Why doesn't this D2006 code to fade a PNG Image work?

问题

This question springs from an earlier one. Most of the code is from suggested answers that probably worked in later versions of Delphi. In D2006 I don't get the full range of opacity, and the transparent part of the image shows as white.

Image is from http://upload.wikimedia.org/wikipedia/commons/6/61/Icon_attention_s.png.
It is loaded from the PNGImageCollection into the TImage at run-time because I have found you have to do this as the image doesn't remain intact after the DFM is saved. For the purposes of demonstrating the behaviour you probably don't need the PNGImageCollection and can just load the PNG image into the TImage at design time and then run it from the IDE.

There are four buttons on the form - each one sets a different value of opacity. Opacity=0 works fine (paintbox image is not visible, opacity=16 looks OK except for the white background, opacity=64, 255 are similar - the opacity seems to saturate at around 10%.

Any ideas as to what's up?

unit Unit18;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, pngimage, StdCtrls, Spin, PngImageList;

type
  TAlphaBlendForm = class(TForm)
    PaintBox1: TPaintBox;
    Image1: TImage;
    PngImageCollection1: TPngImageCollection;
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    Button4: TButton;
    procedure PaintBox1Paint(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
  private
    FOpacity : Integer ;
    FBitmap  : TBitmap ;
    { Private declarations }
  public
    { Public declarations }
  end;

var
  AlphaBlendForm: TAlphaBlendForm;

implementation

{$R *.dfm}

procedure TAlphaBlendForm.Button1Click(Sender: TObject);
begin
FOpacity:= 0 ;
PaintBox1.Invalidate;
end;

procedure TAlphaBlendForm.Button2Click(Sender: TObject);
begin
FOpacity:= 16 ;
PaintBox1.Invalidate;
end;

procedure TAlphaBlendForm.Button3Click(Sender: TObject);
begin
FOpacity:= 64 ;
PaintBox1.Invalidate;
end;

procedure TAlphaBlendForm.Button4Click(Sender: TObject);
begin
FOpacity:= 255 ;
PaintBox1.Invalidate;
end;

procedure TAlphaBlendForm.FormCreate(Sender: TObject);
begin
  Image1.Picture.Assign (PngImageCollection1.Items [0].PNGImage) ;
  FBitmap := TBitmap.Create;
  FBitmap.Assign(Image1.Picture.Graphic);//Image1 contains a transparent PNG
  FBitmap.PixelFormat := pf32bit ;
  PaintBox1.Width := FBitmap.Width;
  PaintBox1.Height := FBitmap.Height;
end;

procedure TAlphaBlendForm.PaintBox1Paint(Sender: TObject);

var
  fn: TBlendFunction;
begin
  fn.BlendOp := AC_SRC_OVER;
  fn.BlendFlags := 0;
  fn.SourceConstantAlpha := FOpacity;
  fn.AlphaFormat := AC_SRC_ALPHA;
  Windows.AlphaBlend(
    PaintBox1.Canvas.Handle,
    0,
    0,
    PaintBox1.Width,
    PaintBox1.Height,
    FBitmap.Canvas.Handle,
    0,
    0,
    FBitmap.Width,
    FBitmap.Height,
    fn
  );
end;

end.

** This code (using graphics32 TImage32) almost works **

unit Unit18;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, pngimage, StdCtrls, Spin, PngImageList, GR32_Image;

type
  TAlphaBlendForm = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    Button4: TButton;
    Image321: TImage32;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  AlphaBlendForm: TAlphaBlendForm;

implementation

{$R *.dfm}

procedure TAlphaBlendForm.Button1Click(Sender: TObject);
begin
Image321.Bitmap.MasterAlpha := 0 ;
end;

procedure TAlphaBlendForm.Button2Click(Sender: TObject);
begin
Image321.Bitmap.MasterAlpha := 16 ;
end;

procedure TAlphaBlendForm.Button3Click(Sender: TObject);
begin
Image321.Bitmap.MasterAlpha := 64 ;
end;

procedure TAlphaBlendForm.Button4Click(Sender: TObject);
begin
Image321.Bitmap.MasterAlpha := 255 ;
end;

end.

** (UPDATE) This code (using graphics32 TImage32) DOES work **

The following code is successful in assigning a PNG image to the Graphics32.TImage32 at run-time. The PNG image with alpha channel is loaded into a TPNGImageCollection (really useful component as it allows mixtures of images of arbitrary size) at design time. On form creation it is written to a stream, then read from the stream into the Image32 using LoadPNGintoBitmap32. Once this is done I can control the opacity by assigning to TImage32.Bitmap.MasterAlpha. No bothering with OnPaint handlers.

procedure TAlphaBlendForm.FormCreate(Sender: TObject);

var
  FStream          : TMemoryStream ;
  AlphaChannelUsed : boolean ;

begin
  FStream := TMemoryStream.Create ;

  try
    PngImageCollection1.Items [0].PngImage.SaveToStream (FStream) ;
    FStream.Position := 0 ;
    LoadPNGintoBitmap32 (Image321.Bitmap, FStream, AlphaChannelUsed) ;
  finally
    FStream.Free ;
    end;

end ;

回答1:

As David commented to the question, the alpha channel information is lost when you assign the graphic to the bitmap. As such there's no point in setting the pixel format to pf32bit after the assignment, apart from preventing AlphaBlend call to fail, there's no per-pixel alpha in the bitmap anyway.

But the png object knows how to draw on a canvas taking into consideration the transparency information. So the solution would involve drawing on the bitmap canvas instead of assigning the graphic, and then, since there's no Alpha channel, remove the AC_SRC_ALPHA flag from the BLENDFUNCTION.

Below is working code here on D2007:

procedure TAlphaBlendForm.FormCreate(Sender: TObject);
begin
  Image1.Picture.LoadFromFile(
      ExtractFilePath(Application.ExeName) + 'Icon_attention_s.png');

  FBitmap := TBitmap.Create;
  FBitmap.Width := Image1.Picture.Graphic.Width;
  FBitmap.Height := Image1.Picture.Graphic.Height;

  FBitmap.Canvas.Brush.Color := Color;      // background color for the image
  FBitmap.Canvas.FillRect(FBitmap.Canvas.ClipRect);

  FBitmap.Canvas.Draw(0, 0, Image1.Picture.Graphic);

  PaintBox1.Width := FBitmap.Width;
  PaintBox1.Height := FBitmap.Height;
end;

procedure TAlphaBlendForm.PaintBox1Paint(Sender: TObject);
var
  fn: TBlendFunction;
begin
  fn.BlendOp := AC_SRC_OVER;
  fn.BlendFlags := 0;
  fn.SourceConstantAlpha := FOpacity;
  fn.AlphaFormat := 0;
  Windows.AlphaBlend(
    PaintBox1.Canvas.Handle,
    0,
    0,
    PaintBox1.Width,
    PaintBox1.Height,
    FBitmap.Canvas.Handle,
    0,
    0,
    FBitmap.Width,
    FBitmap.Height,
    fn
  );
end;

or without using a intermediate TImage:

procedure TAlphaBlendForm.FormCreate(Sender: TObject);
var
  PNG: TPNGObject;
begin
  PNG := TPNGObject.Create;
  try
    PNG.LoadFromFile(ExtractFilePath(Application.ExeName) + 'Icon_attention_s.png');

    FBitmap := TBitmap.Create;
    FBitmap.Width := PNG.Width;
    FBitmap.Height := PNG.Height;

    FBitmap.Canvas.Brush.Color := Color;
    FBitmap.Canvas.FillRect(FBitmap.Canvas.ClipRect);

    PNG.Draw(FBitmap.Canvas, FBitmap.Canvas.ClipRect);

    PaintBox1.Width := FBitmap.Width;
    PaintBox1.Height := FBitmap.Height;
  finally
    PNG.Free;
  end;
end;

来源：https://stackoverflow.com/questions/4910411/why-doesnt-this-d2006-code-to-fade-a-png-image-work