4

I use Delphi 10.3.

This code works just fine. It loads a 32bit BMP with alpha channel and displays with proper transparency:

Bmp := TBitmap.Create;
Bmp.AlphaFormat := afDefined;
Bmp.LoadFromFile('bbb.bmp');    

Image1.Picture.Assign(Bmp);

Bmp.SaveToFile('uuu.bmp');

But this code removes transparency:

Bmp := TBitmap.Create;
Bmp.AlphaFormat := afDefined;
Bmp.LoadFromFile('bbb.bmp');

Bmp.SaveToFile('uuu.bmp'); 
Image1.Picture.Assign(Bmp);

Why?

I even wrote my own saver for pf32:

type TBitmap = class(Vcl.Graphics.TBitmap)
       procedure SaveToFile2(Filename: String);
       procedure LoadFromFile2(Filename: String);
     end;
     PCardinalArray = ^TCardinalArray;
     TCardinalArray = array[0..32767] of Cardinal;

...

procedure TBitmap.SaveToFile2(Filename: String);
var Head: array[0..53] of Byte;
    F: TFileStream;
    Size: Integer;
    P: PCardinalArray;
    y: Integer;
begin
  Size := Width*Height*4 + 54;

  FillChar(Head, 54, 0);

  Head[0] := $42;
  Head[1] := $4D;

  Head[2] := Size and $FF;
  Head[3] := (Size shr 8) and $FF;
  Head[4] := (Size shr 16) and $FF;
  Head[5] := Size shr 24;

  Head[6] := 0;
  Head[7] := 0;
  Head[8] := 0;
  Head[9] := 0;

  Head[10] := 54;
  Head[11] := 0;
  Head[12] := 0;
  Head[13] := 0;

  Head[14] := 40;
  Head[15] := 0;
  Head[16] := 0;
  Head[17] := 0;

  Head[18] := Width and $FF;
  Head[19] := (Width shr 8) and $FF;
  Head[20] := (Width shr 16) and $FF;
  Head[21] :=  Width shr 24;

  Head[22] := Height and $FF;
  Head[23] := (Height shr 8) and $FF;
  Head[24] := (Height shr 16) and $FF;
  Head[25] := Height shr 24;

  Head[26] := 1;
  Head[27] := 0;

  Head[28] := 32;
  Head[29] := 0;

  Head[38] := $12;
  Head[39] := $0B;
  Head[40] := 0;
  Head[41] := 0;

  Head[42] := $12;
  Head[43] := $0B;
  Head[44] := 0;
  Head[45] := 0;

  F := TFileStream.Create(Filename, fmCreate or fmShareDenyWrite);
  F.Write(Head[0], 54);

  for y:=Self.Height-1 downto 0 do begin
    P := Self.ScanLine[y];
    F.Write(P[0], Self.Width*4);
  end;

  F.Free;
end;

I also wrote my own loader for pf32bit:

procedure TBitmap.LoadFromFile2(Filename: String);
var Head: array[0..53] of Byte;
    F: TFileStream;
    P: PCardinalArray;
    y: Integer;
begin
  F := TFileStream.Create(Filename, fmOpenRead or fmShareDenyWrite);

  FillChar(Head, 54, 0);
  F.Read(Head[0], 54);

  if (F.Size < 54) or (Head[0] <> $42) or (Head[1] <> $4D) or (Head[10] <> 54) or (Head[28] <> 32) then begin
    F.Free;
    Exit;
  end;

  Self.Width := Head[18] + (Head[19] shl 8) + (Head[20] shl 16) + (Head[21] shl 24);
  Self.Height := Head[22] + (Head[23] shl 8) + (Head[24] shl 16) + (Head[25] shl 24);
  Self.PixelFormat := pf32bit;
  Self.AlphaFormat := afIgnored;

  for y:=Self.Height-1 downto 0 do begin
    P := Self.ScanLine[y];
    F.Read(P[0], Self.Width*4);
  end;

  Self.AlphaFormat := afDefined;
  F.Free;
end;

Output of this function is different from the input file. Seems the alpha channel is being changed here:

Self.AlphaFormat := afDefined;

So it seems this doesn't change how the bitmap is rendered but it actually changes the bitmap and recalculates pixels. Can this be avoided? I want a bitmap which is displayed with alpha transparency but I want all the pixels to stay intact, like they are in the file.

Tom
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  • Perhaps the `PixelFormat` is being altered during the save? Do you need the saved file to preserve transparency, or just the `TImage`? – Remy Lebeau Apr 19 '20 at 22:42
  • @RemyLebeau Basically I want a bitmap that is reliable and is not doing any conversions without my say. – Tom Apr 20 '20 at 12:42

1 Answers1

0

In the Embarcadero Documentation for Vcl.Graphics.TBitmap.TransparentColor, the transparent color is set to the first pixel in the bitmap image data. Maybe the problem is there. The program reload the picture this way.

Maybe this code will help you to find the problem: http://docwiki.embarcadero.com/CodeExamples/Rio/en/TGraphic_(Delphi)

Lionel Plais
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