There are two images in this post. The left image is a PNG file and the right is a JPEG file. Drag each photo over some text in your browser. You'll notice that the JPEG file has opaque white space in between the black lines. The PNG file has the white space removed.
The image of Da Vinci is in the public domain, and not protected by copyright. Right? But if an artist went about selectively removing the white space from the image and created a PNG file, would that be entitled to protection as a derivative work? The artist has added nothing--he's only subtracted white space. But if he selectively removed white space, would the image then be a unique work of authorship?