Using GLSL shaders + lighting / normals

Question

I've got this not-so-small-anymore tile-based game, which is my first real OpenGL project. I want to render every tile as a 3D object. So at first I created some objects, like a cube and a sphere, provided them with vertex normals and rendered them in immediate mode with flat shading. But since I've got like 10.000 objects per level, it was a bit slow. So I put my vertices and normals into VBOs.

That's where I encountered the first problem: Before using VBOs I just push()ed and pop()ed matrices for every object and used glTranslate / glRotate to place them in my scene. But when I did the same with VBOs, the lighting started to behave strangely. Instead of a fixed lighting position behind the camera, the light seemed to rotate with my objects. When moving around them 180 degrees I could see only a shadow.

So i did some research. I could not find any answer to my specific problem, but I read, that instead of using glTranslate/glRotate one should implement shaders and provide them with uniform matrices.

I thought "perhaps that could fix my problem too" and implemented a first small vertex shader program which only stretched my objects a bit, just to see if I could get a shader to work before focusing on the details.

void main(void)
{
    vec4 v = gl_Vertex;
    v.x = v.x * 0.5;
    v.y = v.y * 0.5;
    gl_Position = gl_ModelViewProjectionMatrix * v;
}

Well, my objects get stretched - but now OpenGLs flat shading is broken. I just get white shades. And I can't find any helpful information. So I got a few questions:

• Can I only use one shader at a time, and when using my own shader, OpenGLs flat shading is turned off? So do I have to implement flat shading myself?

• What about my vector normals? I read somewhere, that there is something like a normal-matrix. Perhaps I have to apply operations to my normals as well when modifying vertices?

Answer 1

That your lighting gets messed up with matrix operations changes means, that your calls to glLightfv(..., GL_POSITION, ...) happen in the wrong context (not the OpenGL context, but state of matrices, etc.).

Well, my objects get stretched - but now OpenGLs flat shading is broken. I just get white shades

I think you mean Gourad shading (flat shading means something different). The thing is: If you're using a vertex shader you must do everthing the fixed function pipeline did. That includes the lighting calculation. Lighthouse3D has a nice tutorial http://www.lighthouse3d.com/tutorials/glsl-tutorial/lighting/ as does Nicol Bolas' http://arcsynthesis.org/gltut/Illumination/Illumination.html