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OpenGL tilemap in 3d not 2d - 3d newb

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So far I have been doing my tilemaps/terrain in 2d via quads the same way I would do them in 2d - that is "blit" each tile that is visible on the screen from left to right - top to bottom. I have come to realize that I could be doing much better looking stuff easier in 3d if I only had a clue. The fact is I can fake 2d with OpenGL pretty easily and still have great performance. But why do this when I could use 3d directly and get even better performance? - Answer- I just don't understand all of the 3d concepts I feel that I should to be able to do this. For instance. I know I should be rendering my terrain as som kind of "mesh" in order to light it properly but everytime I look it assumes I even know what a mesh is and how to make one. I get that it is basicly a "mesh" of vertecies and I can create one from different formats of data but then come the glLoadIdentity and glTranslate calls. Unfortunately I was never any good at calc/trig in school and the matrix math stuff eludes me to be quite honest. So, onto my initial question... How do I go about creating a tilemap ( say 3 screens wide by 4 screens the other way ) using 3d instead of just texturing quads - my current 2d mindset? If I am reading my literature right this will enable me to use 3d world space and lighting and loads of other goodies for virtually free. Please bear in mind the following if you answer - although I have read and read OpenGL tutorials, referenced the red and blue books, I still do not fully understand the concepts of: 1. what matrix mode I am in and why it makes a difference 2. the term "units" is pretty vague when speaking to someone coming from 2d where I speak in pixels although I think I slightly understand this concept. 3. heightmap is a foreign term to me. I know what it is supposed to represent but I have no idea how to load it and put it to use (other than copy paste). 4. loading a texture for anything other than mapping it as it is to a quad (triangles are new to me as well). There is more but I think this is way more than I should have written in my first post.

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It's quite simple to create a map:
Keep it 2D!

At least the structure of it.

Use your basic grid but instead of blitting tile A, draw model A instead.
How to load models/heightmaps are available on the net.

Basically a Heightmap is a grid (much like your gamemap, but with the numbers representing height instead of a tilenr) which is looped through with:

glBegin(GL_TRIANGLES);
for (x < width-1)
for (y < height-1)
glVertex3f(x+1, height(x+1,y+1), y+1); //corner A
glVertex3f(x, height(x,y+1), y+1); //corner B
glVertex3f(x, height(x,y), y); //corner C
glVertex3f(x+1, height(x+1,y), y); //corner D
glEnd();

float height(int x,int y)
{
// reads height from heightmap data (i.e standard grid)
}




Code is not tested, and is pseudo.

Using the various techniques has their own pro/con's:

Heightmap:
Pro: Easy collision detection, quite fast.
Con: No extravagant details, no cliffs etc.

Models:
Pro: Can be any shape
Con: Harder to collision detect.


Hope all goes well!

Cheers!

/Robert


EDIT:

nehe.gamedev.net <-- LOTS of excellent tutorials for basic/immediate and advance usage!

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You must know how a point given in 3D world is mapped to be displayed on the 2D screen. Verbosely, a vertex is a point where faces (usually triangles) of a mesh have a corner. Those vertices of a mesh are specified not in the worlds global co-ordinate system, but in a local co-ordinate system. This is senseful to be able to use the same mesh (i.e. vertices at the respective same vertex positions when seen in local co-ordinates) regardless where in the world the mesh as a whole is located. However, when rendering, the mesh actually appears at a global position, and hence the vertices _are_ to be transformed from the local to the global space. This is done on-the-fly by what OpenGL calls the Model transformation.

Next, you as the observer of the scene be located also somewhere in the global space. If you move you see another cut of the world. So, what you have to display ("what you see") doesn't depend only on the placement of meshes but also of you (most often expressed as a camera). This is what OpenGL calls the View transformation.

Still being in 3D, now its time to map the cut scene onto the screen. This can be done (you already know that) in different modes, mainly in perspective or orthogonal projection.

So coarsly you have to transform a vertex like this:
local (3D) -> global (3D) -> view/camera (3D) -> screen (2D)

In OpenGL you assemble the part local (3D) -> global (3D) -> view (3D) in the so-called MODELVIEW matrix (in D3D these both parts are more distinct), and the part view (3D) -> screen (2D) in the so-called PROJECTION matrix.

Both of these matrices are seperated by the matrix mode. The PROJECTION matrix is normally set up once and used throughout the whole lifetime of the application. The MODELVIEW matrix is normally set up per frame, because the camera stuff is changed all the way.

OpenGL is normally in MODELVIEW matrix mode. Switching is to done by yourself. So somewhere at the (logical) beginnging of the app you switch to the PROJECTION matrix, set it up, switch back to the MODELVIEW matrix, and begin with your game loop. One of the parts of the game loop is setting up the MODELVIEW matrix. First the VIEW portion as a very first part of rendering, then the MODEL portion appropriately for each mesh.

Notice that the order in which the transformations are assembled by their particular matrices (for position, orientation, and perhaps scaling) is a very important thing!

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Thank you very much haegarr... I think the explaination of the matricies and their order has really helped me out a bunch. A few things I didn't understand are starting to click together.

One thing I am still missing as this relates to a tile map. I have read about "stretching a texture accross a mesh". I would think this involves calulating texture coordinates to place with all of the verticies. Not that it is really all that difficult to figure out programaticly I am just wondering if there is something I am missing that makes this part a bit easier?

Also, setting up glFrustum... I can get it to work using gluPerspective (ala NeHe) but I would like to know how to get this to work... Any ideas or suggestions that explain in depth? I have searched google results extensively but as I said in my original post, many tutorials suggest that I have some deeper understanding of 3d concepts.

I can follow along in the NeHe tutorials but I feel like I am copy, pasting and not really understanding why some things work the way they do.

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