I have an application which can capture a regon of the desktop into an image. It uses TBgraBitmap since it has a method for grabbing data from the screen.

So I can assign this to the clipboard and also save to a file:

var
  MyCapture : TBgraBitmap;
  Clip: TRect;
  ...
begin
  ...
  Clip := Bounds(Self.Left, Self.Top, Self.Width, Self.Height);
  MyCapture := TBgraBitmap.Create();
  MyCapture.TakeScreenShot(Clip);
  Clipboard.Assign(MyCapture.Bitmap);
  MyCapture.Bitmap.SaveToFile(ChangeFileExt(ParamStr(0), '.jpg'));
  MyCapture.Free;
  ...
end;

But the SaveToFile results in a file that is NOT a jpeg file at all...

How could I save it to a specific (known) format, like jpg or png?

Essentially you are asking a BGRA Bitmap to save itself to file, it is irrelevant whether the file ends in .jpg or .moonmission (although a TPicture or some custom image components may check the file extension).

What you need is to create a JPEG object, put the data in it and then use its JPEG-machinery to save the file.

I haven't tested this, typing from my head so may make syntax errors or such, but the idea is sound:

var
   jpg : TJPEGImage;

...

     jpg := TJPEGImage.Create;
     jpg.Assign(MyCapture.Bitmap);
    ... perhaps adjust the JPEG quality parameters etc.
     jpg.SaveToFIle('somefile.jpg');
     jpg.Free;

...

Note that this "jpg.SaveToFIle('somefile.jpg')" statement will ALWAYS save JPEG data to the file, even if the given filename ends in  .png or .bmp or .mooseknuckle

Hi,

Replace :
MyCapture.Bitmap.SaveToFile(ChangeFileExt(ParamStr(0), '.jpg'));
by :
MyCapture.SaveToFile(ChangeFileExt(ParamStr(0), '.jpg'));

This work because Bgrabitmap set automatically the image writer based on the extension.

Patrick

Correct!
I could remove *all* the extra code I had added to create jpg, png and bmp containers. Instead I could save directly from the MyCapture object, which has built-in magic to save files correctly based on the commonly used extesnions in the file name.

Problem solved, thanks!