   2.5D game,2D map with 3D charactor on it
  I render the map in the ortho mode,and render charators in the perspective mode.nothing strange in ortho mode,but in perspective mode.I place camera at (0,32,32),and look at (0,0,0),viewport is (0,0,800,600),and 2D(400,300) is the 3D point(0,0,z) that project to.when I want to place a charactor at (100,100),I unproject the coordinate (100,100) to the 3d space,then render the charactor.question is how can I make the charactor same size at any coord?because of perspective projection,the charactor is not the same size when it was placed at diffirent screen coordinate.thanks.   
  sorry,I post this topic at GameProgramming,but no answer,so I post here.   ;  Why in the world are you mixing perspective and orthogonal? You could try some funky hackish scaling parameter based on an object's depth in the scene, but I really don't understand why you'd want to do it that way. Maybe some more explanation of what you are trying to accomplish...   ;  I'm not sure but I think your problem might be with your field of view being too high, move your camera about 1000 units back then make the field of view really low and that should drastically reduce your problem, it might not make it completely dissapear but it should be acceptable.   ;  Have you considered a 2 step approach? I don't think it would be a speed killer. Maybe like so:1.) Render 3d model to texture.2.) Blend texture quad to where you want it.You could even update the texture every other or every few frames, like a dynamic 3d model 2d sprite... thingie.This should solve any 'size' issues caused by projection, since placement won't count during the 3d render portion.   