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.
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

One form, one TPaintBox (preview window), one scrollbox, and one TImage (inside the scrollbox).

procedure TForm1.PaintBox1Paint(Sender: TObject);
      scalex, scaley : single;
      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));

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

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

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.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
You can use the StretchBlt windows api function. I'll try to give a example...

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

Become a Certified Penetration Testing Engineer

This CPTE Certified Penetration Testing Engineer course covers everything you need to know about becoming a Certified Penetration Testing Engineer. Career Path: Professional roles include Ethical Hackers, Security Consultants, System Administrators, and Chief Security Officers.

would be, that is :-)
gragaAuthor Commented:

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 :)

gragaAuthor Commented:
Also, any quick suggestion on centering the grab box on the mouse?
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.
gragaAuthor Commented:

No problem, I will find the way. Thank you for your help.
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today

From novice to tech pro — start learning today.