Delphi Tutorial - Shaping a Form around a Bitmap

Introduction

This article comes thanks to Gerard Oei who provided the Delphi source code.

We have already done a tutorial on how to create non-rectangular windows. This tutorial shows how to create a form that is shaped around a bitmap. Like the previous tutorial it uses the SetWindowRGN function. The clever part here is how the region is created.

Shaping the Form

I will not explain in detail how the code works because I think it will be easier to read the code and let the comments explain what is happening.

As an overview on the form you should place a TImage and set its picture property to be the image that you would like the form to be shaped around. At design time it should look like the following image:

The main part of the code can be found in the function CreateRegion which uses the API functions CreateRectRGN and CombineRGN to great affect. CreateRegion is then called from the form's constructor to shape the form accordingly.

The Sample Code

What follows is the Delphi code for shaping the window. You can also download the code at the bottom of this tutorial.

function TForm1.CreateRegion(Bmp: TBitmap): THandle; 
var
  X, Y, StartX:Integer;
  Excl: THandle;
  Row: PRGBArray;
  TransparentColor: TRGBTriple;
begin
  // Change the format so we know how to compare 
  // the colors 
  Bmp.PixelFormat := pf24Bit;
    
  // Create a region of the whole bitmap 
  // later we will take the transparent   
  // bits away
  Result := CreateRectRGN(0, 0, Bmp.Width, Bmp.Height);

  // Loop down the bitmap   
  for Y := 0 to Bmp.Height - 1 do
  begin
    // Get the current row of pixels
    Row := Bmp.Scanline[Y];

    // If its the first get the transparent
    // color, it must be the top left pixel
    if Y = 0 then
    begin
      TransparentColor := Row[0];
    end;

    // Reset StartX (-1) to indicate we have
    // not found a transparent area yet
    StartX := -1;

    // Loop across the row
    for X := 0 to Bmp.Width do
    begin

      // Check for transparency by comparing the color
      if(X <> Bmp.Width) and
        (Row[X].rgbtRed = TransparentColor.rgbtRed) and
        (Row[X].rgbtGreen = TransparentColor.rgbtGreen) and
        (Row[X].rgbtBlue = TransparentColor.rgbtBlue) then
      begin
        // We have (X <> Bmp.Width) in the clause so that
        // when we go past the end of the row we we can
        // exclude the remaining transparent area (if any)
        // If its transparent and the previous wasn't
        // remember were the transparency started
        if StartX = -1 then
        begin
          StartX := X;
        end;
      end
      else
      begin
        // Its not transparent
        if StartX > -1 then
        begin
          // If previous pixels were transparent we
          // can now exclude the from the region
          Excl := CreateRectRGN(StartX, Y, X, Y + 1);
          try
            // Remove the exclusion from our original region
            CombineRGN(Result, Result, Excl, RGN_DIFF);

            // Reset StartX so we can start searching
            // for the next transparent area
            StartX := -1;
          finally
            DeleteObject(Excl);
          end;
     end;
      end;
    end;
  end; 
end;
    
procedure TForm1.FormCreate(Sender: TObject); 
var
  Bmp: TBitmap;
begin
  Bmp := TBitmap.Create;
  try
    // We use a TImage to hold the bitmap so that 
    // we can see how the form will look at design
    // time
    Bmp.Assign(Image1.Picture);
    FRegion := CreateRegion(Bmp);
    SetWindowRGN(Handle, FRegion, True);
  finally
    Bmp.Free;
  end; 
end;