如何使用过剩和开放GL来改变颜色？

Question

I am trying to teach myself to animate an object without user input, so far I have figured out how to make a scene rotate. How do I get an object to change color though? I thought the code I have would do it, but it remains a white triangle (not even a different color).

How do I get it to change color at the same time that the triangle or perspective rotates?

Here is my current code:

#include <GL/glut.h>

float color1;
float color2;
float color3;

void changeSize(int w, int h) {

// Prevent a divide by zero, when window is too short
// (you cant make a window of zero width).
if (h == 0)
    h = 1;

float ratio = w * 1.0 / h;

// Use the Projection Matrix
glMatrixMode(GL_PROJECTION);

// Reset Matrix
glLoadIdentity();

// Set the viewport to be the entire window
glViewport(0, 0, w, h);

// Set the correct perspective.
gluPerspective(45.0f, ratio, 0.1f, 100.0f);

//changeColor?
color1 += 0.1f;
color2 += 0.3;
color3 += color2;
if (color1 > 1.0)
    color1 = 0;
if (color2 > 1.0)
    color2 = 0;
if (color3 > 1.0)
    color3 = 0;

// Get Back to the Modelview
glMatrixMode(GL_MODELVIEW);
}

float angle = 0.0f;

void renderScene(void) {

// Clear Color and Depth Buffers
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// Reset transformations
glLoadIdentity();
// Set the camera
gluLookAt(0.0f, 0.0f, 10.0f,
    0.0f, 0.0f, 0.0f,
    0.0f, 1.0f, 0.0f);

glRotatef(angle, 0.0f, 1.0f, 0.0f);

glBegin(GL_TRIANGLES);
glColor3f(color1, color2, color3);

glVertex3f(2.0f, -2.0f, 0.0f);
glVertex3f(2.0f, 0.0f, 0.0);
glVertex3f(0.0f, 2.0f, 0.0);
glEnd();

angle += 0.1f;
color1 += 0.1f;
color2 += 0.3;
color3 += color2;

glutSwapBuffers();
}

int main(int argc, char **argv) {

// init GLUT and create window
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowPosition(100, 100);
glutInitWindowSize(320, 320);
glutCreateWindow("tutorial example");

// register callbacks
glutDisplayFunc(renderScene);
glutReshapeFunc(changeSize);

glutIdleFunc(renderScene);

// enter GLUT event processing cycle
glutMainLoop();

return 1;
}

Answer 1

Float colors have to be in range of [0,1] and you only add something to your values and never reset it to zero, so it becomes greater than 1 in first 10 frames (=very fast), OpenGL clamps it to 1, so you see it white.

Ie that bunch of if s in changeSize should actually be in renderScene .

Answer 2

• For something simple like this use a glutTimerFunc() callback with a reasonable timeout
• Update the angle/color in the timer callback & kick off a repaint
• Re-set the projection/modelview matrices each time through the glutDisplayFunc() callback, helps prevent weird matrix problems

All together:

#include <GL/glut.h>

float angle = 0.0f;
float color1 = 0.0f;
float color2 = 0.0f;
float color3 = 0.0f;
void timer( int value )
{
    angle += 3.0f;

    //changeColor?
    color1 += 0.001f;
    color2 += 0.003f;
    color3 += color2;
    if (color1 > 1.0)
        color1 = 0;
    if (color2 > 1.0)
        color2 = 0;
    if (color3 > 1.0)
        color3 = 0;

    glutPostRedisplay();
    glutTimerFunc( 16, timer, 0 );
}

void renderScene()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    gluPerspective(45.0f, w / h, 0.1f, 100.0f);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt
        (
        0.0f, 0.0f, 10.0f,
        0.0f, 0.0f, 0.0f,
        0.0f, 1.0f, 0.0f
        );

    glRotatef(angle, 0.0f, 1.0f, 0.0f);

    glBegin(GL_TRIANGLES);
    glColor3f(color1, color2, color3);
    glVertex3f(2.0f, -2.0f, 0.0f);
    glVertex3f(2.0f, 0.0f, 0.0);
    glVertex3f(0.0f, 2.0f, 0.0);
    glEnd();

    glutSwapBuffers();
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
    glutInitWindowSize(320, 320);
    glutCreateWindow("tutorial example");
    glutDisplayFunc(renderScene);
    glutTimerFunc( 0, timer, 0 );
    glutMainLoop();
    return 1;
}