[Lazarus] How to save screenshot to jpg and png format files

[R.Smith]

On 2021/04/29 17:49, Bo Berglund via lazarus wrote:
> 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