Opacity on/off via alpha of certain layer

[Riccardo Sinti]

I found this expression from dan

target = thisComp.layer(“background”);
target.sampleImage(transform.position, [width, height]/2, true, time)

this works for animating the color of a fill effect turning it black when there is no layer under it but how do I get it into an opacity expression.
I think I need the sample to reduce down to a 1 element array for opacity.
Can anyone help me in this train of thought?

Thank You

[Riccardo Sinti]

I figured it out for myself and this is what I used

target = thisComp.layer(“Audio_PC”);
rgb = target.sampleImage(transform.position, [5, .5], true, time);
hsl = rgbToHsl(rgb);
if (hsl[2] > 0) 100 else 0

I’m sure there is a way to skip the rgb to hsl conversion but I found that in another post and then just added the if else.

I’m happy

Now if some one could tell me how to ad a 10 frame fade out from 100% opacity (on) to off : )

Thank You

[Filip Vandueren]

Hi Riccardo,

using hsl doesn’t really make sense, because in your example you’re actually looking at the brightness of the pixel, not the alpha. They may be the same, but not necessarily when your layer has asoft edge for example

However, sampleimage actually returns a 4 number array: rgba,
so you can just use:

rgb[3] to get the fourth value.

Also this expression didn’t take into account layerspace transform, so I’m not sure how it could work if the layers are not compsized and moving.

I came up with this:


fd=thisComp.frameDuration;
fade=10; //frames

target = thisComp.layer("Audio_PC");
alpha=0;
for (t=0;t

In the 'for'-loop it averages out the sampled alpha's in a 10 frame window, so you get a natural fade-out.
I put the last 2 lines in so, it would flash on immediately when it enters the alpha, but fade out when it leaves. If you want it to also fade in, the last 2 lines should simply be:

alpha*100;

[Filip Vandueren]

After rereading your first post, I realised a difference in my solution: if the layer underneath has an alpha of 50%, the toplayer would also have an opacity of 50, this version should fix that:


fd=thisComp.frameDuration;
fade=10; //frames

target = thisComp.layer("Audio_PC");
alpha=0;
for (t=0;t < fade*fd; t+=fd) {
alpha += target.sampleImage(target.fromComp(toComp(anchorPoint,time-t),time-t), [1, 1], true, time-t)[3]>0 ? 1/fade : 0;
}
alpha_now=target.sampleImage(target.fromComp(toComp(anchorPoint)), [1, 1], true, time)[3]>0 ? 1 : 0;
Math.max(alpha,alpha_now)*100;