Emphasis: Used so our eyes will only concentrate on a certain part of a pictue, whether its been coloured differently than the rest, the background is blurred or black and white its useful to make a certain part stand out.
Value: Value works with lightness and darkness of picture, almost using a black and white frame. With different shading it can make a certain part stand out if needed, so it all fits together. If an item is black and white background, if an item is white add a black background.
Line: Line is known to be the most basic part in design elements, there is a lot of variation to it however, including vertical, horizontal, diagonal, curved, etc. Each line can also be different thickness and length, making each line unique and making it stand out with the rest of the art.
