OpenGL Programming Guide (Addison-Wesley Publishing Company)

OpenGL Programming Guide

Changing Material Properties

Example 6-1 uses the same material properties for all vertices of the only object in the scene (the sphere). In other situations, you might want to assign different material properties for different vertices on the same object. More likely, you have more than one object in the scene, and each object has different material properties. For example, the code that produced Figure J-21 has to draw eight different objects (all spheres), each with different material properties. Example 6-3 shows some of the code in the display() routine.

Example 6-3 : Using Different Material Properties: material.c

GLfloat no_mat[] = { 0.0, 0.0, 0.0, 1.0 };
GLfloat mat_ambient[] = { 0.7, 0.7, 0.7, 1.0 };
GLfloat mat_ambient_color[] = { 0.8, 0.8, 0.2, 1.0 };
GLfloat mat_diffuse[] = { 0.1, 0.5, 0.8, 1.0 };
GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat no_shininess[] = { 0.0 };
GLfloat low_shininess[] = { 5.0 };
GLfloat high_shininess[] = { 100.0 };
GLfloat mat_emission[] = {0.3, 0.2, 0.2, 0.0};

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
/* draw sphere in first row, first column
 * diffuse reflection only; no ambient or specular
 */
glPushMatrix();
    glTranslatef (-3.75, 3.0, 0.0);
    glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat);
    glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
    glMaterialfv(GL_FRONT, GL_SPECULAR, no_mat);
    glMaterialfv(GL_FRONT, GL_SHININESS, no_shininess);
    glMaterialfv(GL_FRONT, GL_EMISSION, no_mat);
    auxSolidSphere();
glPopMatrix();

/* draw sphere in first row, second column
 * diffuse and specular reflection; low shininess; no ambient
 */
glPushMatrix();
    glTranslatef (-1.25, 3.0, 0.0);
    glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat);
    glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
    glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
    glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess);
    glMaterialfv(GL_FRONT, GL_EMISSION, no_mat);
    auxSolidSphere();
glPopMatrix();

/* draw sphere in first row, third column
 * diffuse and specular reflection; high shininess; no ambient
 */
glPushMatrix();
    glTranslatef (1.25, 3.0, 0.0);
    glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat);
    glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
    glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
    glMaterialfv(GL_FRONT, GL_SHININESS, high_shininess);
    glMaterialfv(GL_FRONT, GL_EMISSION, no_mat);
    auxSolidSphere();
glPopMatrix();

/* draw sphere in first row, fourth column
 * diffuse refl.; emission; no ambient or specular reflection
 */
glPushMatrix();
    glTranslatef (3.75, 3.0, 0.0);
    glMaterialfv(GL_FRONT, GL_AMBIENT, no_mat);
    glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
    glMaterialfv(GL_FRONT, GL_SPECULAR, no_mat);
    glMaterialfv(GL_FRONT, GL_SHININESS, no_shininess);
    glMaterialfv(GL_FRONT, GL_EMISSION, mat_emission);
    auxSolidSphere();
glPopMatrix();

As you can see, glMaterialfv() is called repeatedly to set the desired material property for each sphere. Note that it's called only to change a property that needs to be changed. The second and third spheres use the same ambient and diffuse properties as the first sphere, for example, so these properties aren't reset. Since glMaterial*() has a performance cost associated with its use, it's best to minimize material-property changes.

Another technique for minimizing performance costs associated with changing material properties is to use glColorMaterial(). void glColorMaterial(GLenum face, GLenum mode);

Causes the material property (or properties) specified by mode of the specified material face (or faces) specified by face to track the value of the current color at all times. A change to the current color (using glColor*()) immediately updates the specified material properties. The face parameter can be GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK (the default). The mode parameter can be GL_AMBIENT, GL_DIFFUSE, GL_AMBIENT_AND_DIFFUSE (the default), GL_SPECULAR, or GL_EMISSION.

Note that glColorMaterial() specifies two independent values: the first specifies which face or faces are updated, and the second specifies which material property or properties of those faces are updated. OpenGL does not maintain separate mode variables for each face.

After calling glColorMaterial(), you need to call glEnable() with GL_COLOR_MATERIAL as the parameter. Then, you can change the current color using glColor*() (or other material properties, using glMaterial*()) as needed as you draw:

glColorMaterial(GL_FRONT, GL_DIFFUSE);
glEnable(GL_COLOR_MATERIAL);
glColor3f(0.2, 0.5, 0.8);
/* draw some objects here */
glColor3f(0.9, 0.0, 0.2);
/* draw other objects here */
glDisable(GL_COLOR_MATERIAL);

You should use glColorMaterial() whenever you need to change a single material parameter for most vertices in your scene. If you need to change more than one material parameter, as was the case for Figure J-21 , use glMaterial*(). When you don't need the capabilities of glColorMaterial() anymore, be sure to disable it, so that you don't get undesired material properties and so that you don't incur the performance cost associated with it. The performance value in using glColorMaterial() varies, depending on your OpenGL implementation. Some implementations may be able to optimize the vertex routines so that they can quickly update material properties based on the current color.

Example 6-4 shows an interactive program that uses glColorMaterial() to change material parameters. Pressing each of the three mouse buttons changes the color of the diffuse reflection.

Example 6-4 : Using glColorMaterial(): colormat.c

#include <GL/gl.h>
#include <GL/glu.h>
#include "aux.h"

GLfloat diffuseMaterial[4] = { 0.5, 0.5, 0.5, 1.0 };

void myinit(void)
{
    GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
    GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };

    glMaterialfv(GL_FRONT, GL_DIFFUSE, diffuseMaterial);
    glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
    glMaterialf(GL_FRONT, GL_SHININESS, 25.0);
    glLightfv(GL_LIGHT0, GL_POSITION, light_position);

    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    glDepthFunc(GL_LEQUAL);
    glEnable(GL_DEPTH_TEST);

    glColorMaterial(GL_FRONT, GL_DIFFUSE);
    glEnable(GL_COLOR_MATERIAL);
}

void changeRedDiffuse (AUX_EVENTREC *event)
{
    diffuseMaterial[0] += 0.1;
    if (diffuseMaterial[0] > 1.0)
        diffuseMaterial[0] = 0.0;
    glColor4fv(diffuseMaterial);
}

void changeGreenDiffuse (AUX_EVENTREC *event)
{
    diffuseMaterial[1] += 0.1;
    if (diffuseMaterial[1] > 1.0)
        diffuseMaterial[1] = 0.0;
    glColor4fv(diffuseMaterial);
}

void changeBlueDiffuse (AUX_EVENTREC *event)
{
    diffuseMaterial[2] += 0.1;
    if (diffuseMaterial[2] > 1.0)
        diffuseMaterial[2] = 0.0;
    glColor4fv(diffuseMaterial);
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    auxSolidSphere(1.0);
    glFlush();
}

void myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    if (w <= h) 
        glOrtho (-1.5, 1.5, -1.5*(GLfloat)h/(GLfloat)w, 
            1.5*(GLfloat)h/(GLfloat)w, -10.0, 10.0);
    else 
        glOrtho (-1.5*(GLfloat)w/(GLfloat)h, 
            1.5*(GLfloat)w/(GLfloat)h, -1.5, 1.5, -10.0, 10.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}


int main(int argc, char** argv)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA | AUX_DEPTH);
    auxInitPosition (0, 0, 500, 500);
    auxInitWindow (argv[0]);
    myinit();
    auxMouseFunc(AUX_LEFTBUTTON, AUX_MOUSEDOWN, 
        changeRedDiffuse);
    auxMouseFunc(AUX_MIDDLEBUTTON, AUX_MOUSEDOWN, 
        changeGreenDiffuse);
    auxMouseFunc(AUX_RIGHTBUTTON, AUX_MOUSEDOWN, 
        changeBlueDiffuse);
    auxReshapeFunc(myReshape);
    auxMainLoop(display);
}

Try This

Modify Example 6-3 :

Change the global ambient light in the scene. Hint: Alter the value of the GL_LIGHT_MODEL_AMBIENT parameter.
Change the diffuse, ambient, and specular reflection parameters, the shininess exponent, and the emission color. Hint: Use the glMaterial*() command, but avoid making excessive calls.
Use two-sided materials and add an arbitrary clipping plane (see "Additional Clipping Planes" ) so you can see the inside and outside of a row or column of spheres. Hint: Turn on two-sided lighting with GL_LIGHT_MODEL_TWO_SIDE, set the desired material properties, and add a clipping plane.
Remove all the glMaterialfv() calls, and use the more efficient glColorMaterial() calls to achieve the same lighting.

OpenGL Programming Guide

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