ImageScaler | Simple but powerful batch image rescaler | Computer Vision library

I have a problem that I haven't been able to completely understand and thus I am struggling to fix it.

Basically I am busy writing a small game engine for Java Swing, and one of the key components of this engine is the ability to separate design resolution from screen resolution. Meaning if I design a game on a resolution of 400 (w) x 300 (h), and I position an object at the center of the design resolution, then a user can specify the actual resolution they want to play the game at for example 800 (w) x 600 (h) and the object will still be placed correctly at the center of the screen in the current resolution.

This is where I am having trouble, when the design resolution and the current resolution are the same i.e. design resolution 400 x 300 and current resolution is 400 x 300, the object seems to be placed correctly at the center of the screen on start up and the bullet correctly at the center of the player regardless of the players position when moved:

However when the design resolution and current screen resolution are not the same i.e. design resolution 400 x 300 and current resolution is 800 x 600 the object is no longer correctly placed at center of the screen and neither is the bullet centered for the player:

I have a method to generate the center spawn point for all visible objects (the red reference dot, the sprite/player and the bullet) this method is a simple convenience method to help generate a center based coordinate for a Sprite within a container or another Sprite: