OpenGl旋转和翻译

Question

我正在浏览一系列NeHe OpenGK教程。 Tutorial #9做一些花哨的东西;我明白了一切，除了两件事我认为是整个教程的骨干。

在DrawGlScene函数中，我没有理解下面这行。

glRotatef(tilt,1.0f,0.0f,0.0f); // Tilt The View (Using The Value In 'tilt') 

我明白那行代码的作用，它在教程中也很清楚地提到。但我不明白他为什么要倾斜屏幕。

另一件事是他首先倾斜屏幕，然后用星形角度旋转屏幕，然后立即旋转屏幕。那是什么技术？什么需要倾斜？当星星面向用户时，旋转星星。

glRotatef(star[loop].angle,0.0f,1.0f,0.0f); // Rotate To The Current Stars Angle 
glTranslatef(star[loop].dist,0.0f,0.0f); // Move Forward On The X Plane 

glRotatef(-star[loop].angle,0.0f,1.0f,0.0f); // Cancel The Current Stars Angle 
glRotatef(-tilt,1.0f,0.0f,0.0f);    // Cancel The Screen Tilt 

如果有些机构告诉我机制正在进行，我将非常感激。

Answer 1

我不明白他为什么要倾斜屏幕。

倾斜让你看到另一个角度的星星，而不仅仅是“正上方”。

另一件事是他首先倾斜屏幕，然后以星形角度旋转，然后立即旋转屏幕。那是什么技术？

这是因为他想围绕选定平面（在这种情况下是Y平面）旋转恒星，但是（！）他也希望有纹理的四边形面对观看者。让我们假设他将它旋转90度，如果是这样，你只会看到（如他在教程中所述）一条“粗”线。

考虑这些评论：

// Rotate the current drawing by the specified angle on the Y axis 
// in order to get it to rotate. 
glRotatef(star[loop].angle, 0.0f, 1.0f, 0.0f); 

// Rotating around the object's origin is not going to make 
// any visible effects, especially since the star object itself is in 2D. 
// In order to move around in your current projection, a glRotatef() 
// call does rotate the star, but not in terms of moving it "around" 
// on the screen. 
// Therefore, use the star's distance to move it out from the center. 
glTranslatef(star[loop].dist, 0.0f, 0.0f); 

// We've moved the star out from the center, with the specified 
// distance in star's distance. With the first glRotatef() 
// call in mind, the 2D star is not 100 % facing 
// the viewer. Therefore, face the star towards the screen using 
// the negative angle value. 
glRotatef(-star[loop].angle, 0.0f, 1.0f, 0.0f); 

// Cancel the tilt on the X axis. 
glRotatef(-tilt, 1.0f, 0.0f, 0.0f);