使用VBO从OpenGL中的点矢量绘制线条

Question

我有一个简单的OpenGL程序,我试图利用顶点缓冲区对象进行渲染而不是旧的glBegin() - glEnd().基本上用户点击指示起点的窗口,然后按下一个键以生成OpenGL作为一行绘制的后续点.

我已经使用glBegin()和glEnd()实现了这个,但是使用VBO还没有成功.我想知道问题是,在我初始化VBO之后,我添加了更多没有分配内存的顶点,因此不会显示它们.

编辑:另外,我对它如何确切知道顶点结构中哪些值用于x和y以及r,g,b有点困惑.我未能找到一个明确的例子.

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <Math.h>
#include <iostream>
#include <vector>
#include <GL/glew.h>
#include <GL/glut.h>

struct vertex {
    float x, y, u, v, r, g, b;
};

const int D = 10;   // distance
const int A = 10;   // angle
const int WINDOW_WIDTH = 500, WINDOW_HEIGHT = 500;

std::vector<vertex> vertices;
boolean start = false;
GLuint vboId;

void update_line_point() {
    vertex temp;
    temp.x = vertices.back().x + D * vertices.back().u;
    temp.y = vertices.back().y + D * vertices.back().v;
    temp.u = vertices.back().u;
    temp.v = vertices.back().v;
    vertices.push_back(temp);
}

void update_line_angle() {
    float u_prime, v_prime;
    u_prime = vertices.back().u * cos(A) - vertices.back().v * sin(A);
    v_prime = vertices.back().u * sin(A) + vertices.back().v * cos(A);
    vertices.back().u = u_prime;
    vertices.back().v = v_prime;
}

void initVertexBuffer() {
    glGenBuffers(1, &vboId);
    glBindBuffer(GL_ARRAY_BUFFER, vboId);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertex) * vertices.size(), &vertices[0], GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
}

void displayCB() {
    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, WINDOW_WIDTH, 0, WINDOW_HEIGHT);

    if (start) {
        glBindBuffer(GL_ARRAY_BUFFER, vboId);
        glEnableClientState(GL_VERTEX_ARRAY);
        glEnableClientState(GL_COLOR_ARRAY);
        glVertexPointer(2, GL_FLOAT, sizeof(vertex), &vertices[0]);
        glColorPointer(3, GL_FLOAT, sizeof(vertex), &vertices[0]);
        glDrawArrays(GL_LINE_STRIP, 0, vertices.size());
        glDisableClientState(GL_VERTEX_ARRAY);
        glDisableClientState(GL_COLOR_ARRAY);
        glBindBuffer(GL_ARRAY_BUFFER, 0);
    }

    /***** this is what I'm trying to achieve
    glColor3f(1, 0, 0);
    glBegin(GL_LINE_STRIP);
    for (std::vector<vertex>::size_type i = 0; i < vertices.size(); i++) {
            glVertex2f(vertices[i].x, vertices[i].y);
    }
    glEnd();
    *****/

    glFlush();
    glutSwapBuffers();
}


void mouseCB(int button, int state, int x, int y) {
    if (state == GLUT_DOWN) {
        vertices.clear();
        vertex temp = {x, WINDOW_HEIGHT - y, 1, 0, 1, 0, 0};  // default red color
        vertices.push_back(temp);
        start = true;
        initVertexBuffer();
    }
    glutPostRedisplay();
}

void keyboardCB(unsigned char key, int x, int y) {
    switch(key) {
    case 'f':
        if (start) {
            update_line_point();
        }
        break;
    case 't':
        if (start) {
            update_line_angle();
        }
        break;
    }

    glutPostRedisplay();
}

void initCallbackFunc() {
    glutDisplayFunc(displayCB);
    glutMouseFunc(mouseCB);
    glutKeyboardFunc(keyboardCB);
}

int main(int argc, char** argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE|GLUT_DEPTH);
    glutInitWindowSize(WINDOW_WIDTH, WINDOW_HEIGHT);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("Test");
    initCallbackFunc();

    // initialize glew
    GLenum glewInitResult;
    glewExperimental = GL_TRUE;
    glewInitResult = glewInit();
    if (GLEW_OK != glewInitResult) {
        std::cerr << "Error initializing glew." << std::endl;
        return 1;
    }
    glClearColor(1, 1, 1, 0);
    glutMainLoop();

    return 0;
}

Answer 1

如果您有 VBO 绑定，则pointer调用的参数gl*Pointer()将被解释为距 VBO 开头的字节偏移量，而不是实际的指针。不过，您的用法与顶点数组的用法一致。

因此，您的vertex结构x从字节零开始并r从 byte 开始sizeof(float) * 4。

此外，您的鼠标回调会在每次调用时重置您的顶点向量，因此在任何给定时间您永远无法在其中包含多个顶点。它还通过glGenBuffers()in泄露了 VBO 名称initVertexBuffer()。

试一试：

#include <GL/glew.h>
#include <GL/glut.h>
#include <iostream>
#include <vector>

struct vertex 
{
    float x, y;
    float u, v;
    float r, g, b;
};

GLuint vboId;
std::vector<vertex> vertices;
void mouseCB(int button, int state, int x, int y) 
{
    y = glutGet( GLUT_WINDOW_HEIGHT ) - y;
    if (state == GLUT_DOWN) 
    {
        vertex temp = {x, y, 1, 0, 1, 0, 0};  // default red color
        vertices.push_back(temp);
        glBindBuffer(GL_ARRAY_BUFFER, vboId);
        glBufferData(GL_ARRAY_BUFFER, sizeof(vertex) * vertices.size(), &vertices[0], GL_STATIC_DRAW);
        glBindBuffer(GL_ARRAY_BUFFER, 0);
    }
    glutPostRedisplay();
}

void displayCB()
{
    glClearColor(1, 1, 1, 0);
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    glOrtho( 0, w, 0, h, -1, 1 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    if ( vertices.size() > 1 ) 
    {
        glBindBuffer(GL_ARRAY_BUFFER, vboId);
        glEnableClientState(GL_VERTEX_ARRAY);
        glEnableClientState(GL_COLOR_ARRAY);
        glVertexPointer(2, GL_FLOAT, sizeof(vertex), (void*)(sizeof( float ) * 0));
        glColorPointer(3, GL_FLOAT, sizeof(vertex), (void*)(sizeof( float ) * 4));
        glDrawArrays(GL_LINE_STRIP, 0, vertices.size());
        glDisableClientState(GL_VERTEX_ARRAY);
        glDisableClientState(GL_COLOR_ARRAY);
        glBindBuffer(GL_ARRAY_BUFFER, 0);
    }

    glutSwapBuffers();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE|GLUT_DEPTH);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("Test");

    // initialize glew
    glewExperimental = GL_TRUE;
    GLenum glewInitResult = glewInit();
    if (GLEW_OK != glewInitResult) {
        std::cerr << "Error initializing glew." << std::endl;
        return 1;
    }

    glGenBuffers(1, &vboId);

    glutDisplayFunc(displayCB);
    glutMouseFunc(mouseCB);
    glutMainLoop();
    return 0;
}