I am currently drawing a isometric world in 3D, meaning the floors and walls are all quads aligned to the original axis in the world and using a rotated orthogonal camera to "distort" the images into an isometric projection.
The textures of the walls and floors are just rectangular textures on a axis aligned quad, the camera is doing the rest. I am building a single mesh from everything and adding vertices in the order of back to front manually. So no OpenGL sorting is used. The order is important since I am using alpha blending.
The other quad represents an object on a tile but can very well be a dynamic object such as a player or NPC. This is currently drawn along the regular Y(up) axis. It's texture is already painted isometric and since there is an angle it gets distorted. To counteract this I am currently stretching the quad in Y(up) direction. This works fairly well but obviously it creates artifacts, especially when it needs tiling.
I figured my best option is to draw these as camera aligned billboard but I need control of the draw order, just like I am drawing it now so I can fit these objects in between walls and other objects. However I am not sure how to create these billboards properly. I could create a quad with the screen dimensions I need and then perhaps rotate the verts or perhaps change projection matrix when I need to draw a billboard. They obviously have to be aligned properly to the world.
So how would I approach this problem in detail? Perhaps with some code how to transform a quad to face the camera and keeping it aligned with the world? In particular, since I am using a fixed camera projection I would like to leverage this and reduce calculations if that is possible.