The reason seems to be that the transparency is applied two times to the image when loaded with LoadPNGintoBitmap32, giving it a more transparent and greyish look (more on this later).

First the transparency:

这是原来的<代码>LoadPNGintoBitmap32的代码,关键部分有以下评论意见:

 PNGObject := TPngObject.Create;
 PNGObject.LoadFromStream(srcStream);

 destBitmap.Assign(PNGObject);  // <--- paint to destBitmap s canvas with transparency (!)
 destBitmap.ResetAlpha;         

 case PNGObject.TransparencyMode of  // <--- the following code sets the transparency again for the TBitmap32
 { ... }

<代码>destBitmap.Assign Internal do与你以往的做法相同:它让PNG形象反映自己。 这项行动尊重全国过渡政府的甲型频道。 但没有必要这样做,因为甲型频道被分配到<代码>。 TBitmap32s pixels in a second step!

如今,《刑法》的修改如下,关键部分再次以评论标示:

 PNGObject := TPngObject.Create;
 PNGObject.LoadFromStream(srcStream);

 PNGObject.RemoveTransparency;  // <--- paint PNG without any transparency...
 destBitmap.Assign(PNGObject);  // <--- ...here
 destBitmap.ResetAlpha;

 srcStream.Position:=0;
 PNGObject.LoadFromStream(srcStream); // <--- read the image again to get the alpha channel back

 case PNGObject.TransparencyMode of   // <--- this is ok now, the alpha channel now only exists in the TBitmap32
 { ... }

上述解决办法效率不高,因为其形象是两倍。 但这表明,你的第二种做法产生了更透明的形象。

灰色: 原始代码中还有一个问题:destBitmap.Assign首先填满了cl White32的背景,然后以透明的方式描述了图像。 之后,“LoadPNGintoBitmap32出现并增加了另一层透明度。

问题可能是,全国过渡政府错误地转换为TBitmap32,从而失去了过境的透明度信息。 巴布亚新几内亚的图像是常见的情况。 否则,你就不必使用„Bit地图。 Draw Col Col 如果从PNG获得的跨国信息将正确转移到TBitmpa32, 绘画: d) Blend本来会奏效,无需设置外部Color。

What matters most is how did you load a PNG into the TBitmap32. The TPngImage from Vcl.Imaging.pngimage unit (implemented in Delphi XE2 and later) can draw transparently on bitmaps, preserving what was on that bitmaps, combining colors using the PNG alpha layer, etc, but it doesn’t allow to easily convert various formats of PNG transparency (including paletted) into the alpha component of each pixel of TBitmap32. Once TPngImage have drawn an image, you get the combined RGB for each pixel, but the alpha component is not transferred to the target bitmap.

现有的帮助者例行做法试图以透明方式将一个PNG装入一个TBitmap32,但也有缺点:

(1) “LoadPNGintoBitmap32” from http://graphics32.org/wiki/FAQ/ImageFormatRelated - it applies the transparency twice, so the images with alpha values other than 0 or 255 will look differently than in other software (most noticeable on translucent images with glass effects). This code will first apply alpha to RGB and then sets alpha as a separate layer, so when you paint, alpha will be applied again. You can find more information on this issue here: Delphi, GR32 + PngObject: converting to Bitmap32 doesn t work as expected . Besides that, it doesn t convert correctly transparency from paletted images into the alpha layer of TBitmap32. They manually set alpha transparency for the pixels of a certain color of the output bitmap (rendered to RGB) rather doing that before rendering to RGB, so the actual transparency is lost as on your sample image when all white pixels are transparent.

(2) “LoadBitmap32FromPNG” from gr32ex library: https://code.google.com/archive/p/gr32ex/ - a slightly different implementation of the same algorithm as (1), and has the same issues as (1).

So, the solutions are:

1. Do not use TBitmap32; use Vcl.Imaging.pngimage.TPngImage do draw directly on target bitmap (screen, etc.) – this is the most compatible way that deals correctly with various PNG formats.
2. Use a helper routing to transfer transparency information from Vcl.Imaging.pngimage.TPngImage to TBitmap32.
3. Use the GR32 PNG library that can natively load a PNG into TBitmap32 https://sourceforge.net/projects/gr32pnglibrary/ Since you now have all the information on this issue, you may get the right solution for you.

How to load the alpha layer in one pass

Heinrich Ulbricht made a nice suggestion to remove the transparency layer before paining and then to read the image again. To avoid loading the image twice, you can save the alpha layer before calling PNGObject.RemoveTransparency. Here is the code that correctly applies the alpha layer and loads the image only once. Unfortunately, it does not work with paletted images. If you know how to correctly fill the alpha layer of TBitmap32 from any paletted image, without the effects described at Transparent Png to TBitmap32 please let me know.

procedure LoadPNGintoBitmap32(DstBitmap: TBitmap32; SrcStream: TStream; out AlphaChannelUsed: Boolean);
var
  PNGObject: TPngImage;
  PixelPtr: PColor32;
  AlphaPtr: PByte;
  SaveAlpha: PByte;
  I, AlphaSize: Integer;
begin
  AlphaChannelUsed := False;
  PNGObject := TPngImage.Create;
  try
    PNGObject.LoadFromStream(SrcStream);
    AlphaPtr := PByte(PNGObject.AlphaScanline[0]);
    if Assigned(AlphaPtr) then
    begin
      AlphaSize := PNGObject.Width * PNGObject.Height;
      if AlphaSize <= 0 then raise Exception.Create( PNG files with zero dimensions are not supported to be loaded to TBitmap32 );
      GetMem(SaveAlpha, AlphaSize);
      try
        Move(AlphaPtr^, SaveAlpha^, AlphaSize);
        PNGObject.RemoveTransparency;
        DstBitmap.Assign(PNGObject);
        DstBitmap.ResetAlpha;
        PixelPtr := PColor32(@DstBitmap.Bits[0]);
        AlphaPtr := SaveAlpha;
        for I := 0 to AlphaSize-1 do
        begin
          PixelPtr^ := (PixelPtr^ and $00FFFFFF) or (TColor32(AlphaPtr^) shl 24);
          Inc(PixelPtr);
          Inc(AlphaPtr);
        end;
      finally
        FreeMem(SaveAlpha, AlphaSize);
      end;
      AlphaChannelUsed := True;
    end else
    if PNGObject.TransparencyMode = ptmNone then
    begin
      DstBitmap.Assign(PNGObject);
    end else
    begin
      raise Exception.Create( Paletted PNG images are not supported in LoadPNGintoBitmap32, transparency cannot be stored to TBitmap32 );
    end;
  finally
    FreeAndNil(PNGObject);
  end;
end;