1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:
39:

function ResizeJPEG(jpeg: TJpegImage; width, height: integer): TJpegImage;
var
  bmp:TBitmap;
begin
  try
    bmp:=TBitmap.Create;
    try
      bmp.width:=width;
      bmp.height:=height;

      bmp.canvas.StretchDraw(Rect(0,0,bmp.width,bmp.height),jpeg);
      jpeg.assign(bmp);
    finally
      bmp.free;
    end;
    jpeg.CompressionQuality := 100;
    jpeg.Compress;
    result := jpeg;
  finally

  end;
end;

procedure SavePNG(name: string; width: integer; height: integer);
var
  JPG:TJPEGImage;
  BMP:TBitmap;
  PNG:TPNGImage;
begin
  JPG:= TJPEGImage.Create;
  BMP:= TBitmap.Create;
  PNG:= TPngImage.Create;
  JPG.LoadFromFile('img/'+name);
  JPG := ResizeJPEG(JPG, width, height);
  //JPG.SaveToFile('img/'+name);
  BMP.Assign(JPG);
  PNG.Assign(BMP);
  PNG.SaveToFile(ChangeFileExt('img/'+name,'.png'));
end;					

1:
2:

PNG.Assign(JPG);
PNG.SaveToFile(ChangeFileExt('img/'+name,'.png'));					

1:

TJPEGImage kann nicht zu TPNGImage zugewiesen werden.					

1:
2:
3:
4:
5:
6:

bmp.width:=width;
bmp.height:=height;

bmp.canvas.StretchDraw(Rect(0,0,bmp.width,bmp.height),jpeg);
png.Assign(bmp);
png.SaveToFile('test.png');					

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:
39:
40:
41:
42:
43:
44:
45:
46:
47:
48:
49:
50:
51:
52:
53:
54:
55:
56:
57:
58:
59:
60:
61:
62:
63:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, JPEG, StdCtrls, ExtDlgs;

type
  TForm1 = class(TForm)
    OpenPictureDialog1: TOpenPictureDialog;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  bmp: TBitmap;
  jpg: TJpegImage;
  scale: Double;
begin
  if OpenPictureDialog1.execute then
  begin
    jpg := TJpegImage.Create;
    try
      jpg.Loadfromfile(OpenPictureDialog1.filename);
      if jpg.Height > jpg.Width then
        scale := 500 / jpg.Height  //org 50
      else
        scale := 500 / jpg.Width;  //org 50
      bmp := TBitmap.Create;
      try
        {Create thumbnail bitmap, keep pictures aspect ratio}
        bmp.Width := Round(jpg.Width * scale);
        bmp.Height:= Round(jpg.Height * scale);
        bmp.Canvas.StretchDraw(bmp.Canvas.Cliprect, jpg);
        {Draw thumbnail as control}
        Self.Canvas.Draw(100, 10, bmp);
        {Convert back to JPEG and save to file}
        jpg.Assign(bmp);
        jpg.SaveToFile(
          ChangeFileext(OpenPictureDialog1.filename, '_thumb.JPG'));
      finally
        bmp.free;
      end;
    finally
      jpg.free;
    end;
  end;
end;

end.