为什么 glDrawElements 不绘制我的圆圈？

问题描述

我正在学习 opengl 并尝试使用 glDrawEmelents 绘制索引圆，但由于某种原因它不起作用。但是，当我使用 glDrawElements 绘制三角形时（请参阅注释代码），它可以很好地绘制三角形。我认为这与我的元素/索引有关，但我不知道。

SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION,3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MInor_VERSION,1);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK,SDL_GL_CONTEXT_PROFILE_CORE);
SDL_Init(SDL_INIT_VIDEO);
SDL_Window* window = SDL_CreateWindow("GL Window",SDL_WINDOWPOS_UNDEFINED,500,SDL_WINDOW_OPENGL);
SDL_GLContext context =  SDL_GL_CreateContext(window);
glewExperimental = true;
GLenum error = glewInit();

if (error != GLEW_OK)
    return -1;



if (context == NULL)
    return -1;

SDL_Event event;


gluint vShader = 0;
gluint fShader = 0;
static const GLchar* fragText[] = {
    "#version 450 core \n"
    "\n"
    "out vec4 color;\n"
    "void main(void)\n"
    "{\n"
    "   color = vec4(0.5,0.8,1.0,0.7);\n"
    "}\n"
};

static const GLchar* vertText[] = {
    "#version 450 core \n"
    "layout (location = 0) in vec4 position;\n"
    "void main(void)\n"
    "{\n"
    "   gl_Position = position;\n"
    "}\n"
};

vShader = glCreateShader(GL_VERTEX_SHADER);
fShader = glCreateShader(GL_FRAGMENT_SHADER);

glShaderSource(vShader,1,vertText,NULL);

glCompileShader(vShader);

GLint status;
glGetShaderiv(vShader,GL_COMPILE_STATUS,&status);

if (status != GL_TRUE) {
    printf("v bust");
    return -1;
}

glShaderSource(fShader,fragText,NULL);

glCompileShader(fShader);

if (status != GL_TRUE) {
    printf("f bust");
    return -1;
}


gluint programID = glCreateProgram();
glAttachShader(programID,vShader);
glAttachShader(programID,fShader);

glLinkProgram(programID);



glm::vec4 circLeverts[33];
int size = (sizeof(circLeverts) / sizeof(glm::vec4));
circLeverts[0] = glm::vec4(0,1.0f);
for (int i = 1; i < size; i++) {
    float heading = 360.f * ((float)i / (float)(size));
    circLeverts[i] =  glm::vec4(0.5f * glm::cos(heading),0.5f * glm::sin(heading),1);
    printf("%d:(%f,%f,%f)\n",i,circLeverts[i].x,circLeverts[i].y,circLeverts[i].z,circLeverts[i].w);
    printf("heading:%f\n",heading);
}

int circleIndices[96];
size = (sizeof(circleIndices) / sizeof(int));
for (int i = 0; i < size; i++) {
    circleIndices[i] = 0;
    circleIndices[i + 1] = 1;// i % 32;
    circleIndices[i + 2] = 2;// (i + 1) % 32;
}

/* draw triangle
glm::vec4 circLeverts[3];
circLeverts[0] = glm::vec4(0,.5f,1);
circLeverts[1] = glm::vec4(0.5f,-.5f,1);
circLeverts[2] = glm::vec4(-0.5f,-0.5f,1);

glushort circleIndices[3];
circleIndices[0] = 0;
circleIndices[1] = 1;
circleIndices[2] = 2;
*/
gluint vbo;
glGenBuffers(1,&vbo);
glBindBuffer(GL_ARRAY_BUFFER,vbo);
glBufferData(GL_ARRAY_BUFFER,sizeof(circLeverts),circLeverts,GL_STATIC_DRAW);

glVertexAttribPointer(0,4,GL_FLOAT,GL_FALSE,NULL);
glEnabLevertexAttribArray(0);

gluint ibo;
glGenBuffers(1,&ibo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,ibo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER,sizeof(circleIndices),circleIndices,GL_STATIC_DRAW);


glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC1_ALPHA);

while (true) {
    SDL_PollEvent(&event);
    if (event.type == SDL_QUIT) {
        SDL_GL_DeleteContext(context);
        SDL_DestroyWindow(window);
        SDL_Quit();
        return 1;
    }
    glViewport(0,500);
    glClearColor(0,1);
    glClear(GL_COLOR_BUFFER_BIT);

    gluseProgram(programID);

    //glDrawElements(GL_TRIANGLES,3,GL_UNSIGNED_INT,0);
    glDrawElements(GL_TRIANGLES,96,0);
    //glDrawArrays(GL_POINTS,32);
    SDL_GL_SwapWindow(window);
}

它也可以很好地绘制点。

解决方法

glDrawElements 的 type 参数必须与索引的数据类型相对应。
GL_UNSIGNED_INT 对应于 GLuint。如果数据类型为 GLushort，则类型参数必须为 GL_UNSIGNED_USHORT。