在OpenGL中进行转换后获取顶点？

问题描述

我有以下代码：

#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 };
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,1.0f);
    xf = 50.0f;
    yf = 50.0f;
    win = 250.0f;
}

void resize(GLsizei w,GLsizei h)
{
    glViewport(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 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draw函数中将转换应用于基本体。但是，我要做的是将转换后的图元的顶点存储在顶点向量中。是否可以这样做，而不必手动将变换应用于每个顶点？

解决方法

是的，可以使用OpenGL的Transform Feedback。 但是，我看到您使用的是旧版OpenGL，并且“转换反馈”仅在OpenGL 3.0之后可用。新的OpenGL样式非常不同。如果要使用TransformFeedback，则需要删除所有代码并从头开始。

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

捕获TransformFeedback的方法