glLoadIdentity()与glTranslatef()跟glRotatef()-坐标变换

glLoadIdentity()与glTranslatef()和glRotatef()--坐标变换

左边的三角形是第一步绘制的，可以看到该三角形绘制的坐标系，实际上是以(-1.5f,0.0f,-6.0f)为原点的。

第二个三角形绘制的时候，由于使用glLoadIdentity()使原点重新回到屏幕中心来，因此其原点位于屏幕的中心。

glRotatef(angle, x, y, z)
与glTranslatef(x, y, z)类似，glRotatef(angle, x, y, z)也是对坐标系进行操作。
旋转轴经过原点，方向为(x,y,z),旋转角度为angle，方向满足右手定则。
////////////////////////////////////////////////////////////////
glLoadIdentity();
glTranslatef(0.0f,0.0f,-6.0f);
glBegin(GL_TRIANGLES);
glVertex3f(0.0f,0.0f, 0.0f);
glVertex3f(1.0f,0.0f, 0.0f);
glVertex3f(0.0f,1.0f, 0.0f);
glEnd();
////////////////////////////////////////////////////////////////
在未旋转的情况下如图所示：

////////////////////////////////////////////////////////////////
glLoadIdentity();
glRotatef(45,0.0f,0.0f,1.0f);
glTranslatef(0.0f,0.0f,-6.0f);
glBegin(GL_TRIANGLES);
glVertex3f(0.0f,0.0f, 0.0f);
glVertex3f(1.0f,0.0f, 0.0f);
glVertex3f(0.0f,1.0f, 0.0f);
glEnd();
////////////////////////////////////////////////////////////////
绕Z轴正向旋转45度角，因为Z轴正方向由屏幕内指向屏幕外，由右手定则可知方向为逆时针转动。
由于直角顶点即为原点，因此将围绕直角逆时针旋转。

旋转和平移函数都存在的情况下，程序会如何执行呢？只有平移的情况：////////////////////////////////////////////////////////////////?glLoadIdentity();?????????// Reset The Current Modelview Matrix
?glTranslatef(0.0f, 0.0f, -6.0f);
?glBegin(GL_QUADS);
??glVertex3f(-0.01f,-0.01f, 0.0f);
??glVertex3f( 0.01f,-0.01f, 0.0f);
??glVertex3f( 0.01f, 0.01f, 0.0f);
??glVertex3f(-0.01f, 0.01f, 0.0f);
?glEnd();
?glLoadIdentity();
//?glRotatef(45.0f, 0.0f, 0.0f, 1.0f);
?glTranslatef(1.0f,0.0f,-6.0f);
?
?glBegin(GL_TRIANGLES);
??glVertex3f(0.0f,0.0f, 0.0f);
??glVertex3f(1.0f,0.0f, 0.0f);
??glVertex3f(0.0f,1.0f, 0.0f);
?glEnd();??////////////////////////////////////////////////////////////////加上旋转的情况：//////////////////////////////////glLoadIdentity();?????????// Reset The Current Modelview Matrix
?glTranslatef(0.0f, 0.0f, -6.0f);
?glBegin(GL_QUADS);
??glVertex3f(-0.01f,-0.01f, 0.0f);
??glVertex3f( 0.01f,-0.01f, 0.0f);
??glVertex3f( 0.01f, 0.01f, 0.0f);
??glVertex3f(-0.01f, 0.01f, 0.0f);
?glEnd();
?glLoadIdentity();
?glRotatef(45.0f, 0.0f, 0.0f, 1.0f);
?glTranslatef(1.0f,0.0f,-6.0f);
?
?glBegin(GL_TRIANGLES);
??glVertex3f(0.0f,0.0f, 0.0f);
??glVertex3f(1.0f,0.0f, 0.0f);
??glVertex3f(0.0f,1.0f, 0.0f);
?glEnd();?/////////////////////////////////如果旋转和平移语句的顺序调换一下结果会怎么样呢？///////////////////////////glLoadIdentity();?????????// Reset The Current Modelview Matrix
?glTranslatef(0.0f, 0.0f, -6.0f);
?glBegin(GL_QUADS);
??glVertex3f(-0.01f,-0.01f, 0.0f);
??glVertex3f( 0.01f,-0.01f, 0.0f);
??glVertex3f( 0.01f, 0.01f, 0.0f);
??glVertex3f(-0.01f, 0.01f, 0.0f);
?glEnd();
?glLoadIdentity();
?glTranslatef(1.0f,0.0f,-6.0f);
?glRotatef(45.0f, 0.0f, 0.0f, 1.0f);
?
?glBegin(GL_TRIANGLES);
??glVertex3f(0.0f,0.0f, 0.0f);
??glVertex3f(1.0f,0.0f, 0.0f);
??glVertex3f(0.0f,1.0f, 0.0f);
?glEnd();//////////////////////////由这三个例子可知，所有对图形进行平移旋转等操作的语句的执行顺序都是从下到上执行的。而且旋转语句的旋转中心知（0.0f， 0.0f, 0.0f）这个点。

////////////////////////////////////////////////////////////////
glLoadIdentity();
glRotatef(45,0.0f,0.0f,1.0f);
glLoadIdentity();
glTranslatef(0.0f,0.0f,-6.0f);
glBegin(GL_TRIANGLES);
glVertex3f(0.0f,0.0f, 0.0f);
glVertex3f(1.0f,0.0f, 0.0f);
glVertex3f(0.0f,1.0f, 0.0f);
glEnd();
////////////////////////////////////////////////////////////////
在旋转之后加了一个复位的指令，图形就不会旋转了。

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