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我正在尝试使用glFrustum透视投影，据我所知glFrustum，glOrtho可以用来修改我们所需坐标系到真实屏幕坐标的映射（如蓝皮书中所述）。因此，如果我这样做
glFrustum(-1,1,-1,1,1,1000);，它会将坐标更改为
left = -1, right = 1, bottom = -1, top = 1 以笛卡尔坐标的形式。

我试图通过在我的 draw 方法中调用以下函数在这个坐标系中绘制一个简单的房间（有 2 个侧壁），它会在屏幕上适当地绘制出来。

void drawRoomWalls(){
    //Left wall
    glBegin(GL_QUADS);
    glVertex3f(-1.0, 1.0, 0.0);
    glVertex3f(-1.0, 1.0, -0.4);
    glVertex3f(-1.0, -1.0, -0.4);
    glVertex3f(-1.0, -1.0, 0.0);
    glEnd();

    //Right wall
    glBegin(GL_QUADS);
    glVertex3f(1.0, 1.0, 0.0);
    glVertex3f(1.0, 1.0, -0.4);
    glVertex3f(1.0, -1.0, -0.4);
    glVertex3f(1.0, -1.0, 0.0);
    glEnd();
}

该函数被调用如下：

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
    glFrustum(1.0, -1.0, -1.0, 1.0, 1.0, 1000.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0, 0, -1.0);
drawRoomWalls();

随后，我尝试进行离轴投影（通过将鼠标作为输入而不是用户的头部）。代码如下： glMatrixMode(GL_PROJECTION); glPushMatrix(); glMatrixMode(GL_MODELVIEW); glPushMatrix();

double fov, near, far;
double headX, headY, headZ;
float aspectRatio;

near = 0.5f; far = 1000.0f;  aspectRatio = ofGetWidth()/ofGetHeight();
fov = tan(DEG_TO_RAD * 30/2); //tan accepts angle in radians. tan of the half of the fov angle
fov = 0.5; //taken constant for now

double msX = (double)ofGetMouseX();
double msY = (double)ofGetMouseY();
double scrWidth = (double)ofGetWidth();
double scrHeight = (double)ofGetHeight();

headX = (msX / scrWidth) - 0.5; 
headY = ((scrHeight - msY) / scrHeight) - 0.5;
headZ = -2.0;

glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum( near * (-fov * aspectRatio + headX),
               near * (fov * aspectRatio + headX),
               near * (-fov + headY),
               near * (fov + headY),
               near,
               far);

leftValue = near * (-fov * aspectRatio + headX); //for printing out on screen
rightValue = near * (fov * aspectRatio + headX); //for printing out on screen
bottomValue = near * (-fov + headY); //for printing out on screen
topValue = near * (fov + headY); //for printing out on screen

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(headX * headZ, headY * headZ, 0, headX * headZ, headY * headZ, -1, 0, 1, 0);
glTranslatef(0.0, 0.0, headZ);
drawRoomWalls();


我打印了 , , , 的值leftValue，rightValue当bottomValue鼠标topValue位于屏幕中央时（调用glFrustum看起来像glFrustum(-0.25,0.25,-0.25,0.25,1.0,1000.0)
根据上面的调用，我期望坐标系的left=-0.25, right=0.25, bottom=-0.25, top=0.25，我期望墙壁消失（因为它们被绘制在（1.0,1.0,0.0）为例）。但是，墙壁一直出现在屏幕的两侧（场景倾斜，中心与离轴投影基本相同）。为什么会这样墙壁仍然在侧面的位置（即使坐标更改为-0.25,0.25）或者glFrustum调用中有一些关于坐标系的东西我在这里遗漏了？

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1 回答 1

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我才发现你也改成headZ了-2。这解释了这种行为。

如果相机位于z = 2并且您的垂直半视野为 0.5，那么z = 0您已经在 y 方向的正负方向上看到了 1 个单位。这就是你看到墙壁的原因。

可视化

于 2013-05-13T17:43:45.517 回答