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I am developing a graphics application in C++ with the OpenGL API and GLUT.
To add lighting, I made the following changes in my modelview matrix:
glEnable(GL_LIGHTING); glEnable(GL_LIGHT0); // Create light components. GLfloat ambientLight[] = { 0.2f, 0.2f, 0.2f, 1.0f }; GLfloat diffuseLight[] = { 0.8f, 0.8f, 0.8, 1.0f }; GLfloat specularLight[] = { 0.5f, 0.5f, 0.5f, 1.0f }; GLfloat position[] = { 0.0f, 0.0f, 0.0f, 1.0f }; // Assign created components to GL_LIGHT0. glLightfv(GL_LIGHT0, GL_AMBIENT, ambientLight); glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuseLight); glLightfv(GL_LIGHT0, GL_SPECULAR, specularLight); glLightfv(GL_LIGHT0, GL_POSITION, position); The lighting largely works I believe, but the colors of my objects all go away. All I see is a black/white silhouette of my overall figure.
I was wondering why this is?
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In the OpenGL lighting model, the emissive color of a surface adds intensity to the object, but is unaffected by any light sources. Also, the emissive color does not introduce any additional light into the overall scene.
Here are some examples of commands for setting drawing colors in OpenGL: glColor3f(0,0,0); // Draw in black. glColor3f(1,1,1); // Draw in white. glColor3f(1,0,0); // Draw in full-intensity red.
The OpenGL light model presumes that the light that reaches your eye from the polygon surface arrives by four different mechanisms: AMBIENT - light that comes from all directions equally and is scattered in all directions equally by the polygons in your scene.
glColor3f can be called in between glBegin and glEnd. When it is used this way, it can be used to give each vertex its own color. The resulting rectangle is then shaded with an attractive color gradient, as shown on the right.
When lighting is enabled, a vertex' color is not determined from the color set by glColor or glColorPointer, but by the currently set material colors combined with the lights' colors using the lighting computations.
So in order to change an object's color, you need to change the material setting (which by default is a diffuse grey material) before rendering, using the glMaterial functions. There is essentially a corresponding material color for each of the different light colors (GL_DIFFUSE, ...) along with some additional properties to approximate light emitting materials (GL_EMISSION) and controlling the material's roughness (GL_SHININESS). Read some introductory material on OpenGL's lighting features to understand their workings.
What you can do to quickly adapt your code from plain coloring to lighting (or to enable per-vertex material properties) is to use color material. By calling glEnable(GL_COLOR_MATERIAL) and setting an appropriate mapping with glColorMaterial you can configure OpenGL to change a specific material color, whenever you change the current vertex color (using either glColor or glColorPointer).
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