Limit alpha to just a portion of the layer for use with sampleImage

[Dan Ebberts]

I think you probably just need to add sampleImage()’s third parameter (postEffect) and set it to true.

Dan

[Mitch Mann]

Just use a track matte, I’m not sure why you need expressions here.

[David Cabestany]

Thanks Dan, much appreciated as always.
How do I add that parameter, I tried typing it after the point control and before the arrays but didn’t work, can you tell me its exact syntax?

I checked your blog as well Adobe’s entry on sampleImage but I’m not very clear on where to include the third and fourth parameters.

Again, thank you very very much.


alpha=target.sampleImage((thisComp.layer("target pos").effect("Point Control")("Point"))(postEffect=true),[.5,.5])[3];

[David Cabestany]

The object I’m trying to make transparent is an audio spectrum, I might change the expression to drive the height of the peaks instead of the transparency of the layer instead, the same beam also drives an if/the expression on text, when it’s fully opaque it shows some random reading but when it’s transparent it needs to show 0, so a track matte is not helpful in this case.

Thanks,

[David Cabestany]

I think I got the syntax right, but the behavior is still not as expected, I now the expression is functional because if I change the value of alpha to 0 the layer reacts to it, it’s just not changing each frame as I ned it to.

target=thisComp.layer("beam");
alpha=target.sampleImage((thisComp.layer("target pos").effect("Point Control")("Point")),[.5,.5])[3],(postEffect=true);

if
(alpha=1)
{
100
}
else
{
0
}
;

[Xavier Gomez]

Hello, i haven’t read the full thread but i noticed you mispelled your condition: if (alpha=1) should be if (alpha==1). It might fix it.

By the way the whole last statement can be shortened to:

(alpha==1) ? 100 : 0;

Xavier.

[David Cabestany]

Thanks Xavier, I added the extra = sign but no luck, it change the return value to 1% but that was it, it still doesn’t change over time.

My new expression reads like it’s shown below.

Best,
D.

target=thisComp.layer("beam");
alpha=target.sampleImage((thisComp.layer("target pos").effect("Point Control")("Point")),[.5,.5])[3];

(alpha==1) ? 100:0;

[Xavier Gomez]

I dont see how you can get 1% opacity since your expression can only return 0 or 100.
Two things that might happen:
– the alpha at your beam endpoint is never exactly 1, hence your expression always returns 0;
– you are sampling the wrong point (this could come from the point control attached to the null).

I tried this expression on a fake project and it did what it is supposed to do:

target = thisComp.layer(“beam”);
P = thisComp.layer(“beam”).effect(“Beam”)(“Ending Point”);
target.sampleImage(P,[.5,.5],true)[3] * 100;

If it works for you too then you can replace the last line with:
alpha = target.sampleImage(P,[.5,.5],true)[3];
(alpha==1) ? 100 : 0;

Xavier

[David Cabestany]

I don’t know either, but that’s what’s happening, as soon as I change the = to ==, >= or <= the value drops to 1.
I checked the point form where I’m sampling by zooming in as much as I could and reading the alpha value on the info panel, I’m getting absolute 0.
The point I’m sampling is the only thing on the null and I even tried it on a dummy comp with nothing more but a solid, no matter where I placed the point I wasn’t able to get the expected readings, yet I imported an older project with a similar expression but applied to an optical flare and it works as expected.

I’ll try your revised expression first thing tomorrow, thanks a lot for taking the time.

I can post the project if anyone wants to take a look.

I’m completely puzzled about this.
David.

[David Cabestany]

Hey Javier I just tried the expression that you kindly wrote, it works perfect! Thanks a lot, you just saved me a ton of keyframe, if at all possible to keyframe that!

Best,
David.

Below is the revised version to drive the text, in case anyone needs it.

posterizeTime(10);
target = thisComp.layer("beam");
P = thisComp.layer("beam").effect("Beam")("Ending Point");
alpha = target.sampleImage(P,[.5,.5],true)[3];

if (alpha==1)
{

n=random(-16,24);
n.toFixed(3)+" dB]";
}
else{
0+"dB]"
};