How do you do both blending and transparency using only pure VCL on bitmaps?

Question

Like this question, I would like to mix colors as well as bitmap images (png or bmp, but in my case I use png) while maintaining transparency.

Like a related question, I would like (a) not to use third-party libraries, (b) use VCL built into methods where possible, but if necessary use the Win32 GDI API where necessary, and (c) not use GDI + .

This simple code, based on the code in the related question, I see that color mixing works, but the transparency of the PNG file is not preserved when I do this:

Things I tried: - Different pixel formats (pf32bit, pf24bit) - Different values ​​for TBitmap.Transparent.

I tried this on Delphi XE5 and XE6, in this case, but I suspect the same will work for XE2 and higher.

Demo code:

unit BlendUnit2;


{ Investigating VCL alpha blending GDI capability }

interface

uses
   Winapi.Windows,
   Winapi.Messages,
   System.SysUtils,
   System.Variants,
   System.Classes,
   Vcl.Graphics,
   Vcl.Controls,
   Vcl.Forms,
   Vcl.Dialogs,
   Vcl.ExtCtrls,
   Vcl.Imaging.pngimage,
   Vcl.StdCtrls;

type
  TForm2 = class(TForm)
    BackgroundImage: TImage;
    TransparentImageInDFM: TImage;
    GeneratedImage: TImage;
    Label1: TLabel;
    Shape1: TShape;
    Shape2: TShape;
    Label2: TLabel;
    Label3: TLabel;
    procedure FormDestroy(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  public
    { Public declarations }
    procedure Blend(ACanvas:TCanvas);
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

procedure ColorBlend(const ACanvas: TCanvas; const ARect: TRect;
  const ABlendColor: TColor; const ABlendValue: Integer);
var
  bmp: TBitmap;
begin
  bmp := TBitmap.Create;
//  bmp.PixelFormat := pf32bit;
//  bmp.AlphaFormat := afPremultiplied;
//  bmp.Transparent  := True;
  try
    bmp.Canvas.Brush.Color := ABlendColor;
    bmp.Width := ARect.Right - ARect.Left;
    bmp.Height := ARect.Bottom - ARect.Top;
    bmp.Canvas.FillRect(Rect(0,0,bmp.Width, bmp.Height));
    ACanvas.Draw(ARect.Left, ARect.Top, bmp, ABlendValue);
  finally
    bmp.Free;
  end;
end;


procedure TForm2.Blend(ACanvas:TCanvas);
var
  Image: TPNGImage;
begin
  Image := TPNGImage.Create;
  try
    Image.LoadFromFile('..\..\AppIcon2013.png');
    ColorBlend(Image.Canvas, Image.Canvas.ClipRect, $00CCFF80, 85);
    ACanvas.Draw(0, 0, Image);
  finally
    Image.Free;
  end;
end;


procedure TForm2.FormDestroy(Sender: TObject);
begin
//
end;

procedure TForm2.FormCreate(Sender: TObject);
begin
//   GeneratedImage.Picture.Bitmap.Transparent := false;
//   GeneratedImage.Picture.Bitmap.PixelFormat := pf32bit;
   GeneratedImage.Picture.Bitmap.SetSize(280,280);

   Blend( GeneratedImage.Picture.Bitmap.Canvas );
end;

end.

Screenshot showing indelible, but correctly transparent image on the right, and mixed, but no more transparent image on the left:

Note. I think pre-blending the background and foreground with a call to the Win32 API for AlphaBlend MAY be the only way, in which case the VCL does NOT contain the built-in transparency support that is worth mentioning. (Someone from StackOverflow told me that he thought that third-party libraries weren’t worth it because it is built into VCL, and I wondered if it was right, so I only try to do it with VCL.)