使用 SDL2 和 OpenGL 旋转相机和三角形绘制不会显示任何内容？

Question

我只是想在中心创建一个三角形并围绕中心旋转相机。 (0,0,0)。该代码是从多个教程源中提取的，我怀疑问题可能出在三角形绘制调用之前的 glm::perspective/lookat 或 gl:projection/model matrix 中。因为这就是我现在普遍的困惑所在。非常感谢任何帮助，这样我就可以继续我的生活了。提前谢谢你。

//sudo g++ -o sdl main.cpp -lSDL2_image -lGL -lGLU -lglut -lX11 -lGLEW `sdl2-config --cflags --libs`
#include <iostream>
#include <GL/glew.h>
#include <glm/glm.hpp>
#include "glm/gtc/matrix_transform.hpp"
#include <glm/gtc/type_ptr.hpp>
#include <SDL2/SDL.h>
#include <string>
#include <GL/gl.h>



std::string programName = "SDL2/OpenGL";
SDL_Window *mainWindow;
SDL_GLContext mainContext;

void Calculate()
{
    float radius = 2.0f;
    float camX = sin(SDL_GetTicks()*0.001) * radius;
    float camZ = cos(SDL_GetTicks()*0.001) * radius;

    glm::mat4 perspecive_mat = glm::perspective(
        45.0f, 1.0f / 1.0f, 0.1f, 100.0f );

    glm::mat4 view_mat = glm::lookAt(
        glm::vec3(camX, 0.0, camZ), glm::vec3(0.0, 0.0, 0.0), glm::vec3(0.0, 1.0, 0.0) );

    glMatrixMode(GL_PROJECTION);
    glLoadMatrixf(glm::value_ptr(perspecive_mat));

    glMatrixMode(GL_MODELVIEW);
    glLoadMatrixf(glm::value_ptr(view_mat));
}


void Render()
{
    glClearColor(0.0, 0.0, 0.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 

    float vertexCoords[24] = {  // Coordinates for the vertices of a cube.
               1,1,1,   1,1,-1,   1,-1,-1,   1,-1,1,
              -1,1,1,  -1,1,-1,  -1,-1,-1,  -1,-1,1  };

    float vertexColors[24] = {  // An RGB color value for each vertex
               1,1,1,   1,0,0,   1,1,0,   0,1,0,
               0,0,1,   1,0,1,   0,0,0,   0,1,1  };

    int elementArray[24] = {  // Vertex numbers for the six faces.
              0,1,2,3, 0,3,7,4, 0,4,5,1,
              6,2,1,5, 6,5,4,7, 6,7,3,2  };

    glVertexPointer( 3, GL_FLOAT, 0, vertexCoords );
    glColorPointer( 3, GL_FLOAT, 0, vertexColors );

    glEnableClientState( GL_VERTEX_ARRAY );
    glEnableClientState( GL_COLOR_ARRAY );

    glDrawElements( GL_QUADS, 24, GL_UNSIGNED_INT, elementArray );


    SDL_GL_SwapWindow(mainWindow);
}

bool Loop()
{
    while (true ){
        SDL_Event event;
        while ( SDL_PollEvent( &event ) ){
            switch ( event.type ){
                case SDL_QUIT :
                SDL_Quit();
                return 0;
                case SDL_KEYDOWN : 
                    std::cout<<"Key Down"<<std::endl;
                    break;
                case SDL_KEYUP :
                    std::cout<<"Key Up"<<std::endl;
                    break;
                case SDL_MOUSEBUTTONDOWN :
                case SDL_MOUSEBUTTONUP :
                case SDL_MOUSEMOTION :
                default :
                    break;
            }
        }

        Calculate();

        Render();
    }
}


void CheckSDLError(int line = -1){
    std::string error = SDL_GetError();

    if (error != "")
    {
        std::cout << "SLD Error : " << error << std::endl;

        if (line != -1)
            std::cout << "\nLine : " << line << std::endl;

        SDL_ClearError();
    }
}


void Cleanup(){
    SDL_GL_DeleteContext(mainContext);
    SDL_DestroyWindow(mainWindow );
    SDL_Quit();
}

int main(int argc, char *argv[]){
    if (SDL_Init(SDL_INIT_VIDEO) < 0){
        std::cout << "Failed to init SDL\n";
        return false;
    }
    mainWindow = SDL_CreateWindow(programName.c_str(), SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,512, 512, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);
    if (!mainWindow )
    {
        std::cout << "Unable to create window\n"<< std::endl;;
        CheckSDLError(__LINE__);
        return false;
    }

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
    //SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_COMPATIBILITY);

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    mainContext = SDL_GL_CreateContext(mainWindow );

    // This makes our buffer swap syncronized with the monitor's vertical refresh
    //      ( which means it enables v-sync)
    // Setting this to 0 will disable V-sync
    //      Which means our application could run at unlimited fps
    SDL_GL_SetSwapInterval(1);
    // Init GLEW
    glewExperimental = GL_TRUE; 
    glewInit();
    // Enable blending so that we can have transparanet object
    glEnable(GL_BLEND ) ;
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    // Enable depth testing so that closer triangles will hide the triangles farther away
    glEnable( GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);

    Loop();
    Cleanup();

    return 0;
}

Answer 1

请注意，几十年来不推荐使用 glBegin/glEnd 序列和固定功能流水线矩阵堆栈进行绘制。阅读 Fixed Function Pipeline and see Vertex Specification and Shader 了解最先进的渲染方式。

如果您想使用已弃用的绘图方式，则必须使用兼容性配置文件上下文而不是核心配置文件上下文（参见 OpenGL Context）：

~~SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);~~
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_COMPATIBILITY);

另外注意，glm::lookAt and glm::perspective don't set any OpenGL states or even fixed function pipline matrices. glm - OpenGL Mathematics是一个数学库，做OpenGL相关的计算。两者都函数 return 一个 4*4 矩阵。

您可以使用 glLoadMatrixf 通过 glm 函数将矩阵 return 加载到 OpenGL 固定函数管道矩阵堆栈：

#include <glm/glm.hpp>
#include "glm/gtc/matrix_transform.hpp"
#include <glm/gtc/type_ptr.hpp>

void Calculate()
{
    float radius = 2.0f;
    float camX = sin(SDL_GetTicks()*0.001) * radius;
    float camZ = cos(SDL_GetTicks()*0.001) * radius;

    glm::mat4 perspecive_mat = glm::perspective(
        45.0f, 1.0f / 1.0f, 0.1f, 100.0f );

    glm::mat4 view_mat = glm::lookAt(
        glm::vec3(camX, 0.0, camZ), glm::vec3(0.0, 0.0, 0.0), glm::vec3(0.0, 1.0, 0.0) );

    glMatrixMode(GL_PROJECTION);
    glLoadMatrixf(glm::value_ptr(perspecive_mat));

    glMatrixMode(GL_MODELVIEW);
    glLoadMatrixf(glm::value_ptr(view_mat));
}

如果要使用核心配置文件，则必须创建一个简单的 Shader 程序：

#include <string>

std::string sh_vert = R"(
#version 150 core

in vec3 v_pos;
in vec4 v_col;

out vec4 color;

uniform mat4 projection;
uniform mat4 view;

void main()
{
    color = v_col;                    
    gl_Position = projection * view * vec4(v_pos, 1.0);
} 
)";

std::string sh_frag = R"(
#version 150 core

in vec4 color;

void main()
{
    gl_FragColor = color;
} 
)";

编译并link程序：

#include <vector>

GLuint CreateShader(GLenum type, const char *code)
{
    GLuint shaderObj = glCreateShader(type);
    glShaderSource(shaderObj, 1, &code, nullptr);
    glCompileShader(shaderObj);

    GLint status = GL_TRUE;
    glGetShaderiv(shaderObj, GL_COMPILE_STATUS, &status);
    if (status == GL_FALSE)
    {
        GLint logLen;
        glGetShaderiv(shaderObj, GL_INFO_LOG_LENGTH, &logLen);
        std::vector< char >log(logLen);
        GLsizei written;
        glGetShaderInfoLog(shaderObj, logLen, &written, log.data());
        std::cout << "compile error:" << std::endl << log.data() << std::endl;
    }
    return shaderObj;
}

GLuint CreateProgram()
{
    GLuint vShObj = CreateShader(GL_VERTEX_SHADER, sh_vert.c_str());
    GLuint fShObj = CreateShader(GL_FRAGMENT_SHADER, sh_frag.c_str());

    GLuint progObj = glCreateProgram();
    glAttachShader(progObj, vShObj);
    glAttachShader(progObj, fShObj);
    glLinkProgram(progObj);

    GLint status = GL_TRUE;
    glGetProgramiv(progObj, GL_LINK_STATUS, &status);
    if (status == GL_FALSE)
    {
        GLint logLen;
        glGetProgramiv(progObj, GL_INFO_LOG_LENGTH, &logLen);
        std::vector< char >log(logLen);
        GLsizei written;
        glGetProgramInfoLog(progObj, logLen, &written, log.data());
        std::cout << "link error:" << std::endl << log.data() << std::endl;
    }
    return progObj;
}

获取属性和统一位置：

GLuint prog;
GLint pos_attr, col_attr, proj_loc, view_loc;

void Init()
{
    prog = CreateProgram();
    pos_attr = glGetAttribLocation(prog, "v_pos");
    col_attr = glGetAttribLocation(prog, "v_col");
    proj_loc = glGetUniformLocation(prog, "projection");
    view_loc = glGetUniformLocation(prog, "view");

    // ....

 }

创建 Vertex Array Object:

GLuint vao;

void Init()
{
    // ....

    const std::vector<float> varray
    {
    // x      y     z     red   green blue
      -0.5f, -0.5f, 0.0f, 1.0f, 1.0f, 0.0f,
       0.5f,  0.0f, 0.0f, 0.0f, 1.0f, 0.0f,
       0.0f,  0.5f, 0.0f, 0.0f, 0.0f, 1.0f
    };

    GLuint vbo;
    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, varray.size()*sizeof(float), varray.data(), GL_STATIC_DRAW);

    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);
    glVertexAttribPointer(pos_attr, 3, GL_FLOAT, GL_FALSE, 6*sizeof(float), 0);
    glEnableVertexAttribArray(pos_attr);
    glVertexAttribPointer(col_attr, 3, GL_FLOAT, GL_FALSE, 6*sizeof(float), (void*)(3*sizeof(float)));
    glEnableVertexAttribArray(col_attr);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);
}

安装程序对象作为当前渲染状态的一部分并设置统一变量：

void Calculate()
{
    float radius = 2.0f;
    float camX = sin(SDL_GetTicks()*0.001) * radius;
    float camZ = cos(SDL_GetTicks()*0.001) * radius;

    glm::mat4 perspecive_mat = glm::perspective(
        45.0f, 1.0f / 1.0f, 0.1f, 100.0f );

    glm::mat4 view_mat = glm::lookAt(
        glm::vec3(camX, 0.0, camZ), glm::vec3(0.0, 0.0, 0.0), glm::vec3(0.0, 1.0, 0.0) );

    glUseProgram(prog);
    glUniformMatrix4fv(proj_loc, 1, GL_FALSE, glm::value_ptr(perspecive_mat));
    glUniformMatrix4fv(view_loc, 1, GL_FALSE, glm::value_ptr(view_mat));
}

最后画出三角形：

void Render()
{
    glClearColor(0.0, 0.0, 0.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glBindVertexArray(vao);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glBindVertexArray(0);

    SDL_GL_SwapWindow(mainWindow);
}

bool Loop()
{
    Init();

    while (true) {
        // ....

        Calculate();
        Render();
    }
}

使用 SDL2 和 OpenGL 旋转相机和三角形绘制不会显示任何内容？

Using SDL2 and OpenGL to rotate camera and a triangle draw wont display anything?

c++

opengl

glm-math

sdl-2

gl-matrix