How to create a raster image with a clear background in a transparent form?

Question

I am trying to create a fully transparent shape, on top of which I draw a bitmap with alpha transparency. The problem is that I cannot figure out how to set the background of the bitmap in Alpha 0 (full view).

This is what the shape looks like (note that the top right is not transparent).

This is how I want it to look (top right is completely transparent):

Here is 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 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.

Answer 1

You seem to have a misconception about how UpdateLayeredWindow / BLENDFUNCTION . Using UpdateLayeredWindow you use either an alpha pixel or a color key. You name it with ULW_ALPHA as "dwFlags", which means that you intend to use per-pixel alpha and you pass the fully opaque bitmap to your multiplication procedure (all pixels have an alpha value of 255). Your premultiplication procedure does not change the alpha channel, all it does is calculate the red green and blue values ​​according to the alpha channel of the transmitted bitmap. In the end, you have a completely opaque bitmap with correctly calculated r, g, b (also unmodified, since 255/255 = 1). All the transparency you get is from "210", which you assign to SourceConstantAlpha from BLENDFUNCTION . What UpdateLayeredWindow gives with them is a translucent window, with each pixel having the same transparency.

Filling the bitmap area mentioned in the comments to the question seems to work because calling FillRect overwrites the alpha channel. Pixels having alpha 255 now have alpha 0. IMO, usually this should be considered the reason for undefined behavior if you do not fully understand how / why it works.

In its current state, the question requires an answer from using a color key, not for an alpha pixel, or for cutting a shape area ( SetWindowRgn ). If an alpha pixel is used, it should be applied differently for parts of a bitmap image. In the comments on the question, you indicate that the bitmap must be scaled at some point. You must also ensure that the scaling code preserves the alpha channel, if used.