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Control Choke Intensity with Layer Alpha

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Brett Harding
Control Choke Intensity with Layer Alpha
on Feb 25, 2019 at 9:36:29 pm

I am trying create a wipe in which a layer's alpha is unchoked from left to right, so that it appears to grow on. I've tried isolating the area I want choked with a matte, but cannot figure out how to drive the choke so that its intensity is responding to the alpha, not just fading up from no choke to full choke, which is what happens when I feather the alpha. Is there a way to link the simple choker to the alpha channel of the matte so that it chokes in degrees from 0-10 based on the alpha's 0-1 value?


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Dave LaRonde
Re: Control Choke Intensity with Layer Alpha
on Feb 26, 2019 at 5:35:41 am

Can you explain what you intend the viewer to see when you say "grow on"?

Trees grow....
A colony of bacteria grows...
Children grow...
Grass grows...
Crystals grow...

So a little bit of elaboration would be helpful.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Brett Harding
Re: Control Choke Intensity with Layer Alpha
on Feb 26, 2019 at 6:11:34 pm

Grow meaning go from choked to un-choked. Thin to thick. Effect-on to effect-off. But asymmetrically across the text so it if the word were "WORD", then 'w' would start unchoking first, then 'O', then 'R', then 'D', but the thickness should also grow across the letters from one side to the other, not just one character at a time, so I would rather stay away from animating the per character stroke property in the text.

The way I've been trying to do it is building a matte with a ramp inside a pre comp, animated laterally with the position property. This is placed above an adjustment layer that is luma matted to it. This doesn't work because the adjustment layer opacity is what is being controlled by the pre comp, not the intensity of the effect on the adjustment layer, so instead of getting varying width of choke, I get varying opacity of effect layer, effectively mixing between full choke and no choke across the area of the ramp.

Let me know if there's anything else I can be more specific about. Thank you!


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Dave LaRonde
Re: Control Choke Intensity with Layer Alpha
on Feb 26, 2019 at 7:03:15 pm

That helps, but I don't think you'll like my answer.

If I were doing this, I would create the text on individual layers, one layer per character. Then I'd animate multiple instances of Matte choker to get the character to appear at the speed I wanted.

After that, it's just a matter of copy-pasting the instances of the effect, along with the keyframes. Move the keyframes as a group to get the timing of the growth the way you want it.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Brett Harding
Re: Control Choke Intensity with Layer Alpha
on Feb 26, 2019 at 7:15:05 pm

Thanks David, that makes sense. I was hoping for something like an expression that would tie the choke value to the pixel alpha/luma value in the matte, but couldn't figure out a script that would look at each pixel in the layer discreetly.

i.e. if the alpha at a given pixel is .5 the choke effect would be 50%. If it's .75 ten pixels up, then that pixel is 75% of the effect and so on.


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