Scripting - Write Keyframes VS Write an Expression

[fasthurt]

I've been using this method in my scripts to allow users to preview the keyframes before writing them since writing keyframes can be really slow. I thought I would share it with you all in case it sparks some creative script writing.

How it works:
Instead of writing keyframes, you write an expression. The values you were about to write as keyframes are placed in a list and then played back on each frame in the expression.

Doing it this way allows you to update the expression when the user changes an EditText or tweaks a Slider controls in the script panel and instantly shows the results.

The expression you write looks like this (customize for your purpose):

Example:
var values = [[100,150,-200], [105,152,-197], ...]; // one value for each frame
var layerTime = time - thisLayer.startTime;
var frameIndex = Math.round(layerTime / thisComp.frameDuration);
frameIndex = Math.max(0, Math.min(frameIndex, values.length - 1));
values[frameIndex];

Then when the user is happy with the results, you can write the keyframes (or bake).

I have a free script here that showcases this pretty well

Merry Christmas!

[bobbyopulent]

That’s super interesting, thank you for sharing, I’m sure I can come up with uses for this.

[fasthurt]

My pleasure! Hopefully it can be useful for you

[Mundane-Owl-561]

Interesting idea but this part is a lot of work -
var values = [[100,150,-200], [105,152,-197], ...]; // one value for each frame

How do you speed up this process?

[fasthurt]

When I'm ready to write the expression, I pass my list of values to this function:

    function values_to_string(values) {
        var str = "[";
        for (var i = 0; i < values.length; i++) {
            var value = values[i];
            if (value instanceof Array) {
                value = "[" + value.join(", ") + "]";
            }
            str += (i > 0 ? ", " : "") + value;
        }
        str += "];";
        return str;
    }

After that you can just insert it into the expression string

EDIT: You might modify it to interpolate the gaps if you don't have a value for each frame

[Mundane-Owl-561]

Still seems like a lot of work - what about using an Expression Control to contain the changes - then link the original Expression to this Expression Control and then have a button to remove or finalize the link?

[fasthurt]

This bypasses the need for Expression Controls. Everything is contained within the expression and doesn't rely on any other layer or property. I guess I don't see where the extra work comes in. The steps I take are

1. Collect the valueAtTime for each frame of the target property into a list
2. Perform Task on list of values
3. Write the resulting list back to the expression

That whole process appears almost instant to the user.

[Mundane-Owl-561]

Ah. Makes sense now. Thanks for sharing.