在 OpenGL 中转换后获取顶点？

Question

我有如下一段代码：

#include <gl/glut.h>
#include <stdio.h>


GLfloat xf, yf, win;
GLint view_w, view_h;

//auxiliary vector
GLfloat aux[6] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };
GLfloat vertices[768] = { 0.0 };
//count for aux vector
int count = 0;

//Meant to be used for drawing different shapes, store the number of vertecies for each primitive
int primitives[64] = {};

//Index
int vIndex = 0;
int pIndex = 0;

bool flag = false;

//Dedraws the scene using the vertices vector
void Redisplay()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glFlush();     
    int j = 0;
        int k;
        for (int i = 0; i < pIndex; i++)
        {
            glMatrixMode(GL_MODELVIEW);
            glLoadIdentity();
            glBegin(GL_TRIANGLES);
            k = j;
            for(; j < k + 2*primitives[i];j+=2)
                glVertex2f(vertices[j], vertices[j + 1]);
            glEnd();
            glFlush();

        }
}

//Store vertices in the vertices vector
void storePrimitive(GLfloat *vector, int nVertex)
{
    for (int i = 0; i < nVertex * 2; i++)
        vertices[vIndex++] = vector[i];
    primitives[pIndex++] = nVertex;
}

//Draw scene
void draw(void)
{
    if (count > 5)
    {
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
        //If flag is true, apply a transform
        if (flag)
        {
            glPushMatrix();
            glScalef(2, 2, 1);
        }
        glBegin(GL_TRIANGLES);
        glVertex2f(aux[0], aux[1]);
        glVertex2f(aux[2], aux[3]);
        glVertex2f(aux[4], aux[5]);
        storePrimitive(aux, 3);
        if (flag)
            glPopMatrix();
        glEnd();
        glFlush();
        count = 0;
        flag = false;
    }
}

void init(void)
{
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    xf = 50.0f;
    yf = 50.0f;
    win = 250.0f;
}

void resize(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    view_w = w;
    view_h = h;

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(-win, win, -win, win);
}

void keyboardFunc(unsigned char key, int x, int y)
{
    switch (key) {
    case 'r': //Clears the screen
        glClear(GL_COLOR_BUFFER_BIT);
        glFlush();
        break;
    case 'd': //Redraw scene
        Redisplay();
        break;
    case 't': //Apply a transform to the next primitive
        flag = true;
        break;
    }
}

void mouse(int button, int state, int x, int y)
{

    if (button == GLUT_LEFT_BUTTON)
        if (state == GLUT_DOWN) {        
            aux[count] = ((2 * win * x) / view_w) - win;
            count++;
            aux[count] = (((2 * win) * (y - view_h)) / -view_h) - win;
            count++;
        }
    glutPostRedisplay();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(650, 600);
    glutInitWindowPosition(10, 10);
    glutCreateWindow("Example");
    glutDisplayFunc(draw);
    glutReshapeFunc(resize);
    glutKeyboardFunc(keyboardFunc);
    glutMouseFunc(mouse);
    init();
    glutMainLoop();
}

您可以看到可以在绘图函数中对图元应用变换。然而，我想要做的是将转换后的图元的顶点存储在顶点向量中。是否可以在不诉诸手动将变换应用于每个顶点的情况下这样做？

Answer 1

是的，可以使用 OpenGL 的 Transform Feedback。但是，我发现您使用的是旧式 OpenGL，并且变换反馈仅在 OpenGL 3.0 之后可用。新的 OpenGL 风格非常不同。如果您想使用 TransformFeedback，您需要删除所有代码并从头开始。

OP 找到了从 glBegin/End 捕获 TransformFeedback 的方法：https://www.glprogramming.com/red/chapter13.html#name2

在 OpenGL 中转换后获取顶点？

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