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You can right click on a scale, hit Edit, and then enter scale in Pixels, inches , % of Comp, of source....
I would like to set the scale in pixels. How can set the units using expressions?
I tried returning ["200px","200px"], but that is an error.
The size in pixels is easily computed as the source's original dimensions multiplied by the scale. Or in other words
p = d × s
where p=pixel dimensions required, d=source dimensions, s=scale.
So to set the scale so that it takes a given source and makes it conform to a desired dimensions:
s = p / d
In an expression on the scale property you'd use
var p = [123, 456]; / desired output dimension
[ p[0] / width, p[1] / height ] * 100; // the *100 is because scale is a percentage
To be really fancy you could repurpose the scale value input so that it controlled the pixel dimensions. This means if you want the layer's dimension to be [123,456] you'd just dial it in in the scale control:
var d = value; //returns the pre-expression value of the property
[ p[0] / width, p[1] / height ] * 100
