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立方体绘制基本原理

一个立方体是由8个顶点组成，共6个面，所以绘制立方体本质上就是绘制这6个面共12个三角形

顶点的坐标体系如下图所示，三维坐标的中心原点位于立方体的中心，但是要特别注意的是，前后方向表示的是Z轴，上下方向表示的是Y轴

立方体的顶点坐标和绘制顺序

立方体坐标定义如下：

static GLfloat vertices[] = {// 位置           // 颜色-0.5f, -0.5f, -0.5f,  1.0f, 0.0f, 0.0f,  // 左下后 红色0.5f, -0.5f, -0.5f,  0.0f, 1.0f, 0.0f,  // 右下后 绿色0.5f,  0.5f, -0.5f,  0.0f, 0.0f, 1.0f,  // 右上后 蓝色-0.5f,  0.5f, -0.5f,  1.0f, 1.0f, 0.0f,  // 左上后 黄色-0.5f, -0.5f,  0.5f,  0.0f, 1.0f, 1.0f,  // 左下前 青色0.5f, -0.5f,  0.5f,  1.0f, 0.0f, 1.0f,  // 右下前 品红0.5f,  0.5f,  0.5f,  0.5f, 0.5f, 0.5f,  // 右上前 灰色-0.5f,  0.5f,  0.5f,  1.0f, 1.0f, 1.0f   // 左上前 白色
};// 立方体索引数据
static GLuint indices[] = {// 后面0, 1, 2,2, 3, 0,// 前面4, 5, 6,6, 7, 4,// 左面0, 4, 7,7, 3, 0,// 右面1, 5, 6,6, 2, 1,// 底面0, 1, 5,5, 4, 0,// 顶面3, 2, 6,6, 7, 3
};

在这里vertices定义不同的顶点，indices数组中表示不同顶点的绘制顺序，每三个顶点构成一个三角形，最后由 glDrawElements(GL_TRIANGLES,36,GL_UNSIGNED_INT,0);每三个顶点为一组绘制共12个三角形，不同的面和顶点的绑定关系如下：

立方体颜色和着色器

这里为立方体的每个顶点定义不同的颜色，同时在VertexShader中将颜色传递给FragmentShader,这样 FragmentShader会得到经过插值后的每个片元的颜色，并将这个颜色设置为最终的显示，实现的是一个渐变色的效果，shader 程序如下：

static const char* vertexShaderSource ="#version 300 es                            \n""precision mediump float;\n""uniform mat4 u_mvpMatrix;                   \n""layout(location = 0) in vec3 a_position;   \n""layout(location = 1) in vec3 a_color;      \n""out vec3 v_color;                          \n""void main() {\n""    gl_Position = u_mvpMatrix*vec4(a_position,1.0);\n""    v_color = a_color;                     \n""}\n";// Fragment Shader source code
static const char* fragmentShaderSource ="#version 300 es                            \n""precision mediump float;\n""layout(location = 0) out vec4 outColor;             \n""in vec3 v_color;           \n""void main() {\n""   outColor = vec4(v_color, 1.0);\n""}\n";

实现效果和参考代码

最终实现效果：

参考代码如下：

Note: 这里还使用了ModeViewProject Matrix实现了旋转的效果，同时使用VAO,VBO,EBO实现了对顶点内容和顶点Index的缓冲

typedef struct {GLuint programObject;GLuint vboIds[2];GLuint vaoId;uint64_t timeInMiliSeconds;GLint   mvpLoc;GLfloat   angle;GLint  deltaTime;ESMatrix  mvpMatrix;
} UserData;static GLfloat vertices[] = {// 位置           // 颜色-0.5f, -0.5f, -0.5f,  1.0f, 0.0f, 0.0f,  // 左下后 红色0.5f, -0.5f, -0.5f,  0.0f, 1.0f, 0.0f,  // 右下后 绿色0.5f,  0.5f, -0.5f,  0.0f, 0.0f, 1.0f,  // 右上后 蓝色-0.5f,  0.5f, -0.5f,  1.0f, 1.0f, 0.0f,  // 左上后 黄色-0.5f, -0.5f,  0.5f,  0.0f, 1.0f, 1.0f,  // 左下前 青色0.5f, -0.5f,  0.5f,  1.0f, 0.0f, 1.0f,  // 右下前 品红0.5f,  0.5f,  0.5f,  0.5f, 0.5f, 0.5f,  // 右上前 灰色-0.5f,  0.5f,  0.5f,  1.0f, 1.0f, 1.0f   // 左上前 白色
};// 立方体索引数据
static GLuint indices[] = {// 后面0, 1, 2,2, 3, 0,// 前面4, 5, 6,6, 7, 4,// 左面0, 4, 7,7, 3, 0,// 右面1, 5, 6,6, 2, 1,// 底面0, 1, 5,5, 4, 0,// 顶面3, 2, 6,6, 7, 3
};static const char* vertexShaderSource ="#version 300 es                            \n""precision mediump float;\n""uniform mat4 u_mvpMatrix;                   \n""layout(location = 0) in vec3 a_position;   \n""layout(location = 1) in vec3 a_color;      \n""out vec3 v_color;                          \n""void main() {\n""    gl_Position = u_mvpMatrix*vec4(a_position,1.0);\n""    v_color = a_color;                     \n""}\n";// Fragment Shader source code
static const char* fragmentShaderSource ="#version 300 es                            \n""precision mediump float;\n""layout(location = 0) out vec4 outColor;             \n""in vec3 v_color;           \n""void main() {\n""   outColor = vec4(v_color, 1.0);\n""}\n";static int initInternal(ESContext* esContext) {UserData *userData = esContext->userData;// Vertex Shader source codeGLuint programObject = esLoadProgram(vertexShaderSource, fragmentShaderSource);if (programObject == 0) {return GL_FALSE;}// turn on depth testglEnable(GL_DEPTH_TEST);glDepthFunc(GL_LESS);glClearColor(0.0, 0.0, 0.0, 1.0);// Store the program objectuserData->programObject = programObject;userData->vboIds[0] =  userData->vboIds[1] = 0;userData->mvpLoc = glGetUniformLocation(userData->programObject, "u_mvpMatrix");userData->angle = 0.0f;return GL_TRUE;
}static void uMVPMatrixUpdateRotate(ESContext *esContext, GLuint deltaTime)
{UserData *userData = esContext->userData;ESMatrix perspective;ESMatrix modelview;float    aspect;userData->angle +=  1.0f;if (userData->angle >= 360.0f) {userData->angle -= 360.0f;}aspect = (GLfloat) esContext->width / (GLfloat) esContext->height;esMatrixLoadIdentity(&perspective);esMatrixLoadIdentity(&modelview);esPerspective(&perspective, 45.0f, aspect, 1.0f, 10.0f);esTranslate(&modelview, 0.0, 0.0, -4.0);esRotate(&modelview, userData->angle, 0.0, 1.0, 0.0);esMatrixMultiply(&userData->mvpMatrix, &modelview, &perspective);
}static void DrawPrimitiveWithVBOs(ESContext *esContext)
{UserData *userData = esContext->userData;GLuint   offset = 0;// vboIds[0] - used to store vertex attribute data// vboIds[l] - used to store element indicesif (userData->vboIds[0] == 0 && userData->vboIds[1] == 0) {// Only allocate on the first drawglGenBuffers(2, userData->vboIds);glGenVertexArrays(1, &userData->vaoId);printf("gen vbo id:%d %d vao id:%d.\n",userData->vboIds[0],userData->vboIds[1],userData->vaoId);glBindVertexArray(userData->vaoId);glBindBuffer(GL_ARRAY_BUFFER, userData->vboIds[0]);glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);glVertexAttribPointer(0, 3,GL_FLOAT, GL_FALSE, 6*sizeof(GLfloat) , NULL);glEnableVertexAttribArray(0);glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6*sizeof(GLfloat) ,(void*)(3*sizeof(GLfloat)));glEnableVertexAttribArray(1);// notice using GL_ARRAY_BUFFERglBindBuffer(GL_ELEMENT_ARRAY_BUFFER, userData->vboIds[1]);glBufferData(GL_ELEMENT_ARRAY_BUFFER,sizeof(indices), indices, GL_STATIC_DRAW);glBindVertexArray(0);}glBindVertexArray(userData->vaoId);glUniformMatrix4fv(userData->mvpLoc, 1, GL_FALSE, (GLfloat *)&userData->mvpMatrix.m[0][0]);glDrawElements(GL_TRIANGLES,36,GL_UNSIGNED_INT,0);glBindVertexArray(0);
}static int drawLoopInternal(ESContext* esContext) {UserData *userData = esContext->userData;struct timespec currentts;clock_gettime(CLOCK_REALTIME, &currentts);uint64_t milliseconds = currentts.tv_sec * 1000LL + currentts.tv_nsec / 1000000;int periodInMs = milliseconds - userData->timeInMiliSeconds;userData->timeInMiliSeconds = milliseconds;userData->deltaTime++;//printf("current time in milliseconds %lld period:%d\n",milliseconds, (periodInMs > 0 ? periodInMs: -1));// Set the viewportglViewport(0, 0, 640, 480);glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);glUseProgram(userData->programObject);uMVPMatrixUpdateRotate(esContext,userData->deltaTime);DrawPrimitiveWithVBOs(esContext);// Swap bufferseglSwapBuffers(esContext->eglDisplay, esContext->eglSurface);
}static int cleanupInternal(ESContext* esContext) {printf("%s enter!.\n",__FUNCTION__);UserData *userData = esContext->userData;glDeleteProgram(userData->programObject);eglDestroySurface(esContext->eglDisplay, esContext->eglSurface);eglDestroyContext(esContext->eglDisplay, esContext->eglContext);eglTerminate(esContext->eglDisplay);XDestroyWindow(esContext->x_display, esContext->win);XCloseDisplay(esContext->x_display);return GL_TRUE;
}int testbasicDrawCube(ESContext* esContext) {printf("%s enter!.\n", __FUNCTION__);esContext->userData = (UserData*)malloc(sizeof(UserData));esCreateWindow(esContext, esContext->testcaseName,640, 480, ES_WINDOW_DEPTH);initInternal(esContext);while (1) {XEvent xev;while (XPending(esContext->x_display)) {XNextEvent(esContext->x_display, &xev);if (xev.type == KeyPress) {cleanupInternal(esContext);}}drawLoopInternal(esContext);}
}