OpenGL Lighting & Color feature

This is a very good article that clean up my way to get close to OpenGL. This article address the issue about how glColor, glMaterial, glLight and glTexture works together to get the final effect that you have expected.

In OpenGL, when you enable the lighting, the glColor that used to set the vertex color will be overwrite. It seems OpenGL allows only one vertex color channel at all time. At the same time you need to set up the light source with glLight and set up material property for each polygon with glMaterial, of course you should provide vertex normal with glNormal command. All is almost all you need to do with the OpenGL lighting feature. In addition to those settings, you could set up some advance feature like glLightModel, set the light model, whether front and back surface have the same lighting effect or double sided. Most of time the default value will be OK.
Every time you want to have different lighting effect, you need to switch the material settings. The glMaterial function is not effective at all and could not work with some OpenGL advanced feature well. To address this issue, you could enable GL_COLOR_MATEERIAL to use glColor to set up some material lighting components color and gain some good performance at the same time.
At the end of the above pass, you will get some color or no color for each vertex. But how about the final color if I have a texture bind?  Here is the result, that I will continue to tell you:

As you see, there are four result that you will have when you have different settings. The vertex color will always blend with the texture color if have one. And  this will cause another serious problem here is that, when you call glColor at somewhere. Because the OpenGL works like a state machine. It will keep the current state until some one change it later. And the vertex color always there, you could not clean it up at all. On way to fix this problem is that you fill it with a totally white color, like glColor4f(1, 1, 1, 1). Or you could customize the texture stage, and specify the final color will get from the texture instead of vertex color or other color calculating functions. Both of them could address this issue. Of course, the first one solution seems much easier. To get all those staff work together, a demo program with some documents provided here.

 

A Sample Program

This program was written based on the NeHe OpenGL Lesson06. You could press different number key on you keyboard to see the different effect. Some of them are the same, that is normal.

Here some some points I should be mentioned:

1) When glLight for light source position, the current matrix will impact on the light position and direction. The Beast way to do that is like following:

glPushMatrix();
    glLoadIndentity();
    glLightfv(GL_LIGHT0,GL_POSITION,light_position);
     // …
glPopMatrix();

 

2) When you set the GL_POSTION with glLight function, you should take care the fourth component ‘w’ of the position (x, y, z, w). If w was 0, the current light will be a directional light source. If w was non-ZERO (usually 1), then the current light will be a positional light source.