Bitmap shrinken

[bernhard_LA]

ich möchte mit folgendem Code eine Bitmap in der Größe möglichst verlustfrei reduzieren

Delphi-Quellcode:

Function SmartShrinkBitmap0(PBitmap: TBitMap; Scale: Double): Boolean;
var
  Out_X, Out_Y: Integer;
  Source_X, Source_Y: Integer;
  ScanImageBox: TRect;
  ShrinkImageLine: PByteArray;
  SourceImageLine: PByteArray;
  ScaleF: Double;
  PixCount: Integer;
  Local_Red, Local_Green, Local_Blue: Integer;
  Local_Scr_X, local_dest_X: Integer;
  SourceBitmap: TBitMap;
begin
  Result := False;

  //  no work to to ....
  if not assigned(PBitmap) or (Scale > 1) or (Scale < 0) or (PBitmap.Width < 2)
    or (PBitmap.Height < 2) then
    exit;

  if Scale = 1 then
  begin
    exit;
  end;

  if Scale = 0 then
  begin
    PBitmap.Width := 0;
    PBitmap.Height := 0;
    exit;
  end;

  ScaleF := 1 / Scale;

  // Daten von PBitMap in new SourceBitmap Kopieren
  SourceBitmap := TBitMap.Create;
  SourceBitmap.PixelFormat := pf24bit;
  SourceBitmap.Assign(PBitmap);

  //  set values for new out bitmap
  PBitmap.Width := round(SourceBitmap.Width * Scale);
  PBitmap.Height := round(SourceBitmap.Height * Scale);
  PBitmap.PixelFormat := pf24bit;

  try // try...finally
    try // try...except
      // for each pixel in new OutBitmap do...
      for Out_Y := 0 to PBitmap.Height - 1 do
      begin
        ShrinkImageLine := PBitmap.Scanline[Out_Y];
        for Out_X := 0 to PBitmap.Width - 1 do
        begin
          ScanImageBox.Left := trunc(Out_X * ScaleF);
          ScanImageBox.Top := trunc(Out_Y * ScaleF);
          ScanImageBox.Right := trunc((Out_X + 1) * ScaleF);
          ScanImageBox.Bottom := trunc((Out_Y + 1) * ScaleF);
          Local_Red := 0;
          Local_Green := 0;
          Local_Blue := 0;
          PixCount := 0;
          for Source_Y := ScanImageBox.Top to ScanImageBox.Bottom - 1 do
          begin
            SourceImageLine := SourceBitmap.Scanline[Source_Y];
            for Source_X := ScanImageBox.Left to ScanImageBox.Right - 1 do
            begin
            ///   hier entsteht die AV ??? 

              [B]              
              Local_Scr_X := Source_X * 3;
              inc(Local_Red, SourceImageLine[Local_Scr_X]);
              [/B]              
              inc(Local_Green, SourceImageLine[Local_Scr_X + 1]);
              inc(Local_Blue, SourceImageLine[Local_Scr_X + 2]);
              inc(PixCount);
            end;
          end;
          local_dest_X := Out_X * 3;
          If local_dest_X > 32000 then
            Raise Exception.CreateFmt('Variable LDX to high ...(%d)', [local_dest_X]);
          // Raise Exception.Create ('Variable LDX to high ...('+ inttostr (LDX)+')');
          ShrinkImageLine[local_dest_X] := Local_Red div PixCount;
          ShrinkImageLine[local_dest_X + 1] := Local_Green div PixCount;
          ShrinkImageLine[local_dest_X + 2] := Local_Blue div PixCount;
        end;
      end;

      Result := True;

    except
      // If the code produces an Exception ....
      on E: Exception do
        // {--Only for debuging reasons }

        ShowMessageFmt(E.ClassName + ' error raised, with message : ' +
          E.Message + #13#10 + 'Variables values:' + #13#10 +
          '  out_X=' + Inttostr(Out_X) + ', ' + '  out_Y=' + Inttostr(Out_Y) + #13#10 +
          '  src_x=' + Inttostr(Source_X) + ', ' + '  src_Y=' + Inttostr(Source_Y) + #13#10 +
          '  ScanBOX.TOP=' + Inttostr(ScanImageBox.TOP) + #13#10 +
          '  ScanBOX.LEFT=' + Inttostr(ScanImageBox.LEFT) + #13#10 +
          '  ScanBOX.RIGHT=' + Inttostr(ScanImageBox.Right) + #13#10 +
          '  ScanBOX.BOTTOM=' + Inttostr(ScanImageBox.Bottom) + #13#10 +
          '  LDX=' + Inttostr(local_dest_X) + #13#10 +
          '  LSX=' + Inttostr(Local_Scr_X) + #13#10 +
          '  PBitmap=%p, OutBitmap=%p' +  '  DLine=%p, Dimensions='  + #13#10 +
          '  Inbmp Size ' + Inttostr(SourceBitmap.Height) + 'x' + Inttostr(SourceBitmap.Width)+ ' pixel' + #13#10 +
          '  Outbmp Size ' + Inttostr(PBitmap.Height) + 'x' + Inttostr(PBitmap.Width)+ ' pixel' + #13#10 +
          '  Scale :'  + Floattostr(Scale) +
          '  ScaleF:'  + Floattostr(ScaleF),
          [SourceBitmap.Scanline[0], PBitmap.Scanline[0], ShrinkImageLine]);

    end; // Try...except...end;
  finally
    SourceBitmap.free;
  end; // Try...finally...end;
end;

mein Problem : bei machen Bildern erzeugt die Fubktion scanline eine AV beim Zugriff auf bestimmte Pixel, ich kann mir aber keinen Reim darauf machen, auch wenn ich alle Werte ausgeben lasse , siehe screnn Dump.

Sieht jemand einen Fehler in diesem Code ?