如何在 OpenGL 中绘制由 5 个三角形组成的 VAO？

Question

我最近编写了一个程序来绘制一个具有 3 个不同 RGB 值的三角形，我想在同一个程序中对另一个单独的 VAO 做同样的事情，但我希望这个由 5 个三角形组成。这是我的 main.cpp：

void framebuffer_size_callback(GFLWwindow* window, int width, int height);
void processInput(GLFWwindow *window);

// Shaders
const char *vertexShaderSource =
"#version 410\n"
"in vec3 vp;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(aPos, 1.0);\n"
"}\0";

const char *fragmentShader1Source =
"#version 410\n"
"out vec4 FragColor;\n"
"in vec3 myColor;\n"
"void main()\n"
"{\n"
"FragColor = vec4(myColor, 1.0f);\n"
"}\n\0";

int main ()
{
// start GL context and O/S window using the GLFW helper library
if (!glfwInit ())
    {
    fprintf (stderr, "ERROR: could not start GLFW3\n");
    return 1;
    }
    // uncomment these lines if on Apple OS X
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow* window = glfwCreateWindow(640, 480, "LearnOpenGL", NULL, NULL);

    if (!window)
    {
        fprintf(stderr, "ERROR: could not open window with GLFW3\n");
        glfwTerminate();
        return 1;
    }
    glfwMakeContextCurrent(window);

    // start GLEW extension handler
    glewExperimental = GL_TRUE;
    glewInit();

    // get version info
    const GLubyte* renderer = glGetString(GL_RENDERER);
    const GLubyte* version = glGetString(GL_VERSION);

    printf("Renderer: %s\n", renderer);
    printf("OpenGL version supported %s\n", version);

    glEnable(GL_DEPTH_TEST); // enable depth-testing
    glDepthFunc(GL_LESS);

    /* OTHER STUFF GOES HERE */

    // Draw a single triangle VBO
    float points[] = {
    // positions       // colors
    0.0f, 0.5f, 0.0f,  1.0f, 0.0f, 0.0f,
    0.5f, -0.5f, 0.0f, 0.0f, 1.0f, 0.0f,
    -0.5f, -0.5f, 0.0f, 0.0f 0.0f, 1.0f
    };

    GLuint VBO = 0;
    glGenBuffers(1, &VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(points), points, GL_STATIC_DRAW);

    // Generate a VAO.
    GLuint VAO = 0;
    glGenVertexArrays(1, &VAO);
    glBindVertexArray(VAO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(points), points, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
    glEnableVertexAttribArray(0);

    // Compile a Vertex Shader
    int vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);

    // Compile a fragment shader.
    int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);

    // Compile shaders into a executable shader program.
    int shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, fragmentShader);
    glAttachShader(shaderProgram, vertexShader);
    glLinkProgram(shaderProgram);

    // Drawing the triangles aka render loop
    while (!glfwWindowShouldClose(window))
    {
        processInput(window);
        // wipe the drawing surface clear
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        // Draw Triangle
        glUseProgram(shaderProgram);
        glBindVertexArray(VAO);
        glDrawArrays(GL_TRIANGLES, 0, 3);

        // Draw Triangle Fan; unfinished

        // swap buffers and poll IO events
        glfwPollEvents();
        glfwSwapBuffers(window);
    }

    // close GL context and any other GLFW resources
    glfwTerminate();
    return 0;
}

我是否像使用第一个 VBO 或其他东西一样简单地创建另一个浮点“点”矩阵？我遵循的教程在这部分并不完全清楚。

另外，我在我的 Mac 上使用 Xcode，并为我的片段和顶点着色器创建了单独的 .cpp 文件。我应该将它们切换到头文件吗？

Answer 1

您必须指定颜色的输入属性 (aColor) 并将颜色属性从顶点着色器传递到片段着色器 (myColor)。使用Layout Qualifiers 指定属性索引。

#version 330 core

layout(location = 0) in vec3 aPos;
layout(location = 1) in vec3 aColor;

out vec3 myColor;

void main()
{
    myColor = aColor;
    gl_Position = vec4(aPos, 1.0);
}

#version 330 core

out vec4 FragColor;
in vec3 myColor;

void main()
{
    FragColor = vec4(myColor, 1.0f);
}

请注意，您当前的顶点着色器无法编译。通过glGetShaderiv和参数GL_COMPILE_STATUS检查着色器编译是否成功，通过glGetProgramiv和参数GL_LINK_STATUS检查程序链接是否成功。部分代码sn-ps见OpenGL ignores Quads and makes them Triangles的回答。

---

您的顶点是具有 6 个组件（x、y、z、r、g、b）的元组：

float points[] = {
    // positions       // colors
    0.0f, 0.5f, 0.0f,  1.0f, 0.0f, 0.0f,
    0.5f, -0.5f, 0.0f, 0.0f, 1.0f, 0.0f,
   -0.5f, -0.5f, 0.0f, 0.0f 0.0f, 1.0f
};

使用glVertexAttribPointer 指定2 个顶点属性。必须以字节为单位指定步幅和偏移量。步幅为6 * sizeof(float)。顶点坐标的偏移量为0，颜色属性的偏移量为3 * sizeof(float)。例如：

GLuint VBO = 0;
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(points), points, GL_STATIC_DRAW);

// Generate a VAO.
GLuint VAO = 0;
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);

glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);

glBindBuffer(GL_ARRAY_BUFFER, VBO);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), NULL);
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)(3 * sizeof(float)));

---

如果要绘制更复杂的网格，则必须扩展顶点数组。只需将另外 3 个顶点和颜色添加到 points 数组以用于下一个三角形。或者，您可以使用不同的原始类型，例如 GL_TRIANGLE_STRIP 或 GL_TRIANGLE_FAN。见Triangle primitives

示例代码：

#include <iostream>
#include <vector>

// Shaders
const char *vertexShaderSource = R"(#version 330 core

layout(location = 0) in vec3 aPos;
layout(location = 1) in vec3 aColor;

out vec3 myColor;

void main()
{
    myColor = aColor;
    gl_Position = vec4(aPos, 1.0);
}
)";

const char *fragmentShaderSource = R"(#version 330 core

out vec4 FragColor;
in vec3 myColor;

void main()
{
    FragColor = vec4(myColor, 1.0f);
}
)";

bool CompileStatus( GLuint shader );
bool LinkStatus( GLuint program );

float radians( float deg ) { return deg * 3.141529 / 180.0; }

int main ()
{
    // start GL context and O/S window using the GLFW helper library
    if (!glfwInit())
    {
        fprintf (stderr, "ERROR: could not start GLFW3\n");
        return 1;
    }
    // uncomment these lines if on Apple OS X
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow* window = glfwCreateWindow(640, 480, "LearnOpenGL", NULL, NULL);

    if (!window)
    {
        fprintf(stderr, "ERROR: could not open window with GLFW3\n");
        glfwTerminate();
        return 1;
    }
    glfwMakeContextCurrent(window);

    // start GLEW extension handler
    glewExperimental = GL_TRUE;
    glewInit();

    // get version info
    const GLubyte* renderer = glGetString(GL_RENDERER);
    const GLubyte* version = glGetString(GL_VERSION);

    printf("Renderer: %s\n", renderer);
    printf("OpenGL version supported %s\n", version);

    glEnable(GL_DEPTH_TEST); // enable depth-testing
    glDepthFunc(GL_LESS);

    /* OTHER STUFF GOES HERE */

    // Draw a single triangle VBO
    float points[] = {
        // positions                                         // colors
        0.0f,                     0.0f,                      0.0f,   1.0f, 0.0f, 0.0f,
        0.5f,                     0.0f,                      0.0f,   0.0f, 1.0f, 0.0f,
        0.5f * cos(radians(72)),  0.5f * sin(radians(72)),   0.0f,   0.0f, 0.0f, 1.0f,
        0.5f * cos(radians(144)), 0.5f * sin(radians(144)),  0.0f,   1.0f, 1.0f, 0.0f,
        0.5f * cos(radians(216)), 0.5f * sin(radians(216)),  0.0f,   0.0f, 1.0f, 1.0f,
        0.5f * cos(radians(288)), 0.5f * sin(radians(288)),  0.0f,   1.0f, 0.0f, 1.0f,
        0.5,                      0.0f,                      0.0f,   1.0f, 0.5f, 0.0f
    };

    GLuint VBO = 0;
    glGenBuffers(1, &VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(points), points, GL_STATIC_DRAW);

    // Generate a VAO.
    GLuint VAO = 0;
    glGenVertexArrays(1, &VAO);
    glBindVertexArray(VAO);

    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), NULL);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)(3 * sizeof(float)));

    // Compile a Vertex Shader
    int vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    CompileStatus( vertexShader );

    // Compile a fragment shader.
    int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    CompileStatus( fragmentShader );

    // Compile shaders into a executable shader program.
    int shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, fragmentShader);
    glAttachShader(shaderProgram, vertexShader);
    glLinkProgram(shaderProgram);
    LinkStatus( shaderProgram );

    // Drawing the triangles aka render loop
    while (!glfwWindowShouldClose(window))
    {
        //processInput(window);
        // wipe the drawing surface clear
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        // Draw Triangle
        glUseProgram(shaderProgram);
        glBindVertexArray(VAO);
        glDrawArrays(GL_TRIANGLE_FAN, 0, 7);

        // Draw Triangle Fan; unfinished

        // swap buffers and poll IO events
        glfwPollEvents();
        glfwSwapBuffers(window);
    }

    // close GL context and any other GLFW resources
    glfwTerminate();
    return 0;
}

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

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

Answer 2

您可以通过添加创建另一个包含新点的浮点数组并创建另一个 VAO 和 VBO 来完成此操作。由于您想要一个三角形扇形（基于代码中的注释），而不是 5 个单独的三角形，您可以这样：

float points_5_triangles[] = {
    // positions       // colors
    // Original triangle
    x1, y1, z1, r1, g1, b1,    // point 1
    x2, y2, z2, r2, g2, b2,    // point 2
    x3, y3, z3, r3, g3, b3,    // point 3
    // Another triangle made from point 1, 3 and 4
    x4, y4, z4, r4, g4, b4,
    // Another triangle made from point 1, 4 and 5
    x5, y5, z5, r5, g5, b5,
    // Another triangle made from point 1, 5 and 6
    x6, y6, z6, r6, g6, b6,
    // Another triangle made from point 1, 6 and 7
    x7, y7, z7, r7, g7, b7,
};

GLuint VBO_5_triangles = 0;
glGenBuffers(1, &VBO_5_triangles);
glBindBuffer(GL_ARRAY_BUFFER, VBO_5_triangles);
glBufferData(GL_ARRAY_BUFFER, sizeof(points_5_triangles), points_5_triangles, GL_STATIC_DRAW);

// Generate another VAO.
GLuint VAO_5_triangles = 0;
glGenVertexArrays(1, &VAO_5_triangles);
glBindVertexArray(VAO_5_triangles);
glBindBuffer(GL_ARRAY_BUFFER, VBO_5_triangles);
glBufferData(GL_ARRAY_BUFFER, sizeof(points_5_triangles), points_5_triangles, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
glEnableVertexAttribArray(0);

现在在绘制两个对象时，首先绑定目标 VAO，然后渲染，然后继续下一个对象：

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

glBindVertexArray(VAO_5_triangles);
glDrawArrays(GL_TRIANGLE_FAN, 0, 7);

有关如何绘制三角形扇形的更多信息，请参阅Triangle primitives