如何计算glFrustum参数？

Question

我有这个代码：

/*
 * This is a simple program that computes FPS
 * by means of a circular buffer
 */
#include <GL/glut.h>
//#include <numeric>
#include <unistd.h>
#include <time.h>
#include <stdio.h>

// Number of elements in the circular buffer
#define NELS    10

// Number of lines
#define NLINES  10000

// circular buffer used to compute frame rate
float circularBuffer[NELS];
int firstInd = 0, nEls = 0;

// function to get the number of elapsed ticks
uint32_t getTick()
{
    struct timespec ts;
    unsigned theTick = 0U;
    clock_gettime( CLOCK_REALTIME, &ts );
    theTick  = ts.tv_nsec / 1000000;
    theTick += ts.tv_sec * 1000;
    return theTick;
}

// Function to compute real modulus and NOT remained as % does
inline int modulo(int a, int b) {
    const int result = a % b;
    return result >= 0 ? result : result + b;
}

// Compute sum of the elements in the circular buffer
float sumCircularBuffer()
{
    int ind;
    float sum = 0;

    if (nEls > 0) {
        for (ind=1; ind<=nEls; ind++) {
            sum = sum + circularBuffer[modulo(firstInd-ind, NELS)];
        }
    }

    return sum;
}

// accumulate buffer and update window title
void computeAndShowFrameRate(void)
{
    static float lastTime = 0.0f;
    static unsigned int frameCount = 0;
    char windowTitle[100];
    float sumFPS;

    float currentTime = (float)getTick() * 0.001f;
    // Initialize lastTime to the current time
    if (lastTime == 0) {
        lastTime = currentTime;
    }

    // increase frame count
    frameCount++;
    if (currentTime - lastTime > 1.0f) {
        // insert the current fps in the circular buffer
        circularBuffer[firstInd] = ((float)frameCount) / (currentTime - lastTime);

        // update variable lastTime
        lastTime = currentTime;

        //circularBuffer[firstInd] = (float)frameCount;
        firstInd = ((firstInd+1)%NELS);
        if (nEls < NELS) {
            nEls++;
        }
        frameCount = 0;

        // sum elements in circular buffer
        sumFPS = sumCircularBuffer();
        snprintf(windowTitle, 100, "FPS = %6.2f", sumFPS/nEls);
        // update window title
        glutSetWindowTitle(windowTitle);
    }
}

// display function
void display(void)
{
    int currLineInd;

    // get current frame rate
    computeAndShowFrameRate();

    // clear buffer
    glClear (GL_COLOR_BUFFER_BIT);

    for (currLineInd = 0; currLineInd<NLINES; currLineInd++) {
        // draw line
        glBegin(GL_LINES);
        // random color
        glColor3f((float)rand()/RAND_MAX, (float)rand()/RAND_MAX, (float)rand()/RAND_MAX);
        // random first point
        glVertex2f((float)rand()/RAND_MAX, (float)rand()/RAND_MAX);
        // random color
        glColor3f((float)rand()/RAND_MAX, (float)rand()/RAND_MAX, (float)rand()/RAND_MAX);
        // random second point
        glVertex2f((float)rand()/RAND_MAX, (float)rand()/RAND_MAX);
        glEnd();
    }

    glFinish();
    glutPostRedisplay();
}

// initialization function
void init (void)
{
    // Use current time as seed for random generator
    srand(time(0));

    // select clearing color
    glClearColor (0.0, 0.0, 0.0, 0.0);

    // Orthographic projection
    glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
}

// Window size and mode
int main(int argc, char** argv)
{
    // pass potential input arguments to glutInit
    glutInit(&argc, argv);

    // set display mode
    // GLUT_SINGLE = single buffer window
    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);

    glutInitWindowSize (400, 400);
    glutInitWindowPosition (100, 100);
    glutCreateWindow ("OpenGL Window");

    // Call initialization routinesx
    init();
    glutDisplayFunc(display);
    glutMainLoop();
    return 0;
}

我必须用glFrustum替换glOrtho函数并得到相同的结果。

我阅读了khronos上的opengl指南，理解了glOrtho和glFrustum之间的区别，但我不知道如何计算参数。

如何计算传递给glFrustum函数的参数？

Answer 1

将产生的值取决于您的实现和您正在使用的模型的规模。如上所述，如果投影的几何体在近平面的前面或远平面的后面，它将被剪掉，因此它将不可见。

要解决这个问题，您要么必须重新计算glfrustum()函数的参数（这是个坏主意），要么沿着z轴移动相机/场景。

参考资料：

1. http://www.songho.ca/opengl/gl_transform.html
2. https://learnopengl.com/gett-start/cordinate-systems

Answer 2

在透视投影中,距离远近平面的距离必须大于0,

0 < near < far

因为您要定义一个查看截头体：

如果到近平面的距离小于0，则结果是未定义的（通常指令完全没有作用）。

请参阅glfrustum:

void glFrustum( GLdouble left,
    GLdouble right,
    GLdouble bottom,
    GLdouble top,
    GLdouble nearVal,
    GLdouble farVal);

距离左,右,底和顶是在近平面上从视图中心到截头体侧面的距离。near和far指定到截头体的近平面和远平面的距离。

几何图形必须在近平面和远平面之间，否则就会被裁剪。因此，您必须沿z轴向负方向移动移位（在negativ方向，因为视图空间z轴指向视图之外）：

// initialization function
void init (void)
{
    // [...]

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-0.1, 0.1, -0.1, 0.1, 0.1, 50.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(0.0f, 0.0f, -5.0f);
}