Masking is a similar to clipping, except that clipping defines an area of a graphic element
that will be visible and makes a sharp boarder between the visible and clipped areas.
A mask defines an area where each pixel of the mask has a degree of transparency and opaqueness
that lets some of the graphic element show through, making a blurry boundary between the
visible and masked areas.
In the example shown above, a radial gradient is defined and given an id of "fade".
A mask is defined and given an id of "fademask". The radial gradient is then used as the
fill of the mask. The image leaves.jpg is then loaded with xlink:href
and the mask is used with the mask attribute in the leaves image tag.
A mask works by using its alpha channel to allow or block pixels of the image from showing
through. The less opaque a part of the mask is, the less visible that part of the image
will be. you can make the mask stop-colors any color you like, but it will turn out white
in the result because mask uses only the transparency bits of the mask.