Question

I have a Image list assigned to a listview to display transparent images.

There is a slight issue with this regarding some transparent images that are added, and that is they are sometimes hard to see/find in the listview.

See this example image:

You will notice that the images (noticeably the mouse) is barely viewable, infact if a empty image was added you would not even see it, the number captions come to the rescue here to show there is something actually there.

But I would like to make the images visually easier to see. I thought maybe having another image underneath the transparent images would work – of course though it could not affect the actual image.

So with that in mind, I made a bitmap of a chessboard grid:

I feel this would be the most suitable way of representing transparent areas of the images just like Paint.NET etc does.

To further illustrate this example I have modified the original image to show how it would look, if we had the chessboard bitmap as the underlay image:

Having the chessboard there would indicate there is a list item there in the first place, and the bitmap of the chessboard grid could be darker or an altogether different kind of image. As I said earlier if there was no image you would see nothing at all, so better to show an empty chess grid or other bitmap than nothing.

So, how can I display a second image underneath the original images using a imagelist to give a result similar to the example above? The underneath image could be anything – just another loaded bitmap for example.

Answer 1

If you store the chessboard as the first image (with index 0) and make the overlay image from the current image in the OnGetImageIndex event handler, it will do what you want to. However I’m not sure how efficient is to make the overlay image every time the event is fired.

procedure TForm1.ListView1GetImageIndex(Sender: TObject; Item: TListItem);
begin
  // make the overlay (with overlay index 1) from the 
  // image with index Item.Index + 1
  ImageList1.Overlay(Item.Index + 1, 1);
  // use the first image from the list as a background
  Item.ImageIndex := 0;
  // and assign just created overlay index for overlay
  Item.OverlayIndex := 1;
end;