Drawing over a JPEG image

[Aldo Caruso]

Hi,

I've a TImage which is loaded with a JPEG file (with the LoadFormFile
method).
When, in the Form OnPaint event, I try to draw over the image using the
Image.Canvas object (for example Image1.Canvas.Ellipse(x1,y1,x2,y2)), it
raises an exception with the following message: "Can only modify an Image if
it contains a bitmap".
Of course, if I load the TImage with a BMP file, there's no problem.

Thanks in advance,

Aldo

[David J Taylor]

so why not convert the JPEG to a bitmap (in code) and use that?

or Draw the JPEG onto the bitmap (create the bitmap yourself, and set the
size appropriately. Then use the JPEG's Draw method to put the pixel data
from the JPEG onot the bitmap). Check the TJPEGImage in the help file:

"TJPEGImage implements the protected Draw method introduced in TGraphic, so
it can draw itself on the canvas of another object."

David

"Aldo Caruso" <juanc...@yahoo.com> wrote in message
news:3a7c08ba_2@dnews...

[Bob Villiers]

Aldo,

The trick is to convert the jpeg to a bitmap, do any drawing on the
bitmap and then convert back to a jpeg. Below is something I put
together when I was working through these things.

Bob
----

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
Dialogs,
Jpeg, StdCtrls, ExtCtrls;

type
TForm1 = class(TForm)
Image1: TImage;
Button1: TButton;
Button2: TButton;
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
var
JPEG: TJPEGImage;
BMP : TBitmap;
begin
JPEG := TJPEGImage.create;
BMP := TBitmap.create;

JPEG.LoadFromFile( 'factory.jpg' );
{If you need to draw on a jpeg, convert to bmp and draw on the bitmap
canvas
BMP.Height := JPEG.Height;
BMP.Width := JPEG.Width;
BMP.Canvas.Draw(10,10, JPEG); }
BMP.Assign(JPEG);
Image1.Picture.Graphic:= BMP;
JPEG.free;
Bmp.free;
end;

procedure TForm1.Button2Click(Sender: TObject);
var
jp: TJpegImage;
bmp: TBitmap;
const filename = 'factory.bmp';
begin
bmp:= Tbitmap.Create;
try
bmp.Loadfromfile(filename);
jp:= TJpegImage.Create;
try
jp.Assign( bmp );
jp.SaveToFile( ChangeFileExt( filename, '.JPG' ));
ShowMessage('Bitmap converted to JPeg');
finally
jp.free
end;
finally
bmp.free
end;
end;

end.

[Aldo Caruso]

Thanks for your advice.

Aldo

[Aldo Caruso]

Bob,

Thanks for your code. But there's something I want to ask you. You've put
two alternatives (one commented and the other not) about implementing it:

{ First Alternative }

{If you need to draw on a jpeg, convert to bmp and draw on the bitmap
canvas
BMP.Height := JPEG.Height;
BMP.Width := JPEG.Width;
BMP.Canvas.Draw(10,10, JPEG); }

{ Second Alternative }

BMP.Assign(JPEG);

Why don't use allways the second one ? Is there any difference ?

Thanks in advance,
Aldo

[Bob Villiers]

Aldo,

Example. Drawing a cross on a JPEG.

var jpg: TJpegImage;
bmp: TBitmap;
begin
jpg:= TJpegImage.Create;
bmp:= TBitmap.Create;
try
jpg.LoadFromFile('test1.jpg');
bmp.width:= jpg.width;
bmp.Height:= jpg.Height;
with bmp.Canvas do
begin
Draw(0,0,jpg);
pen.color:= clRed;
MoveTo(0,0);
LineTo(bmp.width,bmp.height);
moveto(bmp.width,0);
lineTo(0,bmp.height);
end;
jpg.Assign(bmp);
jpg.SaveToFile('test2.jpg');
finally
jpg.Free;
bmp.free;
end;
end;

Bob
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[Aldo Caruso]

Thanks Bob,
Aldo