How do I create a bitmap that has a clear background on a transparent form?

I am trying to create a form that is completely transparent on top of which I draw a bitmap with alpha transparency. The problem is that I can't figure out how to set the background of the bitmap to Alpha 0 (totally see through).

Here's how the form looks now (note top right not transparent).

Here's how I want it to look (top right totally transparent):

Here's my source:

unit frmMain;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, ActiveX,

  GDIPObj, GDIPAPI, Vcl.StdCtrls, Vcl.ExtCtrls;

type
  TForm7 = class(TForm)
    Panel1: TPanel;
    Edit1: TEdit;
    Button1: TButton;
    Button2: TButton;
    procedure Button2Click(Sender: TObject);
  private
    function CreateTranparentForm: TForm;
  end;

var
  Form7: TForm7;

implementation

{$R *.dfm}

// Thanks to Anders Melander for the transparent form tutorial
// (http://melander.dk/articles/alphasplash2/2/)
function CreateAlphaBlendForm(AOwner: TComponent; Bitmap: TBitmap; Alpha: Byte): TForm;

  procedure PremultiplyBitmap(Bitmap: TBitmap);
  var
    Row, Col: integer;
    p: PRGBQuad;
    PreMult: array[byte, byte] of byte;
  begin
    // precalculate all possible values of a*b
    for Row := 0 to 255 do
      for Col := Row to 255 do
      begin
        PreMult[Row, Col] := Row*Col div 255;

        if (Row <> Col) then
          PreMult[Col, Row] := PreMult[Row, Col]; // a*b = b*a
      end;

    for Row := 0 to Bitmap.Height-1 do
    begin
      Col := Bitmap.Width;

      p := Bitmap.ScanLine[Row];

      while (Col > 0) do
      begin
        p.rgbBlue := PreMult[p.rgbReserved, p.rgbBlue];
        p.rgbGreen := PreMult[p.rgbReserved, p.rgbGreen];
        p.rgbRed := PreMult[p.rgbReserved, p.rgbRed];

        inc(p);
        dec(Col);
      end;
    end;
  end;

var
  BlendFunction: TBlendFunction;
  BitmapPos: TPoint;
  BitmapSize: TSize;
  exStyle: DWORD;
  PNGBitmap: TGPBitmap;
  BitmapHandle: HBITMAP;
  Stream: TMemoryStream;
  StreamAdapter: IStream;
begin
  Result := TForm.Create(AOwner);

  // Enable window layering
  exStyle := GetWindowLongA(Result.Handle, GWL_EXSTYLE);

  if (exStyle and WS_EX_LAYERED = 0) then
    SetWindowLong(Result.Handle, GWL_EXSTYLE, exStyle or WS_EX_LAYERED);

  // Load the PNG from a resource
  Stream := TMemoryStream.Create;
  try
    Bitmap.SaveToStream(Stream);

    // Wrap the VCL stream in a COM IStream
    StreamAdapter := TStreamAdapter.Create(Stream);
    try
      // Create and load a GDI+ bitmap from the stream
      PNGBitmap := TGPBitmap.Create(StreamAdapter);
      try
        // Convert the PNG to a 32 bit bitmap
        PNGBitmap.GetHBITMAP(MakeColor(0,0,0,0), BitmapHandle);

        // Wrap the bitmap in a VCL TBitmap
        Bitmap.Handle := BitmapHandle;
      finally
        FreeAndNil(PNGBitmap);
      end;
    finally
      StreamAdapter := nil;
    end;
  finally
    FreeAndNil(Stream);
  end;

  // Perform run-time premultiplication
  PremultiplyBitmap(Bitmap);

  // Resize form to fit bitmap
  Result.ClientWidth := Bitmap.Width;
  Result.ClientHeight := Bitmap.Height;

  // Position bitmap on form
  BitmapPos := Point(0, 0);
  BitmapSize.cx := Bitmap.Width;
  BitmapSize.cy := Bitmap.Height;

  // Setup alpha blending parameters
  BlendFunction.BlendOp := AC_SRC_OVER;
  BlendFunction.BlendFlags := 0;
  BlendFunction.SourceConstantAlpha := Alpha;
  BlendFunction.AlphaFormat := AC_SRC_ALPHA;

  UpdateLayeredWindow(Result.Handle, 0, nil, @BitmapSize, Bitmap.Canvas.Handle,
    @BitmapPos, 0, @BlendFunction, ULW_ALPHA);
end;

procedure CopyControlToBitmap(AWinControl: TWinControl; Bitmap: TBitmap; X, Y: Integer);
var
 SrcDC: HDC;
begin
  SrcDC := GetDC(AWinControl.Handle);
  try
    BitBlt(Bitmap.Canvas.Handle, X, Y, AWinControl.ClientWidth, AWinControl.ClientHeight, SrcDC, 0, 0, SRCCOPY);
  finally
     ReleaseDC(AWinControl.Handle, SrcDC);
  end;
end;

function MakeGDIPColor(C: TColor; Alpha: Byte): Cardinal;
var
  tmpRGB : TColorRef;
begin
  tmpRGB := ColorToRGB(C);

  result := ((DWORD(GetBValue(tmpRGB)) shl  BlueShift) or
             (DWORD(GetGValue(tmpRGB)) shl GreenShift) or
             (DWORD(GetRValue(tmpRGB)) shl   RedShift) or
             (DWORD(Alpha) shl AlphaShift));
end;

procedure TForm7.Button2Click(Sender: TObject);
begin
  CreateTranparentForm.Show;
end;

function TForm7.CreateTranparentForm: TForm;
const
  TabHeight = 50;
  TabWidth = 150;
var
  DragControl: TWinControl;
  DragCanvas: TGPGraphics;
  Bitmap: TBitmap;
  ControlTop: Integer;
  DragBrush: TGPSolidBrush;
begin
  DragControl := Panel1;

  Bitmap := TBitmap.Create;
  try
    Bitmap.PixelFormat := pf32bit;

    Bitmap.Height := TabHeight + DragControl.Height;
    Bitmap.Width := DragControl.Width;
    ControlTop := TabHeight;

    // <<<< I need to clear the bitmap background here!!!

    CopyControlToBitmap(DragControl, Bitmap, 0, ControlTop);

    DragCanvas := TGPGraphics.Create(Bitmap.Canvas.Handle);
    DragBrush := TGPSolidBrush.Create(MakeGDIPColor(clBlue, 255));
    try
      // Do the painting...
      DragCanvas.FillRectangle(DragBrush, 0, 0, TabWidth, TabHeight);
    finally
      FreeAndNil(DragCanvas);
      FreeAndNil(DragBrush);
    end;

    Result := CreateAlphaBlendForm(Self, Bitmap, 210);
    Result.BorderStyle := bsNone;
  finally
    FreeAndNil(Bitmap);
  end;
end;

end.

...and the DFM:

object Form7: TForm7
  Left = 0
  Top = 0
  Caption = 'frmMain'
  ClientHeight = 300
  ClientWidth = 635
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object Panel1: TPanel
    Left = 256
    Top = 128
    Width = 321
    Height = 145
    Caption = 'Panel1'
    TabOrder = 0
    object Edit1: TEdit
      Left = 40
      Top = 24
      Width = 121
      Height = 21
      TabOrder = 0
      Text = 'Edit1'
    end
    object Button1: TButton
      Left = 40
      Top = 64
      Width = 75
      Height = 25
      Caption = 'Button1'
      TabOrder = 1
    end
  end
  object Button2: TButton
    Left = 16
    Top = 16
    Width = 75
    Height = 25
    Caption = 'Go'
    TabOrder = 1
    OnClick = Button2Click
  end
end

Thanks.

---

You seem to have a misconception of how UpdateLayeredWindow / BLENDFUNCTION works. With UpdateLayeredWindow , you either use per-pixel alpha or a color-key. You're calling it with ULW_ALPHA as 'dwFlags' which means you intend to use per-pixel alpha, and you pass a fully opaque bitmap to your premultiplication routine (all pixels have alpha value of 255). Your premultiplication routine does not modify alpha channel, all it does is to calculate the red green and blue values according to the alpha channel of the passed bitmap. In the end, what you've got is a fully opaque bitmap with properly calculated r, g, b (also unmodified, since 255/255 = 1). All the transparency you will get is from the '210' that you assign to SourceConstantAlpha of BlendFunction . What UpdateLayeredWindow gives with these is a semi-transparent window, every pixel having the same transparency.

Filling a region of the bitmap, mentioned in the comments to the question, seems to work because the FillRect call overwrites the alpha channel. Pixels having an alpha of 255 now have an alpha of 0. IMO, normally this should be considered to cause undefined behavior unless you fully understand how/why it works.

The question, in its current state, calls for an answer of using a color-key rather than per-pixel alpha, or cutting a region of the form ( SetWindowRgn ). If per-pixel alpha is to be used, it should be applied differently to parts of the bitmap. In the comments to the question, you mention the bitmap is to be scaled at some point. You've also got to be sure the scaling code preserves alpha channel, if it is used.

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