Fake distance scaling

I'm trying to create an effect where object shrinks as it goes farther from camera (upwards on screen) and gets bigger when it gets closer (downwards). There needs to be a set Y position that determines where the object's size is at default. I just can't seem to wrap my head around how to do this.

Choose where Y = default size 'Ydef'.

Choose a scaling rate 'Srate'

Sprite > set size

Width = lerp( Sprite.ImageWidth / Srate , Sprite.ImageWidth , (Sprite.Y/Ydef) )

Height = lerp( Sprite.ImageHeight / Srate , Sprite.ImageHeight , (Sprite.Y/Ydef) )

In my head this works.

Lerp would certainly work, although it might be easier to just calculate a scale factor...

if the top of the screen is 0 and the bottom is 800. And you want it to be full size (100%) at the bottom, and say 10% at the top then: (whenever you change the y value)

sprite > set scale to Sprite.Y/800 * 0.9 + 0.1

if the screen bottom is 1024 and you want it to be 20% when at the top:

sprite > set scale to Sprite.Y/1024 * 0.8 + 0.2

You would also want to adjust the speed as it shrinks to help sell the idea that it is moving away from you...

EDIT: ok, just read your post again, you mention that there would be a set point where it would be full size... so you want to clamp it

at the top of screen set it to 10% and at y=500 and below have it at 100%:

sprite > set scale to Clamp(Sprite.Y/500 * 0.9 + 0.1, 1, 0.1)

clamp will now prevent it from getting bigger than 100%, or smaller than 10%, so it will stay 100% between 500 and the bottom of the screen, but will scale smoothly towards the top of the screen once y is less than 500...