Resize selection/layer/object, but keep number of pixels?

[arthurpopular]

Let's say I've got a red rectangle, 200x300px.

I copy the lower half of it (200x150px), paste that as a new layer, and make it black...

Now I want to be able to transform that black layer, but keep the exact number of pixels.

In other words, I want to literally change the shape, but not scale it up or down.

For example: if I tried to make it higher, I'd want the width to automatically decrease.

Is this at all possible?

[creative explorer]

@arthurpopular top of my head, I don't think so. What you're trying to achieve—changing the shape of a layer while keeping the exact number of pixels—is not a built-in feature of Photoshop's transform tools. The transform tools scale pixels, meaning they either add or remove them as you resize. There isn't a function that automatically calculates and adjusts one dimension to compensate for a change in the other in order to maintain a constant pixel count.

[Glenn 8675309]

Why?   Seems like waaaaaay to much work to me.   What is your goal?  Yes "chage the shape" What shape?    Your sizes mentioned indicate an icon / thumbnail size.

[arthurpopular]

The sizes I mentioned aren't really relevant, I just pulled them from nowhere to present an easy example of what I'm trying to accomplish. My ultimate goal is to get a certain percentage of a full image, and then be able to play around with that to see where else in the full image it could fit by changing the shape of it, but not decreasing/increasing the size of it.

[Semaphoric]

This reminds me of when I was comparing the areas of old Standard Def CRTs to LCD HDTVs. You could use a calculator to find the area of a selection, Original Width × Original Height, divide this by New Width or Height to get the other dimension, and enter this in the Options for Select > Transform Selection. You can do similar with a rectangular  Shape or Path.

Granted, this is a somewhat Stone Age way of doing it, but it should be scriptable.

[D Fosse · Accepted solution]

@arthurpopular  Short answer:

A Photoshop document is defined by the number of pixels w x h.

Altering the size of a part of it, will by definition alter the number of pixels it occupies within the document.