Using a Foreground Alpha image provides the control you
might need for specific keying purposes.
The alpha image you used was created in Adobe Photoshop from the original Fountain
image. It has had its sky painted black, while everything else is painted white. The white
areas block the objects from rendering, enabling the foreground image to show, thereby
making it look as if the moon is behind the buildings and trees.
Now you can see that the rendered image appears much as it did with the Foreground Key,
but this time using an alpha image, which is a bit more precise.Using an alpha image gives
you much more flexibility in determining where your foreground image appears. It is also
more accurate than using a range of colors to clip the image.However, both methods are
suitable depending on the project and the images at hand.
Using alpha images when compositing gives you the most control over your scene because
the alpha image can be used to shape the foreground image into any shape you desire.
The situation outlined in the previous exercise would be fine if your 3D object needed
only to be placed behind the buildings and in front of the sky.
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