Diablo 2 is a game with isometric graphics. However, unlike other isometric games, D2 has the option to activate perspective. Obviously, the D2 developers found a clever trick to convincingly fake perspective in their isometric engine. It has to be a fake, because D2's graphics assets consist of pre-rendered isometric 2D tiles.
I want to implement a similar effect in my isometric graphics engine, but was unable to find out how to deal with the more subtle issues of applying perspective. A similar question has been asked on this site before. Unfortunately, the answer to that question does not help with the specific problems I am facing.
Let me explain my approach in detail. I use two dimensional tiles for floor and walls that look like this: (Note that the floor tile is not actually part of the wall tiles. I added it just for clarity.)
These tiles can be assembled in 2D to create an isometric scene:
It is actually pretty easy to fake perspective in such an isometric scene. Slightly enlarging objects at the bottom of the screen and shrinking objects at the top makes them seem close or distant, and creates a subtle illusion of perspective. This is easily achieved in a vertex shader and works very well for floor tiles: . The floor looks good, but the walls do not. Vertical lines get distorted. I can fix this by applying uniform scaling to objects, instead of distorting them like floor tiles. The scale factor depends on the object's y-position. This works well for character sprites and front facing walls, but fails for diagonal walls and more complex objects.
Now the actual question: A front facing wall can be scaled by the y-coordinate of points A and B, which is the same. However, this does not work for corners or diagonal walls where the points have different y-coordinates. It seems, for an accurate perspective effect, each wall tile needs to be treated differently so they can fit seamlessly together. This does not seem feasible to me. Ideally, I want to treat all wall tiles equally.
Is it possible to scale wall tiles so they fulfill the following criteria?
- they should become continuously smaller the further up on the screen they are
- they should not appear to move or jitter when the viewpoint changes
- they should seamlessly fit together
Any help is appreciated. I've been worrying about this for days, but maybe I'm thinking too complicated :)
update: Isometric view with increasing perspective. Note what happens when wall tiles are simply scaled. This works well for front facing walls only.
I want to approximate perspective correction so that all wall tiles fit seamlessly.