Fill or swatch that makes layers underneath transparent?

Question

I'm working on a vector with Illustrator CS3, and soon I'm planning to have many of the elements animate. What I'm looking for is a swatch or something that I can fill a shape with that will occlude everything on every layer underneath that shape.
To explain further, let's say I have (in ascending order) two layers. Layer 1 is filled entirely with yellow. Layer 2 has a shape of a circle with a black stroke and no fill. Since it has no fill, the yellow can be seen through the inside of the circle. What I want to do is turn the circle into something akin to a window or an X-ray device by using something (what it is, I'm not sure) that, if I fill the circle with it, will let me see through the yellow on Layer 1. And I want to be able to do this without permanently or "physically" altering Layer 1 (such as selecting the innards of the circle, switching to Layer 1, and deleting the selection), so that I can move the circle around and it will remain occluding the objects underneath.
Does such a thing exist? Or are there any effective alternatives that I can use? If so, I'd be most grateful for your advice.

Answer 1

Nicholas,
It is unclear from your description what you want the scope of the cutaway effect to be.
If your entire file consists of those two layers, then there is nothing below the yellow on Layer 1, so what would it mean to "see through" the yellow?
On the other hand, if there is something below the yellow on Layer 1, then you would need some way to limit the scope of the "x-ray lens" circle so that it didn't cutaway the entire file down to the bottom.
So let's assume that what you want is a circle that will cut away (or knockout) everything below it in the same layer (where the "same layer" may contain other sublayers.) In your example, both the circle and the yellow square would be in the same layer.
To get this effect, you would open the Layers panel, and click on the target circle for that layer. Then open the Transparency panel, and turn on the "Knockout Group" checkbox. This option means that all objects with reduced opacity in that layer fully knock out everything below them in the same layer. (The objects keep whatever transparency they have with respect to objects outside that layer.)
Then select the circle, and give it any fill color, e.g., white. Then go to the Transparency panel, and reduce the Opacity of that circle to 0%. It will then knock out the objects below it in the same layer, and since it is fully transparent, it will otherwise seem invisible. (Since white and invisible look the same if there is nothing below them, it might make it more apparent what is going on if you fill the circle with a color like red and give it 10% opacity.)
To better observe the knockout behavior, you will want to have another layer below that layer with some stuff that you can see through the circle as it moves around.
You can save the 100% transparent white fill as a style in the Graphic Styles panel, but you would need to separately turn on the Knockout Group attribute of the layer or group that contains the knockout objects. (Swatches cannot contain both color and transparency, but graphic styles can.)
This feature has been available ever since AI 9, when transparency was first introduced.
It also works with groups instead of layers, I just described it with layers since that's the way your query was posed.