I am making UI graphics for a screen with non-square pixels. To be exact, the horizontal distance between pixel centers is larger than the vertical distance.
If I draw a circle in Photoshop and export to a bitmap, it ends up a bit oval on the device screen.
I can fix this by setting the Pixel Aspect Ratio in Photoshop to match the screen. Shift-dragging a circle or square then does the right thing. It looks circular in Photoshop, the exported PNG looks oval on a PC, but then on the device screen it looks circular again.