Question

I am working with delphi.
I have one scroll box in which I am putting TImage control. Now I wanted to zoom the image rendered into TImage control. So, I am using stretchDraw method of TCanvas. My code is –

   if sbZoom.Down then begin
      rct := imgmain.Picture.Bitmap.Canvas.ClipRect;
      rct := Rect(rct.Left * 2,rct.Top * 2,rct.Right * 2,rct.Bottom * 2);
      imgmain.Picture.Bitmap.Canvas.StretchDraw(rct,imgmain.Picture.Bitmap);
      imgmain.Repaint;
   end;

It is correctly zooming the image, my problem is I want the size of scroll box also should be changed with zooming of image.
Also explain me parameters of Canvas.StretchDraw method. I am little confused with it.
Thank You.

Answer 1

Uwe Raabe is giving you the right way to do it. Here’s why your way doesn’t work: A scroll box will show scrollbars and help you see whole controls. In your case, it will only show scrollbars when the TImage object grows larger then the Scrollbox. The Scrollbox can’t possibly know the internals of TImage so it doesn’t care about TImage.Picture, it only cares about the control. And a TImage that has AutoSize = False doesn’t care about it’s Picture, it’s size stays the same at all times.

Your code repaints the base bitmap onto itself. The problem is, the bitmap has fixed Width and Height: if you paint outside the bitmap’s area you’re basically silently ignored. When you’re “zooming” by StretchDrawing the bitmap onto itself (and I’m surprised it worked to start with!) you’re not making the bitmap larger and the stuff that doesn’t fit gets silently clipped away. If you do want the internal bitmap to change size then you’ll first need to create a new, larger bitmap, draw your enlarged image to the new bitmap and then assign the bitmap to your TImage. If you do this, make sure TImage.AutoSize = True.