TScrollBox and TImage , Pan algorithm

[Tommi]

I have a TImage inserted in  a TScrollBox and I would like to implement a PAN function when I move the mouse on the image.

Here my actual code:

Code: [Select]

procedure TForm6.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
 // form6.Caption:=inttostr(X)+'   '+inttostr(Y);
  if ssLeft in Shift then
  begin
    ScrollBox1.VertScrollBar.Position:=Y-trunc(scrollBox1.ClientHeight/2);
    scrollBox1.HorzScrollBar.Position:=X-trunc(scrollBox1.ClientWidth/2);
  end;
end;

It works but it isn't a real pan function. My intent is to keep the image attached to the mouse while it is moving.

Could someone help me ?

Thank you

[Mr.Madguy]

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, ExtCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Image1: TImage;
16.     ScrollBox1: TScrollBox;
17.     procedure Image1MouseDown(Sender: TObject; Button: TMouseButton;
18.       Shift: TShiftState; X, Y: Integer);
19.     procedure Image1MouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer
20.       );
21.     procedure Image1MouseUp(Sender: TObject; Button: TMouseButton;
22.       Shift: TShiftState; X, Y: Integer);
23.   private
24.     { private declarations }
25.   public
26.     { public declarations }
27.     Panning:Boolean;
28.     PanX, PanY:Integer;
29.   end;
30.  
31. var
32.   Form1: TForm1;
33.  
34. implementation
35.  
36. {$R *.lfm}
37.  
38. { TForm1 }
39.  
40. procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
41.   Shift: TShiftState; X, Y: Integer);
42. begin
43.   Panning := True;
44.   PanX := X;
45.   PanY := Y;
46. end;
47.  
48. procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
49.   Y: Integer);
50. begin
51.   if Panning then begin
52.     ScrollBox1.HorzScrollBar.Position := ScrollBox1.HorzScrollBar.Position + PanX - X;
53.     ScrollBox1.VertScrollBar.Position := ScrollBox1.VertScrollBar.Position + PanY - Y;
54.   end;
55. end;
56.  
57. procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
58.   Shift: TShiftState; X, Y: Integer);
59. begin
60.   Panning := False;
61. end;
62.  

It's assumed, that TScrollBox has some internal Position value range handling. It works, but only bad thing - it looks like, that if one of scroll bars isn't visible, it's width is still being added to virtual with or height of TScrollBox client area.

[jamie]

Lets see..

 Lets assume for the moment you use a mouse click on the image, every time you do this you can record the
Point using a Tpoint. This gives you the starting point and you should record to what the scrollbox.client locations are
not the timage using the MOUSE record, this gives you screen coordinates.

 As you process the mouse moves while the mouse is being held down, you then take another reading from the
MOUSE record, calculate the offset difference in movement from the starting point when you initiated the click.
 
 Reposition the Timage Left,Top using the these offsets + or - and you'll get your panning effects..

 Just remember that when you release the mouse, you need to restore the LEFT,TOP of the timage back to 0,0
or where ever you want it.

[Tommi]

Ok, thank you. It works.

I just changed:

Code: [Select]

  if Panning then begin
    ScrollBox1.HorzScrollBar.Position := ScrollBox1.HorzScrollBar.Position + PanX - X;
    ScrollBox1.VertScrollBar.Position := ScrollBox1.VertScrollBar.Position + PanY - Y;
  end;

with:

Code: [Select]

  if ssLeft in Shift then
  begin
    ScrollBox1.HorzScrollBar.Position := ScrollBox1.HorzScrollBar.Position + PanX - X;
    ScrollBox1.VertScrollBar.Position := ScrollBox1.VertScrollBar.Position + PanY - Y;
  end;


[Mr.Madguy]

Ok, thank you. It works.

I just changed:

Code: [Select]

  if Panning then begin
    ScrollBox1.HorzScrollBar.Position := ScrollBox1.HorzScrollBar.Position + PanX - X;
    ScrollBox1.VertScrollBar.Position := ScrollBox1.VertScrollBar.Position + PanY - Y;
  end;

with:

Code: [Select]

  if ssLeft in Shift then
  begin
    ScrollBox1.HorzScrollBar.Position := ScrollBox1.HorzScrollBar.Position + PanX - X;
    ScrollBox1.VertScrollBar.Position := ScrollBox1.VertScrollBar.Position + PanY - Y;
  end;


Bad idea, as X and Y are set inside OnMouseDown and your code will pan image even without processing this event.