OpenGL入门笔记（三）

今天主要学习二维图形的着色和旋转。

int DrawGLScene(GLvoid)                                    // Here's Where We Do All The Drawing
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    // Clear Screen And Depth Buffer
    glLoadIdentity();    
    glTranslatef(-1.5f,0.0f,-6.0f);                        // Move Left 1.5 Units And Into The Screen 6.0
    glRotatef(rtri,0.0f,1.0f,0.0f);
    glBegin(GL_TRIANGLES);                                // Drawing Using Triangles
        glColor3f(1.0f,0.0f,0.0f);
        glVertex3f( 0.0f, 1.0f, 0.0f);                    // Top
        glColor3f(0.0f,1.0f,0.0f);
        glVertex3f(-1.0f,-1.0f, 0.0f);                    // Bottom Left
        glColor3f(0.0f,0.0f,1.0f);
        glVertex3f( 1.0f,-1.0f, 0.0f);                    // Bottom Right
    glEnd();                                            // Finished Drawing The Triangle
    glLoadIdentity();
    glTranslatef(1.5f,0.0f,-6.0f);                        // Move Right 3 Units
    glRotatef(rquad,1.0f,0.0f,0.0f);
    glBegin(GL_QUADS);                                    // Draw A Quad
        glColor3f(1.0f,0.0f,0.0f);
        glVertex3f(-1.0f, 1.0f, 0.0f);                    // Top Left
        glColor3f(0.0f,1.0f,0.0f);
        glVertex3f( 1.0f, 1.0f, 0.0f);                    // Top Right
        glColor3f(0.0f,0.0f,1.0f);
        glVertex3f( 1.0f,-1.0f, 0.0f);                    // Bottom Right
        glColor3f(1.0f,1.0f,1.0f);
        glVertex3f(-1.0f,-1.0f, 0.0f);                    // Bottom Left
    glEnd();                                            // Done Drawing The Quad// Reset The Current Modelview Matrix
    rtri+=0.2f;
    rquad-=0.15f;
    return TRUE;                                        // Everything Went OK
}

glColor3f(r,g,b)。括号中的三个参数依次是红、绿、蓝三色分量。取值范围可以从0,0f到1.0f。因为每个顶点有不同的颜色，因此看起来颜色从每个角喷出，并刚好在三角形的中心汇合，三种颜色相互混合。

glRotatef(Angle,Xvector,Yvector,Zvector)负责让对象绕某个轴旋转。这个命令有很多用处。Angle通常是个变量代表对象转过的角度。Xvector、Yvector、Zvector三个参数则共同决定旋转轴的方向。比如(1,0,0)所描述的矢量经过X坐标轴的1个单位处并且方向向右。(-1,0,0)所描述的矢量经过X坐标轴的1个单位处，但方向向左。

代码中我们增加了另一个glLoadIdentity()调用。目的是为了重置模型观察矩阵。如果我们没有重置，直接调用glTranslate的话，会出现意料之外的结果。