I think you jumped the gun a little. Were you able to use the screenshots after you quit the dvd player? I didn't think so.
That's because (at least this is what it looks like) as far as MacOS is concerned, the DVD window is just a gray square. The DVD software paints over that gray square with the DVD content. So when you take a screenshot, you get an image of that gray square. Now, when you open that image, guess what you get. If you open it immediately after taking the screenshot, it looks like you were successful. But now try moving the window around. Lo and behold, the image you thought you took a screenshot of is still dependent on the DVD player window. Try it and you'll see what I mean.
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That's because (at least this is what it looks like) as far as MacOS is concerned, the DVD window is just a gray square. The DVD software paints over that gray square with the DVD content. So when you take a screenshot, you get an image of that gray square. Now, when you open that image, guess what you get. If you open it immediately after taking the screenshot, it looks like you were successful. But now try moving the window around. Lo and behold, the image you thought you took a screenshot of is still dependent on the DVD player window. Try it and you'll see what I mean.
If there is a way, please prove me wrong! ;)