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Bitmap navigating - preview window for large images

I have a large bitmap which is placed on image in a scrroll box.

I want to have another, small image with scaled down version of the same bitmap, which will allow me to select area to be displayed in the large bitmap.

The small bitmap should have a rectangle which will represent the area displayed in the large bitmap.

Draging this rectangle within the small bitmap will result in scrolling into view corresponding area of the large bitmap.

I'm sure someone has done it already but I can't find it anywhere.

I need working example or a link to working example.
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graga
Asked:
graga
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1 Solution
 
gmayoCommented:
One form, one TPaintBox (preview window), one scrollbox, and one TImage (inside the scrollbox).

procedure TForm1.PaintBox1Paint(Sender: TObject);
var
      scalex, scaley : single;
begin
      with PaintBox1.Canvas do begin
            StretchDraw(ClipRect, Image1.Picture.Graphic);
            Brush.Style := bsClear;
            Pen.Mode := pmNOT;
            scalex := PaintBox1.Width / Image1.Width;
            scaley := PaintBox1.Height / Image1.Height;
            Rectangle(Round(ScrollBox1.HorzScrollBar.Position * scalex),
                  Round(ScrollBox1.VertScrollBar.Position * scaley),
                  Round(ScrollBox1.HorzScrollBar.Position * scalex + ScrollBox1.Width * scalex),
                  Round(ScrollBox1.VertScrollBar.Position * scaley + ScrollBox1.Height * scaley));
      end;
end;

procedure TForm1.PaintBox1MouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
      ScrollBox1.HorzScrollBar.Position := Round((X / PaintBox1.Width) * Image1.Width);
      ScrollBox1.VertScrollBar.Position := Round((Y / PaintBox1.Height) * Image1.Height);
      PaintBox1.Invalidate;
end;

procedure TForm1.PaintBox1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
      if (ssLeft in Shift) then PaintBox1MouseDown(Sender, mbLeft, Shift, X, Y);
end;

There's no error or sanity checking but it scrolls perfectly on mine! An improvement you may wish to make is to centre the grab box on the mouse, rather than top-left as I have done.

Geoff M.
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classmateCommented:
You can use the StretchBlt windows api function. I'll try to give a example...

classmate.
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classmateCommented:
Ok, geoff seems to have a solution, so i won't spend time on it unless there would further problems ;-)

classmate
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classmateCommented:
would be, that is :-)
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gragaAuthor Commented:
Geoff,

Everything works nicely except the paintbox flickers quite badly. If you can tell me how to fix it, you will get the points, if you don't, you will get the points anyway :)

Graga
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gragaAuthor Commented:
Also, any quick suggestion on centering the grab box on the mouse?
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gmayoCommented:
Not sure on the first one, except maybe drawing it less frequently.

The second one, well I'm at home so I can't test it. You need to subtract half the width of the rectangle off the X, equivalent for Y too. Make sure they are not negative. Do this in the PaintBoxMouseDown event.

Geoff M.
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gragaAuthor Commented:
Geoff,

No problem, I will find the way. Thank you for your help.
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