Doubts about copying an image onto a canvas

[simsee]

I want to copy the contents of a TImage onto a scrollbox canvas. I tried with Canvas.Draw and with Canvas.CopyRect, but I don't see anything in both cases. Here is the code:

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, ExtCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     ScrollBox1: TScrollBox;
16.     procedure FormShow(Sender: TObject);
17.   private
18.     Image : TImage;
19.     DestRect, SourceRect : TRect;
20.   public
21.  
22.   end;
23.  
24. var
25.   Form1: TForm1;
26.  
27. implementation
28.  
29. {$R *.lfm}
30.  
31. { TForm1 }
32.  
33. procedure TForm1.FormShow(Sender: TObject);
34. begin
35.   Image:=TImage.Create(Self);
36.   Image.Picture.LoadFromFile('project1.ico');
37.   with SourceRect do
38.     begin
39.       Left:=0;
40.       Top:=0;
41.       Width:=Image.Picture.Width;
42.       Height:=Image.Picture.Height;
43.     end;
44.   with DestRect do
45.     begin
46.       Left:=100;
47.       Top:=100;
48.       Width:=Image.Picture.Width;
49.       Height:=Image.Picture.Height;
50.     end;
51.   ScrollBox1.Canvas.Draw(0,0,Image.Picture.Bitmap);             //It does not work
52.   ScrollBox1.Canvas.CopyRect(DestRect,Image.Canvas,SourceRect); //It does not work
53. end;
54.  
55. end.

Where did I go wrong? Thank you.

[wp]

Code: Pascal  [Select][+][-]

1. procedure TForm1.FormShow(Sender: TObject);
2. begin
3.   ...
4.   ScrollBox1.Canvas.Draw(0,0,Image.Picture.Bitmap);             //It does not work
5.   ScrollBox1.Canvas.CopyRect(DestRect,Image.Canvas,SourceRect); //It does not work
6. end;

Accessing a canvas method in the OnShow event handler is not a good idea. In some operating systems this is strictly forbidden and will crash the application, and with even more tolerant ones the drawing will not be persistent, i.e. when after this event handler the OS requests the Scrollbox to repaint itself again it will not know that it should draw the image...

Golden rule for user graphics: Do drawing operations only in the OnPaint handlers of controls because this is exactly what the control must execute when it should redraw itself:

Code: Pascal  [Select][+][-]

1. procedure TForm1.FormShow(Sender: TObject);
2. begin
3.   Image:=TImage.Create(Self);
4.   Image.Picture.LoadFromFile('project1.ico');
5. end;
6.  
7. procedure TForm1.Scrollbox1Paint(Sender: TObject);
8. var
9.   SourceRect, DestRect: TRect;     // No need to make them global variables!
10. begin
11.   with SourceRect do
12.     begin
13.       Left:=0;
14.       Top:=0;
15.       Width:=Image.Picture.Width;
16.       Height:=Image.Picture.Height;
17.     end;
18.   with DestRect do
19.     begin
20.       Left:=100;
21.       Top:=100;
22.       Width:=Image.Picture.Width;
23.       Height:=Image.Picture.Height;
24.     end;
25.   ScrollBox1.Canvas.Draw(0,0,Image.Picture.Bitmap);            // only one of them needed.
26. //  ScrollBox1.Canvas.CopyRect(DestRect,Image.Canvas,SourceRect);
27. end;

[TRon]

Where did I go wrong? Thank you.

you forgot to mention FPC & Lazarus version and platform. Also take note of the remark at the documentation of the scrollbox OnPaint handler.

nvm as I crossed wp's post.

[simsee]

Thanks for the reply and sorry for the delay in mine.

Yes, you're right, I knew the rule that you have to draw inside the OnPaint event handler.

In reality my problem (I'm on Windows) was mainly with CopyRect method (I didn't realize, due to my mistake, that Draw method works within OnPaint).

In fact, if I use CopyRect inside OnPaint, as in your code, I get a run time error (see attached screenshot). What's the reason?