I need a transparent Trackbar on my map.

The underlying transparent component works fine, and is transparent, I just cannot get the background of the TTrackbar to become transparent, no matter what tricks I add from the internet.

When I put a Trackbar on a Form, and set the Form AlphaBlend, then the Trackbar is transparent (together with the whole Form of course). To me that means it should be able to do it.

This is the code I have at the moment:

{code}
TMyTrackBar = class(ComCtrls.TTrackBar)
  private
    procedure WMEraseBkGnd(var Message: TWMEraseBkGnd); message WM_EraseBkGnd;
  protected
    procedure CreateParams(var Params: TCreateParams); override;
    // procedure Paint; override;
  public
    constructor Create(AOwner: TComponent); override;
  end;

  TBlendBar = class(TCustomControl)
	protected
		procedure Paint; override;
		procedure WMEraseBkgnd(var Message: TWMEraseBkgnd); message WM_ERASEBKGND;
	private
		BlendBitmap: TBitmap;
	public
  	Value:integer;
    TrackBar: TMyTrackBar;

		constructor Create(AOwner: TComponent); override;
		destructor Destroy; override;
	end;



// ----------------------- TBlendBar --------------------------------------------

constructor TMyTrackBar.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  ControlStyle := ControlStyle - [csOpaque];
end;

procedure TMyTrackBar.CreateParams(var Params: TCreateParams);
begin
  inherited CreateParams(Params);
  Params.ExStyle := Params.ExStyle or WS_EX_TRANSPARENT;
end;

procedure TMyTrackBar.WMEraseBkGnd(var Message: TWMEraseBkGnd);
begin
  SetBkMode (Message.DC, TRANSPARENT);
  Message.Result := 1;
end;

constructor TBlendBar.Create(AOwner: TComponent);
begin
	inherited;
  self.Brush.Style := bsClear;
	BlendBitmap := TBitmap.Create;
  //BlendBitmap.Height := 152;
  //BlendBitmap.Width := 24;
  TrackBar := TMyTrackBar.Create(self);
  TrackBar.Parent := Self;
  TrackBar.Orientation := trVertical;
  TrackBar.Width := Width;
  TrackBar.Height := Height;
  Trackbar.TickMarks := tmTopLeft;
  Trackbar.TickStyle := tsNone;
  Trackbar.ShowSelRange := false;
  TrackBar.ThumbLength := 20;
  TrackBar.Min := 0;
  TrackBar.Max := 255;
  TrackBar.Position := 128;
  TrackBar.OnChange := MapIntForm.BlendTrackBarChange;
  TrackBar.Hint := _('Change the transparency of Overlays which can be Blended');
  TrackBar.ShowHint := true;
end;


destructor TBlendBar.Destroy;
begin
	FreeAndNil(BlendBitmap);
  TrackBar.Free;
	inherited;
end;


procedure TBlendBar.Paint;
var
  SrcRect: TRect;
	bf: BLENDFUNCTION;
begin
	// if not assigned(BlendBitmap) then exit; // Seems to happen on shutdown.
  if not self.Visible then exit;
  if not assigned(canvas) then exit;

	with Canvas do
	begin
		Brush.Style := bsClear;
    // Brush.Style := bsSolid;
	end;

  Width := 40;
  Height := 150;
  Trackbar.Width := 25;
  Trackbar.Height := 150;

	Left := MapIntForm.MapBox.width - Self.width - 24;
	Top := MapIntForm.MapBox.Top + MapIntForm.MapBox.ClientHeight - 180;

	// Copy from MapImage what is beneath the canvas
	SrcRect.Left := Self.Left - MapIntForm.MapBox.FMapImage.Left -
		MapIntForm.MapBox.Left - 2;
	SrcRect.Top := Self.Top - MapIntForm.MapBox.FMapImage.Top -
		MapIntForm.MapBox.Top - 2;
	SrcRect.Right := SrcRect.Left + width;
	SrcRect.Bottom := SrcRect.Top + height;

	Canvas.CopyMode := cmSrcCopy;
	Canvas.CopyRect(Rect(0, 0, Self.width, Self.height),
		MapIntForm.MapBox.FMapImage.Canvas, SrcRect);

	BlendBitmap.width := Self.width;
	BlendBitmap.height := Self.height;
	BlendBitmap.Canvas.Brush.Color := clBlack; //clWhite;
	// BlendBitmap.Canvas.Rectangle(0,0,BlendBitmap.Width,BlendBitmap.Height);

	bf.BlendOp := AC_SRC_OVER;
	bf.BlendFlags := 0;
	bf.SourceConstantAlpha := 100; // AlphaValue
	bf.AlphaFormat := 0;
	AlphaBlend(Canvas.Handle, 0, 0, width, height, BlendBitmap.Canvas.Handle, 0,
		0, width, height, bf);

  Trackbar.Repaint;
end;


procedure TBlendBar.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
	Message.result := 1;
end;


{code}

--
Bart Kindt, CEO and developer
SARTrack Limited New Zealand
http://www.sartrack.co.nz/

Bart Kindt wrote:

> I need a transparent Trackbar on my map.
> 
> The underlying transparent component works fine, and is transparent,
> I just cannot get the background of the TTrackbar to become
> transparent, no matter what tricks I add from the internet.
> 
> When I put a Trackbar on a Form, and set the Form AlphaBlend, then
> the Trackbar is transparent (together with the whole Form of course).
> To me that means it should be able to do it.

You are comparing apples and peaches <g>. The alpha blending on the
form level is a Windows feature and it only works on the top window
level (including all child windows of the top window), not only on a
child window (control) level.

> This is the code I have at the moment:
> 
> {code}
> TMyTrackBar = class(ComCtrls.TTrackBar)
>   private
>     procedure WMEraseBkGnd(var Message: TWMEraseBkGnd); message
> WM_EraseBkGnd;   protected

The trackbar is a windows common control under the hood, which means
you are at Window's mercy in regard to how it draws the control. You
block the background fill, but that only works if the control actually
responds to the WM_ERASEBKGND message by drawing its background there.
The trackbar does not do that, it draws itself completely in WM_PAINT,
and it may even take shortcuts and draw while you drag the slider with
the mouse, in response to the mouse messages.

To custom draw common controls you have to use a different approach.
See
https://msdn.microsoft.com/en-us/library/windows/desktop/bb760155%28v=vs.85%29.aspx

for a start. You probably have to add a handler for WM_NOTIFY (or
CN_NOTIFY, the VCL equivalent) to modify the drawing process.

You also have to keep in mind that, for a control to act transparent,
its *parent* control has to draw the area under the transparent
control, which it will not do by default (the child window area is
excluded from the parent's drawing region automatically). The VCL has a
mechanism to handle that for pure VCL controls (like a TPanel, see the
ParentBackground property), but this cannot be easily retrofitted to a
Windows control. The best strategy here is usually to trap
WM_ERASEBKGND and there use the parent's PaintTo method to get it to
draw the part of itself covered by the control to the control's HDC.



-- 
Peter Below (TeamB)

> {quote:title=Peter Below wrote:}{quote}

> To custom draw common controls you have to use a different approach.
> See
> https://msdn.microsoft.com/en-us/library/windows/desktop/bb760155%28v=vs.85%29.aspx

At first sight, I do not see how this helps me, but I will study it some more.

> 
> for a start. You probably have to add a handler for WM_NOTIFY (or
> CN_NOTIFY, the VCL equivalent) to modify the drawing process.
> 
> You also have to keep in mind that, for a control to act transparent,
> its *parent* control has to draw the area under the transparent
> control,

I do this manually, I copy the original background, and paint this back in the control. This works fine.
The problem is that the Trackbar's background then completely overwrites this again. 
This despite the fact that I tried this as well:
{code}
ABlendBar := TBlendBar.Create(Self);
  ABlendBar.Parent := Self;
  SetWindowLong(ABlendBar.Handle, GWL_STYLE, GetWindowLong
      (ABlendBar.Handle, GWL_STYLE) and not WS_ClipChildren);
{code}

Which is supposed to deal with the issue of the parent not painting the area under the control, as I understand it. But it has no effect. 
I don't think this is in fact relevant because I manually paint the background in TBlendBar.Paint; After which I call the paint of the TrackBar, which then overwrites the lot again.

> Peter Below (TeamB)

--
Bart Kindt, CEO and developer
SARTrack Limited New Zealand
http://www.sartrack.co.nz/