View Full Version : Lighting objects

Hi,

I am at a point where I am lighting objects now and I noticed that if you creat one light source(diffuse, ambient, position), and you draw two different cubes, one using the command glutSolidCube, and the other using GL_QUADS, and rotate both of them the lighting have very different effects. the one with glutSolidCube reacts fine to the light, but the one with GL_QUADS gets dark once it rotates a certain amount, then lights up again, and it recures with every full rotation. can somebody explain this.

thanks.

same results with nehes lighting tutorial

Michael Steinberg

09-04-2001, 10:28 AM

You need to build vertex normals in order to perform lighting! They are specified with glNormal*(*) and tell opengl in which direction the vertex is "pointing". For a cube, you will want flat shading, which means that the normal for every of the 4 vertices of the quad are the same, which will get you sharp edges. You can get the normal with a crossproduct of two adjacent edges of the quad. Note that the crossproduct is a right handed system, and that the vectors given to opengl must have the length one.

thanks for you help, I got it working now. here is what i did:

frontface

glNormal3f(0,0,1)

backface

glNormal3f(0,0,-1)

topface

glNormal3f(0,1,0)

bottomface

glNormal3f(0,-1,0)

leftface

glNormal3f(-1,0,0);

rightface

glNormal3f(1,0,0);

this is the first time doing normals in openGL, is normal just a vector that is perpendicular and pointed in the front direction of a plane? when I did a search, there is a lot( alot is not a word http://www.opengl.org/discussion_boards/ubb/smile.gif ) of talk of calculating normals, why calculated them if you can tell by looking at the plane? is there something more to this?

DFrey

09-04-2001, 12:00 PM

Yes, there are basically two different important types of normals. Face normals, and vertex normals. It is not always obvious what a vertex normal is, so you calculate them using the face normals and connectivity info.

can you please give me an example of a vertex normal and explain its applications??

Michael Steinberg

09-04-2001, 12:48 PM

When lighting with plane normals, you will always be able to distinguish the faces (think of a sphere for this example).

Now, in opengl you can average the normals in a certain vertex. So you have calculated the lighting per vertex. Then opengl interpolates the resulting vertex color over the face. Thus, you have a "smooth" looking sphere which you couldn't get with a normal per face. You will still be able to see the edges at the outline of the sphere though...

This technuiqe is called "Gouraud-shading".

http://www.xmission.com/%7Enate/smooth.html

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