Victor Maldonado describes how to use Final Cut Pro filters with Aperture.

The levels adjustment control has a nice row of draggable knobs along the bottom and Aperture users are familiar with these. What is not so obvious is that the four points across the top that just look like markers are draggable too. It's an odd interface design that gives otherwise identical controls a different appearance.

Here is the levels adjustment control with the quarter points shown (click the button below the cog to switch between one and three control points):

And after moving the output (top) points, still leaving the source (bottom) points alone:

Unfortunately there is no numerical display or input on the output points, and that reinforces the idea that these points are fixed. How do I communicate the values that I have chosen to someone else?

There is more than simple dragging. Option-dragging will keep the lines at the same angle and move both end points. That is how I got this:

Command-clicking an end point will straighten the line keeping the clicked point stationary, like this:

And this gives a way of figuring out the numerical value of the output points. Command click on one of them to straighten the line, then read off the value from the box at the bottom. Use command Z to undo that, and then repeat with the other two points.

To set the output points given a set of values, type the values into the boxes at the bottom, straighten the lines by command-clicking on the bottom (source) points, then adjust the source points to their correct values.