opengl-球体的绘制

球体

opengl-pbr 光照一讲里，以球体为案例进行说明。球体的绘制第一次接触理解花了点时间

一、球体坐标分解

球体坐标分解

按照经纬度来理解，经度是y轴上的切面，维度是x-z 平面的弧度

经纬度

将y轴切分成64等分，x-z平面切分成64等分的扇形
y轴从上到下取值为[0, 180]，即[0, π]，x-z平面扫描取值[0, 360]即[0, 2π]
详细计算，参考如下代码，把一个球体拆分成65 * 65 个点
注意：
y = cos(ySegment * PI);
x = sin(ySegment * PI) * cos(xSegment * 2 * PI);
x = sin(ySegment * PI) * sin (xSegment * 2 * PI);
按照上面这个顺序来理解比较容易

     const unsigned int X_SEGMENTS = 64;
        const unsigned int Y_SEGMENTS = 64;
        const float PI = 3.14159265359;
        for (unsigned int y = 0; y <= Y_SEGMENTS; ++y)
        {
            for (unsigned int x = 0; x <= X_SEGMENTS; ++x)
            {
                float xSegment = (float)x / (float)X_SEGMENTS;
                float ySegment = (float)y / (float)Y_SEGMENTS;
                float xPos = std::cos(xSegment * 2.0f * PI) * std::sin(ySegment * PI);
                float yPos = std::cos(ySegment * PI);
                float zPos = std::sin(xSegment * 2.0f * PI) * std::sin(ySegment * PI);

                positions.push_back(glm::vec3(xPos, yPos, zPos));
                uv.push_back(glm::vec2(xSegment, ySegment));
                normals.push_back(glm::vec3(xPos, yPos, zPos));
            }
        }

二、绘制三角形

这里有个坑，一开始没理解GL_TRIANGLE_STRIP，走了弯路，实际上理解了GL_TRIANGLE_STRIP，就明白代码中绘制的逻辑
glDrawElements(GL_TRIANGLE_STRIP, indexCount, GL_UNSIGNED_INT, 0);
三角形绘制有三种方式：
GL_TRIANGLES、GL_TRIANGLE_STRIP和GL_TRIANGLE_FAN

GL_TRIANGLES比较好理解，按照一个个三角形来绘制，每次给三个坐标。
GL_TRIANGLE_STRIP，起始三个点，后面没增加一个点就增加一个三角形

三角形绘制模式

为保证所有三角形绘制都遵循顺时针原则(剔除中用到)，要求当前增加的点角标为奇数：绘制顺序T=[n-1, n-2, n]
为偶数：绘制顺序T=[n-2, n-1, n]
参考下图，自己拿笔画一画就明白了

绘制顺序

绘制球体的其他代码就很好理解了不一一说明

unsigned int sphereVAO = 0;
unsigned int indexCount;
void renderSphere()
{
    if (sphereVAO == 0)
    {
        glGenVertexArrays(1, &sphereVAO);

        unsigned int vbo, ebo;
        glGenBuffers(1, &vbo);
        glGenBuffers(1, &ebo);

        std::vector<glm::vec3> positions;
        std::vector<glm::vec2> uv;
        std::vector<glm::vec3> normals;
        std::vector<unsigned int> indices;

        const unsigned int X_SEGMENTS = 64;
        const unsigned int Y_SEGMENTS = 64;
        const float PI = 3.14159265359;
        for (unsigned int y = 0; y <= Y_SEGMENTS; ++y)
        {
            for (unsigned int x = 0; x <= X_SEGMENTS; ++x)
            {
                float xSegment = (float)x / (float)X_SEGMENTS;
                float ySegment = (float)y / (float)Y_SEGMENTS;
                float xPos = std::cos(xSegment * 2.0f * PI) * std::sin(ySegment * PI);
                float yPos = std::cos(ySegment * PI);
                float zPos = std::sin(xSegment * 2.0f * PI) * std::sin(ySegment * PI);

                positions.push_back(glm::vec3(xPos, yPos, zPos));
                uv.push_back(glm::vec2(xSegment, ySegment));
                normals.push_back(glm::vec3(xPos, yPos, zPos));
            }
        }

        bool oddRow = false;
        for (unsigned int y = 0; y < Y_SEGMENTS; ++y)
        {
            if (!oddRow) // even rows: y == 0, y == 2; and so on
            {
                for (unsigned int x = 0; x <= X_SEGMENTS; ++x)
                {
                    indices.push_back(y       * (X_SEGMENTS + 1) + x);
                    indices.push_back((y + 1) * (X_SEGMENTS + 1) + x);
                }
            }

          // 这里奇偶分开添加是有道理的，奇偶分开添加，就能首位相连，自己可以拿笔画一画
            else
            {
                for (int x = X_SEGMENTS; x >= 0; --x)
                {
                    indices.push_back((y + 1) * (X_SEGMENTS + 1) + x);
                    indices.push_back(y       * (X_SEGMENTS + 1) + x);
                }
            }
            oddRow = !oddRow;
        }
        indexCount = indices.size();

        std::vector<float> data;
        for (unsigned int i = 0; i < positions.size(); ++i)
        {
            data.push_back(positions[i].x);
            data.push_back(positions[i].y);
            data.push_back(positions[i].z);
            if (uv.size() > 0)
            {
                data.push_back(uv[i].x);
                data.push_back(uv[i].y);
            }
            if (normals.size() > 0)
            {
                data.push_back(normals[i].x);
                data.push_back(normals[i].y);
                data.push_back(normals[i].z);
            }
        }
        glBindVertexArray(sphereVAO);
        glBindBuffer(GL_ARRAY_BUFFER, vbo);
        glBufferData(GL_ARRAY_BUFFER, data.size() * sizeof(float), &data[0], GL_STATIC_DRAW);
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(unsigned int), &indices[0], GL_STATIC_DRAW);
        float stride = (3 + 2 + 3) * sizeof(float);
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, stride, (void*)0);
        glEnableVertexAttribArray(1);
        glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, stride, (void*)(3 * sizeof(float)));
        glEnableVertexAttribArray(2);
        glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, stride, (void*)(5 * sizeof(float)));
    }

    glBindVertexArray(sphereVAO);
    glDrawElements(GL_TRIANGLE_STRIP, indexCount, GL_UNSIGNED_INT, 0);
}

三、线条模式绘制动画