问

glDrawElements只绘制由indices指定的第一个三角形

手机用户2502876173 发布于 2023-01-11 20:19

编译

我刚刚开始学习OpenGL,我在使用glDrawElements时遇到了一些麻烦.我试图使用带有GL_TRIANGLE的glDrawElements绘制两个三角形,但只出现一个三角形.

预期结果:glDrawElements绘制两个带顶点的三角形

(0,0)(1,1)(-1,1)和(0,0)( - 1,-1)(1,-1)

会发生什么:glDrawElements绘制一个带顶点的三角形(0,0)(1,1)( - 1,1)

短,自包含,正确(可编译),示例:

#include 
#include 

int main(int argc, char *argv[])
{
    glfwInit();
    GLFWwindow* window = glfwCreateWindow(640, 480, "Sample code does not display two triangles", NULL, NULL);
    glfwMakeContextCurrent(window);

    glewInit();

    GLfloat vertices[] {
        +0.0f, +0.0f, //0
        +1.0f, +1.0f, //1
        -1.0f, +1.0f, //2
        -1.0f, -1.0f  //3
        +1.0f, -1.0f  //4
    };

    GLuint vertexBufferID;
    glGenBuffers(1, &vertexBufferID);
    glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, 0);

    GLuint indexBufferID;
    GLushort indices[] {0,1,2, 0,3,4};
    glGenBuffers(1, &indexBufferID);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBufferID);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    while (!glfwWindowShouldClose(window))
    {
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, 0);
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glfwTerminate();
    return 0;
}

此示例打开一个640x480 GLFW窗口,显示一个三角形,其顶角和顶部位于窗口中间的第三个顶点.缺少另一个三角形,其中两个底角中的顶点和中间的第三个顶点.这是为什么？

规格:
操作系统:Linux Mint 17
GPU:nVidia GTX 275
GPU驱动程序:331.67
GLEW版本:1.10.0
GLFW版本:3.0.4

1 个回答

逗号很重要.这个:

GLfloat vertices[] =
{
    +0.0f, +0.0f, //0
    +1.0f, +1.0f, //1
    -1.0f, +1.0f, //2
    -1.0f, -1.0f  //3
    +1.0f, -1.0f  //4
};
size_t elements = sizeof( vertices ) / sizeof( GLfloat ) = 9(!)

与此不一样:

GLfloat vertices[] =
{
    +0.0f, +0.0f, //0
    +1.0f, +1.0f, //1
    -1.0f, +1.0f, //2
    -1.0f, -1.0f, //3
    +1.0f, -1.0f  //4
};
size_t elements = sizeof( vertices ) / sizeof( GLfloat ) = 10

请注意行尾添加的逗号3.

如果vertices只有9元素,那么当glDrawElements()试图读取第五个顶点时,只有X坐标才有效.如果你幸运的话,Y坐标将包含垃圾,如果你不幸,则会出现段错误.

此外,如果不指定某些着色器,则不应使用通用顶点属性函数.

全部一起:

#include <GL/glew.h>
#include <GLFW/glfw3.h>

int main(int argc, char *argv[])
{
    glfwInit();
    GLFWwindow* window = glfwCreateWindow(640, 480, "Sample code does not display two triangles", NULL, NULL);
    glfwMakeContextCurrent(window);

    glewInit();

    GLfloat vertices[] =
    {
        +0.0f, +0.0f, //0
        +1.0f, +1.0f, //1
        -1.0f, +1.0f, //2
        -1.0f, -1.0f, //3
        +1.0f, -1.0f, //4
    };

    GLuint vertexBufferID;
    glGenBuffers(1, &vertexBufferID);
    glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glEnableClientState( GL_VERTEX_ARRAY );
    glVertexPointer( 2, GL_FLOAT, 0, 0 );

    GLuint indexBufferID;
    GLushort indices[] = { 0,1,2, 0,3,4 };
    glGenBuffers(1, &indexBufferID);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBufferID);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    while (!glfwWindowShouldClose(window))
    {
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, 0);
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glfwTerminate();
    return 0;
}

2023-01-11 20:24 回答

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