Drawing on top of components

Is it possible to draw (a rectangle or something) on a form so that the rectangle would be displayed on top of all the components on the form?

Painting in Paint() or when receiving WM_PAINT message seems to result in the drawing being covered by components (buttons, panels... doesn't seem to matter).

I know that I could place a component representing a rectangle on top of all the other components, but that is not sufficient in this particular situation.
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fvaConnect With a Mentor Commented:
This is what I did in a similar case:

Declare a new derivated control like this:

type TFloatImage = class(TCustomControl)
       procedure WMEraseBkgnd(var Message: TWmEraseBkgnd); message WM_ERASEBKGND;
       procedure CreateParams(var Params:TCreateParams);override;
        procedure Paint;override;

procedure TFloatImage.CreateParams(var Params:TCreateParams);
     Params.ExStyle:=Params.ExStyle or WS_EX_TRANSPARENT;

procedure TFloatImage.WMEraseBkgnd(var Message: TWmEraseBkgnd);
// NO     inherited;

procedure TFloatImage.Paint;
     with Canvas do begin
          // draw here whatever you please

and then place one such control over all your other controls. You can install it in the IDE and do that at design time or you can instantiate it at runtime in the Create of the carrying form.

It "almost" works seamlessly, i.e. some standard controls are not supporting well the presence of this "overlay" and do produce unavoidable side-effects. One notable example would be a TreeView "covered" by this object. When the TreeView scrolls, the scroll is made partially by shifting the pixels on the screen rather than redrawing the TreeView and that messes with the pixels "overdrawn" by the transparent control. You can partially fix that by handling the redraw notify events from the covered controls but it's very form-dependent and still not very reliable.

Hi Keijo,

you can draw directly on desktop in the same coordinates where your form located. It will look like drawing over your form.

hello Keijo, do you want this rect to be redrawn in the Onpaint of your form. or just drawn temporarily over other controls (until a paint happens)? If you want to draw over controls then putting a control over the other controls (as you said) will do it, if you can't do that then you can hide the controls that are in the same place as your rect and show them when the rect is gone, if you have partially covered controls then they will dissapear (a few controls could be resized to still be visible). I think it's posible to change the update area of a window but if you take the rect area out of the Main Form's update area then the rect won't be drawn. It seems the windows OS is very good at making sure all visible controls are drawn where they are suppose to be. Also you might try a timer event triggered by the OnPaint that allows a few microseconds and then Paints the Rectangle on the Forms Canvas.You may look at windows API

    HWND hWnd,     // handle of window
    CONST RECT *lpRect     // address of validation rectangle coordinates

which removes a rect from update

you could try something like
ValidateRect(Button1.Handle, @SmallerRect);
where SmallerRect is the Bounds rect minus the part covered by the Rectangle.

I couldn't get that to be persistant since the update area is renewed on each paint and is not useful on non wincontrols.
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There are two kinds of controls in delphi, wincontrols and non-windowed controls, Windowed controls will always show on top of non windowed ones.

Here are a couple of methods not already discussed..

In win2k and above you can use 'alpha blending' and transparency. I think Delphi 6 comes with a transparancy demo, also the delphi magazine did an article on it. If you use a transparent form or panel on top of the image you want then you will see the image but the (invisible) buttons will still work.

Marco Cantu used to have some kind of transparency demo too. I remember some kind of 'click through' where the mouse events get passed through to the windows underneath.

A final way is to use owner draw controls and bitmap buttons, so that each control paints its own piece of the picture, The controls know where they are so they could copy the corresponding parts from a TImage.


Oh hang on, here is an easy way..

Put a panel on top of the whole form and the enabled property to false.. All the buttons and Edits still work, you just cant see them. If you want you can grab the underlying form image and blend it with the picture you are displaying, like this.

procedure TForm1.FormPaint(Sender: TObject);
if not panel1.Visible then exit;

It feels very wierd.
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