point - plane collision without the glutLookAt* functions

Question

Hi,

As I have understood, it is recommended to use glTranslate / glRotate in favour of glutLootAt. I am not going to seek the reasons beyond the obvious HW vs SW computation mode, but just go with the wave. However, this is giving me some headaches as I do not exactly know how to efficiently stop the camera from breaking through walls. I am only interested in point-plane intersections, not AABB or anything else.

So, using glTranslates and glRotates means that the viewpoint stays still (at (0,0,0) for simplicity) while the world revolves around it. This means to me that in order to check for any intersection points, I now need to recompute the world's vertices coordinates (which was not needed with the glutLookAt approach) for every camera movement. As there is no way in obtaining the needed new coordinates from GPU-land, they need to be calculated in CPU land by hand. For every camera movement ... :(

It seems there is the need to retain the current rotations aside each of the 3 axises and the same for translations. There is no scaling used in my program. My questions:

1 - is the above reasoning flawed ? How ? 2 - if not, there has to be a way to avoid such recalculations. The way I see it (and by looking at http://www.glprogramming.com/red/appendixf.html) it needs one matrix multiplication for translations and another one for rotating (only aside the y axis needed). However, having to compute so many additions / multiplications and especially the sine / cosine will certainly be killing FPS. There are going to be thousands or even tens of thousands of vertices to compute on. Every frame... all the maths... After having computed the new coordinates of the world things seem to be very easy - just see if there is any plane that changed its 'd' sign (from the planes equation ax + by + cz + d = 0). If it did, use a lightweight cross products approach to test if the point is inside the space inside each 'moving' triangle of that plane.

Thanks

edit: I have found about glGet and I think it is the way to go but I do not know how to properly use it:

    // Retains the current modelview matrix
//glPushMatrix();
glGetFloatv(GL_MODELVIEW_MATRIX, m_vt16CurrentMatrixVerts);
//glPopMatrix();

m_vt16CurrentMatrixVerts is a float[16] which gets filled with 0.f or 8.67453e-13 or something similar. Where am I screwing up ?

Answer 1

gluLookAt is a very handy function with absolutely no performance penalty. There is no reason not to use it, and, above all, no "HW vs SW" consideration about that. As Mk12 stated, glRotatef is also done on the CPU. The GPU part is : gl_Position = ProjectionMatrix x ViewMatrix x ModelMatrix x VertexPosition.

"using glTranslates and glRotates means that the viewpoint stays still" -> same thing for gluLookAt

"at (0,0,0) for simplicity" -> not for simplicity, it's a fact. However, this (0,0,0) is in the Camera coordinate system. It makes sense : relatively to the camera, the camera is at the origin...

Now, if you want to prevent the camera from going through the walls, the usual method is to trace a ray from the camera. I suspect this is what you're talking about ("to check for any intersection points"). But there is no need to do this in camera space. You can do this in world space. Here's a comparison :

• Tracing rays in camera space : ray always starts from (0,0,0) and goes to (0,0,-1). Geometry must be transformed from Model space to World space, and then to Camera space, which is what annoys you
• Tracing rays in world space : ray starts from camera position (in world space) and goes to (eyeCenter - eyePos).normalize(). Geometry must be transformed from Model space to World space.

Note that there is no third option (Tracing rays in Model space) which would avoid to transform the geometry from Model space to World space. However, you have a pair of workarounds :

• First, your game's world is probably still : the Model matrix is probably always identity. So transforming its geometry from Model to World space is equivalent to doing nothing at all.
• Secondly, for all other objets, you can take the opposite approach. Intead of transforming the entire geometry in one direction, transform only the ray the other way around : Take your Model matrix, inverse it, and you've got a matrix which goes from world space to model space. Multiply your ray's origin and direction by this matrix : your ray is now in model space. Intersect the normal way. Done.

Note that all I've said is standard techniques. No hacks or other weird stuff, just math :)