Creating a tiled world with OpenGL

Question

I'm planning to create a tiled world with OpenGL, with slightly rotated tiles and houses and building in the world will be made of models. Can anybody suggest me what projection(Orthogonal, Perspective) should I use, and how to setup the View matrix(using OpenGL)?

If you can't figure what style of world I'm planning to create, look at this game: http://www.youtube.com/watch?v=i6eYtLjFu-Y&feature=PlayList&p=00E63EDCF757EADF&index=2

Answer 1

The projection used by that video game looks Oblique to me. There are many different projections, not just perspective and orthographic. See here for a list of the most common ones: http://en.wikipedia.org/wiki/File:Graphical_projection_comparison.png

You definitely want perspective, with a fixed rotation around the X-axis only. Around 45-60 degrees or thereof. If you don't care about setting up the projection code yourself, the gluPerspective function from the GLU library is handy. Assuming OpenGL 2.1:

glMatrixMode(GL_PROJECTION); //clip matrix
glLoadIdentity();
gluPerspective(90.0, width/height, 1.0, 20.0);
glMatrixMode(GL_MODELVIEW); //world/object matrix
glLoadIdentity();
glRotatef(45.0f, 1.0f, 0.0f, 0.0f);
/* render   */

The last two parameters to gluPerspective is the distance to the near and far clipping planes. Their values depend on the scale you use for the environment.

Answer 2

Using Orhtogonal vs Perspective projection is entirely an art style choice. The Pokemon serious you're talking about is orthogonal -- in fact, it's entirely layered 2D sprites (no 3D involved).

OpenGL has no VIEW matrix. It has a MODELVIEW matrix and a PROJECTION matrix. For Pokemon-style levels, I suggest using simple glOrtho for the projection.

Let's assume your world is in XY space (coordinates for tiles, cameras, and other objects are of the form [x, y, 0]). If a single tile is sized 1,1, then something like glOrtho(12, 9, -10, 10) would be a good projection matrix (12 wide, 9 tall, and Z=0 is the ground plane).

For MODELVIEW, you can start by loading identity, glTranslate() by the tile position, and then glTranslate() by the negative of the camera position, before you draw your geometry. If you want to be able to rotate the camera, you glRotate() by the negative (inverse) of the camera rotation between the two Translate()s. In the end, you end up with the following matrix chain:

output = Projection × (CameraTranslation^-1 × CameraRotation^-1 × ModelLocation × ModelRotation) × input

The parts in parens are MODELVIEW, and the "^-1" means "inverse" which really is negative for translation and transpose for rotation.

If you want to rotate your models, too, you generally do that first of all (before the first glTranslate().

Finally, I suggest the OpenGL forums (www.opengl.org) or the OpenGL subforums of www.gamedev.net might be a better place to ask this question :-)