Can a keyframed blur be limited by a vignette?

Question

I've just worked through Lesson 8 in the Wohl/Gross book and now have a question on the limiting power of vignettes.
The effect I would like to achieve is a kind of digital rack focus, I guess. I have a guy sitting on a chair in the background in the middle of the frame. The guy is framed on either side by two other guys in the foreground. The entire frame is in focus. I would like to start the clip with the background guy a little out of focus, and then have him come into focus over maybe 20 frames.
I am able to keyframe a blur to achieve this focus animation in the Color FX room -- except that the blur applies to the entire frame, not just the background guy.
I'm wondering if this Color FX room animated blur node can be linked up with a limiting, soft-edged vignette in the Secondaries room? My attempt to use the vignette node in the Color FX room to limit the blur to just the background guy was a total failure. Of course, it could be newbie error.
Any advice much appreciated,
Jim

Answer 1

Thanks so much for the advice, Ola!
I was able to get the effect by running the vignette node into the right side of the alpha blend node, and the blur node into the middle of the alpha blend. That limited the blur to the inside of the vignette. Putting the blur node in the left side of the alpha limited the blur to the outside of the vignette. So with a keyframed blur going from blurry to in-focus, limited to the inside of the vignette, there it was, sort of a rack focus effect limited to part of the frame.
Just as a strange aside, I thought that checking the "invert" box on the vignette node would swap the blur effect from the outside to the inside of the vignette, but it did not appear to do anything. So I'm puzzled as to what the check box does.
And, for anyone else thinking about a digital rack focus effect, Ola's suggestion got me to thinking about an alternate way, that, in my case, gave me a little more control over the area that goes from blurry to in-focus.
I took my 25-frame clip that I wanted to rack focus on. In Final Cut Pro, add a defocus filter and animate it to go from 16 blur (seemingly the limit of the effect) to 0 over the 25 frames. Then export that clip as a quicktime movie. So this entire clip goes from blurry to in-focus. This is your "clean plate." Export the same 25-frame clip without the blur. Now you have an overlay clip that is identical to the "clean plate," except it is always in focus.
Bring both of these clips into Shake. Hook the always-in-focus clip to the left side of an Over node. Hook the "clean plate" blurry-to-in-focus clip into the right side of the Over node.
So now the in-focus clip is layered on top of the blurry clip.
Now rotoscope out the sections of the in-focus clip that you want to be replaced by the keyframed blur clip.
Add a rotoshape node. Attach a blur node to the rotoshape, if you prefer to fuzz the edges of the rotoshape. Add an invert node to the blur node to flip the rotoshape to it lets the blurred clip show through, that is, it knocks out the in-focus clip wherever you draw your rotoshape.
Now attach the invert node to the right-hand side mask input of the in-focus clip.
Now click on the right-side of the rotoshape node, and the left side of the over node.
You are ready to draw your rotoshape. In my case, I drew the rotoshape around my background elements. My foreground elements were two guys standing in a forest. This kept the two guys in focus throughout the 25-frame clip. But the rotoshape allowed the blurry clip to show through the background area of the in-focus clip, and it went from blurry to in-focus over the course of the 25 frames. Then you export this composite with a file-out node to get your final 25-frame movie.
The finished effect, then, is the two guys in the foreground are always in focus, but the background of the clip moves from out-of-focus to in-focus over the 25 frames.
Really, though, the only difference I found in doing this in Shake as opposed to Color is, I had a little more control in drawing my rotoshape, which had a lot of intricate cutting out to do, around the two in-focus guys, whereas the vignette in Color (I guess it is like the equivalent of the rotoshape in Shake) is just sort of a circle that you can change the shape of, rather than adding bezier points to define the shape.
To the real Shake pros, I apologize for even posting such a baby Shake script, but I am a real newbie and for me, this qualified as a breakthrough.
Anyway, thanks, Ola, for the advice on using the alpha blend node in the Color FX room. I had no idea what that was, but I can see where it is useful.
Jim Bee