Everyone knows that Sonic was a platformer in his glory days. Zooming from left to right; running through loops and corkscrews, jumping up and down collecting rings and smashing badniks at breakneck speeds. Well... maybe not so much at that speed. Sonic ranked high in platforming games up until the mid 1990s. 1998 saw him make the transition to 3D and he sort of lost his appeal ever since. It’s taken along time to grasp that the best place for Sonic in terms of level design and game play should be 2.5D; and with some areas in 3D like Sonic Colors and Unleashed.
ADR was originally going down the path of the adventure series, primarily in 3D. This was proving quite a challenge in terms of planning levels.
The game world for any map inside the Unreal Engine is limited to 10 'Real world' square kilometres. In order to make better usage of the space we have; we had to resolve certain design limitations. Render times and world space Vs speed.
A full 3d level has to render what is in front of the player. Occlusion is good for the most part but when there is a lot of geometry and action going on in front of the player this can cause computer slowdowns. In the 2D perspective, anything in the distance can be very low poly. Add a post process volume to blur the distance slightly and no one would be the wiser if it was low poly or high poly intensive. Meaning: the more polygons on a single model, the more time it takes to render the screen frame by frame.
Space Vs Speed
With sonic not being the typical platform character with his speed boosting ability, running from one side of the world map to the other took approximately 3 to 4 minutes. This in game play terms isn’t quite good. He was too damn fast.
Slowing him down was an option but that also made him lack a certain degree of character.
Scaling down all in game assets and the player character was another option, but that can also cause a lot of problems on the programming side of things, like breaking something with the physics code calculations and can also defiantly confuse the rendering engine.
From watching various Sonic: Unleashed videos it looked like the game levels encompass several miles due to the speed at which Sonic runs at. Level design had to practically move to a more orientated 2D perspective instead of 3D. Thus sort of keeping his speed, bringing rendering times down and bring ease to level designing.
In the end it was decided 2.5D took more precedence over 3D in ADR.