/** 
createSquare is used to create the Vertex Array Object which will hold our square. We will 
be hard coding in the vertices for the square, which will be done in this method. 
*/  
void OpenGLContext::createSquare(void) {  
float* vertices = new float[18];  // Vertices for our square  
  
vertices[0] = -0.5; vertices[1] = -0.5; vertices[2] = 0.0; // 左下  1
vertices[3] = -0.5; vertices[4] = 0.5; vertices[5] = 0.0;  // 左上  2 
vertices[6] = 0.5; vertices[7] = 0.5; vertices[8] = 0.0;   // 右上  3
  
vertices[9] = 0.5; vertices[10] = -0.5; vertices[11] = 0.0; // 右下 4 
vertices[12] = -0.5; vertices[13] = -0.5; vertices[14] = 0.0; //左下1

vertices[15] = 0.5; vertices[16] = 0.5; vertices[17] = 0.0; // 右上 3  

delete [] vertices; // Delete our vertices from memory  
}

glGenVertexArrays(1, &vaoID[0]); // Create our Vertex Array Object  
glBindVertexArray(vaoID[0]); // Bind our Vertex Array Object so we can use it  
  
glGenBuffers(1, vboID); // Generate our Vertex Buffer Object  
glBindBuffer(GL_ARRAY_BUFFER, vboID[0]); // Bind our Vertex Buffer Object  
glBufferData(GL_ARRAY_BUFFER, 18 * sizeof(GLfloat), vertices, GL_STATIC_DRAW); // Set the size and data of our VBO and set it to STATIC_DRAW  
  
glVertexAttribPointer((GLuint)0, 3, GL_FLOAT, GL_FALSE, 0, 0); // Set up our vertex attributes pointer  
  
glEnableVertexAttribArray(0); // Disable our Vertex Array Object  
glBindVertexArray(0); // Disable our Vertex Buffer Object  
  
delete [] vertices; // Delete our vertices from memory

glBindVertexArray(vaoID[0]); // Bind our Vertex Array Object  
  
glDrawArrays(GL_TRIANGLES, 0, 6); // Draw our square  
  
glBindVertexArray(0); // Unbind our Vertex Array Object

void glGenVertexArrays(GLSize n, GLuint * *arrays);

GLvoid glBingVertexArray(GLuint array);

void glVertexAttribPointer()

void glDrawArrays(GLenum type ,GLint start, GLint count);

void glDrawArrays(GLenum type, GLint start, GLint count)
{
    for(GLint element = start; element < start + count; element++)
    {
        VertexShader(positionAttribArray[element], colorAttribArray[element]);
    }
}

void glDrawElements(GLenum type, GLint count, GLenum type, GLsizeiptr indices)

GLvoid *elementArray;

void glDrawElements(GLenum type, GLint count, GLenum type, GLsizeiptr indices)
{
    GLtype *ourElementArray = (type*)((GLbyte *)elementArray + indices);

    for(GLint elementIndex = 0; elementIndex < count; elementIndex++)
    {
        GLint element = ourElementArray[elementIndex];
        VertexShader(positionAttribArray[element], colorAttribArray[element]);
    }
}

const int numberOfVertices = 36;

#define RIGHT_EXTENT 0.8f
#define LEFT_EXTENT -RIGHT_EXTENT
#define TOP_EXTENT 0.20f
#define MIDDLE_EXTENT 0.0f
#define BOTTOM_EXTENT -TOP_EXTENT
#define FRONT_EXTENT -1.25f
#define REAR_EXTENT -1.75f

#define GREEN_COLOR 0.75f, 0.75f, 1.0f, 1.0f
#define BLUE_COLOR     0.0f, 0.5f, 0.0f, 1.0f
#define RED_COLOR 1.0f, 0.0f, 0.0f, 1.0f
#define GREY_COLOR 0.8f, 0.8f, 0.8f, 1.0f
#define BROWN_COLOR 0.5f, 0.5f, 0.0f, 1.0f

const float vertexData[] = {
    //Object 1 positions
    LEFT_EXTENT,    TOP_EXTENT,        REAR_EXTENT,
    LEFT_EXTENT,    MIDDLE_EXTENT,    FRONT_EXTENT,
    RIGHT_EXTENT,    MIDDLE_EXTENT,    FRONT_EXTENT,
    RIGHT_EXTENT,    TOP_EXTENT,        REAR_EXTENT,

    LEFT_EXTENT,    BOTTOM_EXTENT,    REAR_EXTENT,
    LEFT_EXTENT,    MIDDLE_EXTENT,    FRONT_EXTENT,
    RIGHT_EXTENT,    MIDDLE_EXTENT,    FRONT_EXTENT,
    RIGHT_EXTENT,    BOTTOM_EXTENT,    REAR_EXTENT,

    LEFT_EXTENT,    TOP_EXTENT,        REAR_EXTENT,
    LEFT_EXTENT,    MIDDLE_EXTENT,    FRONT_EXTENT,
    LEFT_EXTENT,    BOTTOM_EXTENT,    REAR_EXTENT,

    RIGHT_EXTENT,    TOP_EXTENT,        REAR_EXTENT,
    RIGHT_EXTENT,    MIDDLE_EXTENT,    FRONT_EXTENT,
    RIGHT_EXTENT,    BOTTOM_EXTENT,    REAR_EXTENT,

    LEFT_EXTENT,    BOTTOM_EXTENT,    REAR_EXTENT,
    LEFT_EXTENT,    TOP_EXTENT,        REAR_EXTENT,
    RIGHT_EXTENT,    TOP_EXTENT,        REAR_EXTENT,
    RIGHT_EXTENT,    BOTTOM_EXTENT,    REAR_EXTENT,

    //Object 2 positions
    TOP_EXTENT,        RIGHT_EXTENT,    REAR_EXTENT,
    MIDDLE_EXTENT,    RIGHT_EXTENT,    FRONT_EXTENT,
    MIDDLE_EXTENT,    LEFT_EXTENT,    FRONT_EXTENT,
    TOP_EXTENT,        LEFT_EXTENT,    REAR_EXTENT,

    BOTTOM_EXTENT,    RIGHT_EXTENT,    REAR_EXTENT,
    MIDDLE_EXTENT,    RIGHT_EXTENT,    FRONT_EXTENT,
    MIDDLE_EXTENT,    LEFT_EXTENT,    FRONT_EXTENT,
    BOTTOM_EXTENT,    LEFT_EXTENT,    REAR_EXTENT,

    TOP_EXTENT,        RIGHT_EXTENT,    REAR_EXTENT,
    MIDDLE_EXTENT,    RIGHT_EXTENT,    FRONT_EXTENT,
    BOTTOM_EXTENT,    RIGHT_EXTENT,    REAR_EXTENT,
                    
    TOP_EXTENT,        LEFT_EXTENT,    REAR_EXTENT,
    MIDDLE_EXTENT,    LEFT_EXTENT,    FRONT_EXTENT,
    BOTTOM_EXTENT,    LEFT_EXTENT,    REAR_EXTENT,
                    
    BOTTOM_EXTENT,    RIGHT_EXTENT,    REAR_EXTENT,
    TOP_EXTENT,        RIGHT_EXTENT,    REAR_EXTENT,
    TOP_EXTENT,        LEFT_EXTENT,    REAR_EXTENT,
    BOTTOM_EXTENT,    LEFT_EXTENT,    REAR_EXTENT,

    //Object 1 colors
    GREEN_COLOR,
    GREEN_COLOR,
    GREEN_COLOR,
    GREEN_COLOR,

    BLUE_COLOR,
    BLUE_COLOR,
    BLUE_COLOR,
    BLUE_COLOR,

    RED_COLOR,
    RED_COLOR,
    RED_COLOR,

    GREY_COLOR,
    GREY_COLOR,
    GREY_COLOR,

    BROWN_COLOR,
    BROWN_COLOR,
    BROWN_COLOR,
    BROWN_COLOR,

    //Object 2 colors
    RED_COLOR,
    RED_COLOR,
    RED_COLOR,
    RED_COLOR,

    BROWN_COLOR,
    BROWN_COLOR,
    BROWN_COLOR,
    BROWN_COLOR,

    BLUE_COLOR,
    BLUE_COLOR,
    BLUE_COLOR,

    GREEN_COLOR,
    GREEN_COLOR,
    GREEN_COLOR,

    GREY_COLOR,
    GREY_COLOR,
    GREY_COLOR,
    GREY_COLOR,
};

const GLshort indexData[] =
{
    0, 2, 1,
    3, 2, 0,

    4, 5, 6,
    6, 7, 4,

    8, 9, 10,
    11, 13, 12,

    14, 16, 15,
    17, 16, 14,
};

GLuint vertexBufferObject;
GLuint indexBufferObject;

GLuint vaoObject1, vaoObject2;

void InitializeVertexBuffer()
{
    glGenBuffers(1, &vertexBufferObject);

    glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObject);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertexData), vertexData, GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    glGenBuffers(1, &indexBufferObject);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBufferObject);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indexData), indexData, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
}

void InitializeVertexArrayObjects()
{
    glGenVertexArrays(1, &vaoObject1);
    glBindVertexArray(vaoObject1);

    size_t colorDataOffset = sizeof(float) * 3 * numberOfVertices;

    glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObject);
    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
    glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, (void*)colorDataOffset);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBufferObject);

    glBindVertexArray(0);

    glGenVertexArrays(1, &vaoObject2);
    glBindVertexArray(vaoObject2);

    size_t posDataOffset = sizeof(float) * 3 * (numberOfVertices/2);
    colorDataOffset += sizeof(float) * 4 * (numberOfVertices/2);

    //Use the same buffer object previously bound to GL_ARRAY_BUFFER.
    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)posDataOffset);
    glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, (void*)colorDataOffset);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBufferObject);

    glBindVertexArray(0);
}

    glBindVertexArray(vaoObject1);
    glUniform3f(offsetUniform, 0.0f, 0.0f, 0.0f);
    glDrawElements(GL_TRIANGLES, ARRAY_COUNT(indexData), GL_UNSIGNED_SHORT, 0);

    glBindVertexArray(vaoObject2);
    glUniform3f(offsetUniform, 0.0f, 0.0f, -1.0f);
    glDrawElements(GL_TRIANGLES, ARRAY_COUNT(indexData), GL_UNSIGNED_SHORT, 0);

    glBindVertexArray(0);

#include <stdlib.h>
#include <stdio.h>
/* Ensure we are using opengl's core profile only */
#define GL3_PROTOTYPES 1
#include <GL3/gl3.h>
 
#include <SDL.h>
 
#define PROGRAM_NAME "Tutorial2"
 
/* A simple function that will read a file into an allocated char pointer buffer */
char* filetobuf(char *file)
{
    FILE *fptr;
    long length;
    char *buf;
 
    fptr = fopen(file, "rb"); /* Open file for reading */
    if (!fptr) /* Return NULL on failure */
        return NULL;
    fseek(fptr, 0, SEEK_END); /* Seek to the end of the file */
    length = ftell(fptr); /* Find out how many bytes into the file we are */
    buf = (char*)malloc(length+1); /* Allocate a buffer for the entire length of the file and a null terminator */
    fseek(fptr, 0, SEEK_SET); /* Go back to the beginning of the file */
    fread(buf, length, 1, fptr); /* Read the contents of the file in to the buffer */
    fclose(fptr); /* Close the file */
    buf[length] = 0; /* Null terminator */
 
    return buf; /* Return the buffer */
}
 
/* A simple function that prints a message, the error code returned by SDL, and quits the application */
void sdldie(char *msg)
{
    printf("%s: %s\n", msg, SDL_GetError());
    SDL_Quit();
    exit(1);
}
 
void setupwindow(SDL_WindowID *window, SDL_GLContext *context)
{
    if (SDL_Init(SDL_INIT_VIDEO) < 0) /* Initialize SDL's Video subsystem */
        sdldie("Unable to initialize SDL"); /* Or die on error */
 
    /* Request an opengl 3.2 context.
     * SDL doesn't have the ability to choose which profile at this time of writing,
     * but it should default to the core profile */
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 2);
 
    /* Turn on double buffering with a 24bit Z buffer.
     * You may need to change this to 16 or 32 for your system */
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
 
    /* Create our window centered at 512x512 resolution */
    *window = SDL_CreateWindow(PROGRAM_NAME, SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
        512, 512, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);
    if (!*window) /* Die if creation failed */
        sdldie("Unable to create window");
 
    /* Create our opengl context and attach it to our window */
    *context = SDL_GL_CreateContext(*window);
 
    /* This makes our buffer swap syncronized with the monitor's vertical refresh */
    SDL_GL_SetSwapInterval(1);
}
 
void drawscene(SDL_WindowID window)
{
    int i; /* Simple iterator */
    GLuint vao, vbo[2]; /* Create handles for our Vertex Array Object and two Vertex Buffer Objects */
    int IsCompiled_VS, IsCompiled_FS;
    int IsLinked;
    int maxLength;
    char *vertexInfoLog;
    char *fragmentInfoLog;
    char *shaderProgramInfoLog;
 
    /* We're going to create a simple diamond made from lines */
    const GLfloat diamond[4][2] = {
    {  0.0,  1.0  }, /* Top point */           |
    {  1.0,  0.0  }, /* Right point */         | 
    {  0.0, -1.0  }, /* Bottom point */
    { -1.0,  0.0  } }; /* Left point */
 
    const GLfloat colors[4][3] = {
    {  1.0,  0.0,  0.0  }, /* Red */
    {  0.0,  1.0,  0.0  }, /* Green */
    {  0.0,  0.0,  1.0  }, /* Blue */
    {  1.0,  1.0,  1.0  } }; /* White */
 
    /* These pointers will receive the contents of our shader source code files */
    GLchar *vertexsource, *fragmentsource;
 
    /* These are handles used to reference the shaders */
    GLuint vertexshader, fragmentshader;
 
    /* This is a handle to the shader program */
    GLuint shaderprogram;
 
    /* Allocate and assign a Vertex Array Object to our handle */
    glGenVertexArrays(1, &vao);
 
    /* Bind our Vertex Array Object as the current used object */
    glBindVertexArray(vao);
 
    /* Allocate and assign two Vertex Buffer Objects to our handle */
    glGenBuffers(2, vbo);
 
    /* Bind our first VBO as being the active buffer and storing vertex attributes (coordinates) */
    glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
 
    /* Copy the vertex data from diamond to our buffer */
    /* 8 * sizeof(GLfloat) is the size of the diamond array, since it contains 8 GLfloat values */
    glBufferData(GL_ARRAY_BUFFER, 8 * sizeof(GLfloat), diamond, GL_STATIC_DRAW);
 
    /* Specify that our coordinate data is going into attribute index 0, and contains two floats per vertex */
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, 0);
 
    /* Enable attribute index 0 as being used */
    glEnableVertexAttribArray(0);
 
    /* Bind our second VBO as being the active buffer and storing vertex attributes (colors) */
    glBindBuffer(GL_ARRAY_BUFFER, vbo[1]);
 
    /* Copy the color data from colors to our buffer */
    /* 12 * sizeof(GLfloat) is the size of the colors array, since it contains 12 GLfloat values */
    glBufferData(GL_ARRAY_BUFFER, 12 * sizeof(GLfloat), colors, GL_STATIC_DRAW);
 
    /* Specify that our color data is going into attribute index 1, and contains three floats per vertex */
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, 0);
 
    /* Enable attribute index 1 as being used */
    glEnableVertexAttribArray(1);
 
    /* Read our shaders into the appropriate buffers */
    vertexsource = filetobuf("tutorial2.vert");
    fragmentsource = filetobuf("tutorial2.frag");
 
    /* Create an empty vertex shader handle */
    vertexshader = glCreateShader(GL_VERTEX_SHADER);
 
    /* Send the vertex shader source code to GL */
    /* Note that the source code is NULL character terminated. */
    /* GL will automatically detect that therefore the length info can be 0 in this case (the last parameter) */
    glShaderSource(vertexshader, 1, (const GLchar**)&vertexsource, 0);
 
    /* Compile the vertex shader */
    glCompileShader(vertexshader);
 
    glGetShaderiv(vertexshader, GL_COMPILE_STATUS, &IsCompiled_VS);
    if(IsCompiled_VS == FALSE)
    {
       glGetShaderiv(vertexshader, GL_INFO_LOG_LENGTH, &maxLength);
 
       /* The maxLength includes the NULL character */
       vertexInfoLog = (char *)malloc(maxLength);
 
       glGetShaderInfoLog(vertexshader, maxLength, &maxLength, vertexInfoLog);
 
       /* Handle the error in an appropriate way such as displaying a message or writing to a log file. */
       /* In this simple program, we'll just leave */
       free(vertexInfoLog);
       return;
    }
 
    /* Create an empty fragment shader handle */
    fragmentshader = glCreateShader(GL_FRAGMENT_SHADER);
 
    /* Send the fragment shader source code to GL */
    /* Note that the source code is NULL character terminated. */
    /* GL will automatically detect that therefore the length info can be 0 in this case (the last parameter) */
    glShaderSource(fragmentshader, 1, (const GLchar**)&fragmentsource, 0);
 
    /* Compile the fragment shader */
    glCompileShader(fragmentshader);
 
    glGetShaderiv(fragmentshader, GL_COMPILE_STATUS, &IsCompiled_FS);
    if(IsCompiled_FS == FALSE)
    {
       glGetShaderiv(fragmentshader, GL_INFO_LOG_LENGTH, &maxLength);
 
       /* The maxLength includes the NULL character */
       fragmentInfoLog = (char *)malloc(maxLength);
 
       glGetShaderInfoLog(fragmentshader, maxLength, &maxLength, fragmentInfoLog);
 
       /* Handle the error in an appropriate way such as displaying a message or writing to a log file. */
       /* In this simple program, we'll just leave */
       free(fragmentInfoLog);
       return;
    }
 
    /* If we reached this point it means the vertex and fragment shaders compiled and are syntax error free. */
    /* We must link them together to make a GL shader program */
    /* GL shader programs are monolithic. It is a single piece made of 1 vertex shader and 1 fragment shader. */
    /* Assign our program handle a "name" */
    shaderprogram = glCreateProgram();
 
    /* Attach our shaders to our program */
    glAttachShader(shaderprogram, vertexshader);
    glAttachShader(shaderprogram, fragmentshader);
 
    /* Bind attribute index 0 (coordinates) to in_Position and attribute index 1 (color) to in_Color */
    /* Attribute locations must be setup before calling glLinkProgram. */
    glBindAttribLocation(shaderprogram, 0, "in_Position");
    glBindAttribLocation(shaderprogram, 1, "in_Color");
 
    /* Link our program */
    /* At this stage, the vertex and fragment programs are inspected, optimized and a binary code is generated for the shader. */
    /* The binary code is uploaded to the GPU, if there is no error. */
    glLinkProgram(shaderprogram);
 
    /* Again, we must check and make sure that it linked. If it fails, it would mean either there is a mismatch between the vertex */
    /* and fragment shaders. It might be that you have surpassed your GPU's abilities. Perhaps too many ALU operations or */
    /* too many texel fetch instructions or too many interpolators or dynamic loops. */
 
    glGetProgramiv(shaderprogram, GL_LINK_STATUS, (int *)&IsLinked);
    if(IsLinked == FALSE)
    {
       /* Noticed that glGetProgramiv is used to get the length for a shader program, not glGetShaderiv. */
       glGetProgramiv(shaderprogram, GL_INFO_LOG_LENGTH, &maxLength);
 
       /* The maxLength includes the NULL character */
       shaderProgramInfoLog = (char *)malloc(maxLength);
 
       /* Notice that glGetProgramInfoLog, not glGetShaderInfoLog. */
       glGetProgramInfoLog(shaderprogram, maxLength, &maxLength, shaderProgramInfoLog);
 
       /* Handle the error in an appropriate way such as displaying a message or writing to a log file. */
       /* In this simple program, we'll just leave */
       free(shaderProgramInfoLog);
       return;
    }
 
    /* Load the shader into the rendering pipeline */
    glUseProgram(shaderprogram);
 
    /* Loop our display increasing the number of shown vertexes each time.
     * Start with 2 vertexes (a line) and increase to 3 (a triangle) and 4 (a diamond) */
    for (i=2; i <= 4; i++)
    {
        /* Make our background black */
        glClearColor(0.0, 0.0, 0.0, 1.0);
        glClear(GL_COLOR_BUFFER_BIT);
 
        /* Invoke glDrawArrays telling that our data is a line loop and we want to draw 2-4 vertexes */
        glDrawArrays(GL_LINE_LOOP, 0, i);
 
        /* Swap our buffers to make our changes visible */
        SDL_GL_SwapWindow(window);
 
        /* Sleep for 2 seconds */
        SDL_Delay(2000);
    }
 
    /* Cleanup all the things we bound and allocated */
    glUseProgram(0);
    glDisableVertexAttribArray(0);
    glDisableVertexAttribArray(1);
    glDetachShader(shaderprogram, vertexshader);
    glDetachShader(shaderprogram, fragmentshader);
    glDeleteProgram(shaderprogram);
    glDeleteShader(vertexshader);
    glDeleteShader(fragmentshader);
    glDeleteBuffers(2, vbo);
    glDeleteVertexArrays(1, &vao);
    free(vertexsource);
    free(fragmentsource);
}
 
void destroywindow(SDL_WindowID window, SDL_GLContext context)
{
    SDL_GL_DeleteContext(context);
    SDL_DestroyWindow(window);
    SDL_Quit();
}
 
/* Our program's entry point */
int main(int argc, char *argv[])
{
    SDL_WindowID mainwindow; /* Our window handle */
    SDL_GLContext maincontext; /* Our opengl context handle */
 
    /* Create our window, opengl context, etc... */
    setupwindow(&mainwindow, &maincontext);
 
    /* Call our function that performs opengl operations */
    drawscene(mainwindow);
 
    /* Delete our opengl context, destroy our window, and shutdown SDL */
    destroywindow(mainwindow, maincontext);
 
    return 0;
}