop, X, Y);
    end;
end;
But I get transparent rectangles instead (the background behind the
image become visible were the rectangle should be drawn).

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Ross · 2011-05-18 19:52

Hi Circular,
Thanks for looking at this. 
I can't tell if its something I'm doing wrong or if it is a
TImage.Picture.Bitmap.Canvas bug or a bug in the TPen.
I had a simular problem with the Canvas.FloodFill method and had to
use the WinApi method ExtFloodFill instead to get it to work (pmCopy).

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circular · 2011-05-19 08:31

About the difference when the picture is reloaded, it is because Jpeg
doesn't handle transparency, so it becomes black.

So I think it is a bug of TImage.Picture.Bitmap.Canvas. I suppose the
same program would work well when drawing directly on the form canvas
or on a panel (using OnPaint events for mac compatibility).