Automatically track and adjust exposure values based on other source? Expression maybe?

[Roger Alexander]

That didn’t answer my question at all. It kind of feels like a troll reply. The city background will be far away and there will be no practical lights affecting the subject in the foreground.

My goal is that the background needs to light up when the foreground lights up. Is there an automated way to do this that anyone knows of?

[Kalleheikki Kannisto]

(Dave probably read “lighting” instead of “lightning,” that’s what I did the first time. With lighting it wouldn’t really matter.)

Ok, so you want to sample the foreground layer to see when the lightning strikes and apply it to the background as a color adjustment.

The basic method would be as follows.

1) Add a point control to the foreground layer. This is used to pick the point where you want to sample.

2) Add Brightness Contrast Effect to the background layer and use this expression for the Brightness value:

targetLayer = thisComp.layer(thisLayer.index-1);
samplePoint = targetLayer.effect("Point Control")("Point");
sampleRadius = [10,10];
sampledColor_8bpc = 255 * targetLayer.sampleImage(samplePoint, sampleRadius);
R = Math.round(sampledColor_8bpc[0]);
G = Math.round(sampledColor_8bpc[1]);
B = Math.round(sampledColor_8bpc[2]);
V = (R+G+B)/3;
V

That code will take a 20 by 20 pixel square area around the point set by a point control on the layer above (index-1), average the RGB value to a single value and apply it to the Brightness of the layer below. ANY shift in top layer’s brightness will affect the brightness of the bottom layer. Thus, for practical purposes, you’d want to give “V” a threshold level, that occurs only during the actual flashes, above which it would affect brightness. And you could sample a smaller or larger area as needed. And if there are more layers in between you foreground and background layers, you’d have to change the layer selection value (index-[number of layers here]).

[Roger Alexander]

This is exactly what I was looking for! Thanks Kalle I will try this out and let you know how it goes!

[Roger Alexander]

Hello Kalle,

This works pretty well but I have a few questions. I have tried to research this myself, but alot of it is confusing to me. I’m not really a coder so it’s like a different language and many times more complicated than what I’m looking for.

1. How do I set a minimum value for the brightness effect in the expression. For instance, my point control might set the brightness at 10 via the expression, but I want to set it so the brightness can never be below 20. So instead of 10 it stops at 20. Vice versa for max?

2. How do I set a threshold for the brightness, for instance, the values coming from the point control needs to be over “ABC” before raising the brightness.

3. How do I effect the ratios/increments for increasing the brightness. For instance a value of 1000 from the point control might result in raising the brightness value to 100. How do I modify this to where a point control value of 1000 translates to a value of 50 in brightness?

Thanks in advance!

[Kalleheikki Kannisto]

Hi Roger,

“V” (in this code) is the brightness value that you need to do the math with.

1A. Minimum value: V = Math.max(20,V); (Using Max for minimum value may seem counter-intuitive at first, but you want the higher value of these two)

1B. Maximum value: V = Math.min(100,V); (If you want to set the maximum value to 100)

2. Threshold. This conflicts with the Minimum value code in 1A but basically:if (V<20) {V = 0};
To fix the conflict, you can combine minimum and threshold into one expression: if (V<20) {V = 0} else {V = Math.max(20,V)};

3. Ratio. Linear is a nice way to do this, for example: V = linear(V,20,100,20,50);
This will remap your input values (20-100) to a new range, in this case 20-50. The new range can be any two numbers.

So, it should work by adding this to the end of the existing code (I didn’t test it):

if (V<20) {V = 0} else {V = Math.max(20,V)};
V = Math.min(100,V);
V = linear(V,20,100,20,50);

(You can drop the “V=” on the last line, as the result is used for the brightness value directly.)

Actually, that linear expression already takes care of your minimum and maximum input values. In other words, the whole thing boils down to adding this to the existing code (replacing the last line “V”):

if (V < 20) 0 else linear(V,20,100,20,50);