我是OpenGL的新手，并一直关注Jeffrey Chastine的YouTube教程。作为个人练习，我想使用索引缓冲区创建一个简单的方块。可悲的是，我的尝试，我只看到一个三角形绘制（当我期待两个）。我不确定我错过了什么。这是我的代码，我试着在评论中解释我的逻辑/推理。如果我错了，请随时纠正我：
#include <GL/glew.h>
#include <GL/freeglut.h>
#include <iostream>
#define BUFFER_OFFSET(i) ((char *)NULL + (i))
GLuint shaderProgramID;
GLuint vao = 0;
GLuint vbo;
GLuint positionID, colorID;
GLuint indexBufferID;
#pragma region SHADER_FUNCTIONS
static char* readFile(const char* fileName)
{
//Open the file
FILE* fp = fopen(fileName, "r");
//Move the file pointer to the end of the file and determining the length
fseek(fp, 0, SEEK_END);
long file_length = ftell(fp);
fseek(fp, 0, SEEK_SET);
char* contents = new char[file_length + 1];
//zero out memory
for(int i = 0; i < file_length+1; i++)
{
contents[i] = 0;
}
//Here's the actual read
fread(contents, 1, file_length, fp);
contents[file_length + 1] = '\0';
fclose(fp);
return contents;
}
bool compiledStatus(GLint shaderID){
GLint compiled = 0;
glGetShaderiv(shaderID, GL_COMPILE_STATUS, &compiled);
if (compiled) {
return true;
}
else {
GLint logLength;
glGetShaderiv(shaderID, GL_INFO_LOG_LENGTH, &logLength);
char* msgBuffer = new char[logLength];
glGetShaderInfoLog(shaderID, logLength, NULL, msgBuffer);
printf ("%s\n", msgBuffer);
delete (msgBuffer);
return false;
}
}
//Takes in source code as string
GLuint makeVertexShader(const char* shaderSource)
{
//Call GL to make a vertex shader and get the ID
GLuint vertexShaderID = glCreateShader(GL_VERTEX_SHADER);
//Bind the provided source code to the shader ID
glShaderSource(vertexShaderID, 1, (const GLchar**)&shaderSource, NULL);
//Compile the vertex Shader
glCompileShader(vertexShaderID);
bool compiledCorrect = compiledStatus(vertexShaderID);
if(compiledCorrect)
{
return vertexShaderID;
}
return -1;
return vertexShaderID;
}
GLuint makeFragmentShader(const char* shaderSource)
{
//Call GL to make a fragment shader and get the ID
GLuint fragmentShaderID = glCreateShader(GL_FRAGMENT_SHADER);
//Bind the provided source code to the shader ID
glShaderSource(fragmentShaderID, 1, (const GLchar**)&shaderSource, NULL);
//Compile the fragment Shader
glCompileShader(fragmentShaderID);
bool compiledCorrect = compiledStatus(fragmentShaderID);
if(compiledCorrect)
{
return fragmentShaderID;
}
return -1;
return fragmentShaderID;
}
#pragma endregion SHADER_FUNCTIONS
void changeViewport(int w, int h)
{
glViewport(0, 0, w, h);
}
//This is the function we are using each time the window needs to be redrawn
void render()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glDrawElements(GL_TRIANGLES, 4, GL_UNSIGNED_INT, NULL);
glutSwapBuffers();
}
GLuint makeShaderProgram(GLuint vertextShaderID, GLuint fragmentShaderID)
{
GLuint shaderID = glCreateProgram();
//Attach the vertex shader to the shader program
glAttachShader(shaderID, vertextShaderID);
//Attatch the fragment shader to the shader program
glAttachShader(shaderID, fragmentShaderID);
//Link all the shaders together
glLinkProgram(shaderID);
return shaderID;
}
int main (int argc, char** argv)
{
//Standards
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA);
glutInitWindowSize(800, 600);
glutCreateWindow("Test");
glutReshapeFunc(changeViewport);
glutDisplayFunc(render);
glewInit();
//Verticies
GLfloat vertices_0[] = {-0.5f, -0.5f, 0.0f, //0 -> Bottom Left
-0.5f, 0.5f, 0.0f, //1 -> Top Left
0.5f, -0.5f, 0.0f, //2 -> Bottom Right
0.05f, 0.5f, 0.0f};//3 -> Top Right
//Colors
GLfloat colors_0[] = {1.0f, 0.0, 0.0f, 1.0f,//0
0.0f, 1.0f, 0.0f, 1.0f,//1
0.0f, 0.0f, 1.0f, 1.0f,//2
1.0f, 0.0f, 1.0f, 1.0f};//3
//Indicies -> Triangle 1 -> 0 1 2, Traingle 2 -> 1 3 2
GLuint indicies_0[] = {0, 1, 2, 1, 3, 2};
//Read the vertex shader
char* vertexShaderSourceCode = readFile("vertexShader.vsh");
//Read the fragment shader
char* fragmentShaderSourceCode = readFile("fragmentShader.fsh");
//Make Vertex Shader
GLuint vertexShaderID = makeVertexShader(vertexShaderSourceCode);
//Make Fragment Shader
GLuint fragmentShaderID = makeFragmentShader(fragmentShaderSourceCode);
//Make Shader Program
shaderProgramID = makeShaderProgram(vertexShaderID, fragmentShaderID);
printf("Vertex Shader ID is %d\n", vertexShaderID);
printf("Fragment Shader ID is %d\n", fragmentShaderID);
printf("Shader Program ID is %d\n", shaderProgramID);
printf("s_vPosition's ID is %d\n", positionID);
//Create vertex array object
glGenVertexArrays(1, &vao);
//Bind Vertex array object
glBindVertexArray(vao);
//Create vertex buffer object
glGenBuffers(1, &vbo);
//Bind vertex buffer object
glBindBuffer(GL_ARRAY_BUFFER, vbo);
//Create Buffer ->7 Values for 4 vertices
glBufferData(GL_ARRAY_BUFFER, 7*4*sizeof(GLfloat), NULL, GL_STATIC_DRAW);
//Starting at the beggining of the buffer, place the position data (3 values for 4 verticies)
glBufferSubData(GL_ARRAY_BUFFER, 0, 3*4*sizeof(GLfloat), vertices_0);
//Starting after the placement of position data, place the color data (4 values for 4 verticies)
glBufferSubData(GL_ARRAY_BUFFER, 3*4*sizeof(GLfloat), 4*4*sizeof(GLfloat), colors_0);
//Generate the index buffer
glGenBuffers(1, &indexBufferID);
//Bind the buffer
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBufferID);
//Place index buffer data for the 6 indicies
glBufferData(GL_ELEMENT_ARRAY_BUFFER, 6*sizeof(GLuint), indicies_0, GL_STATIC_DRAW);
//Get the position attribute from the shader
positionID = glGetAttribLocation(shaderProgramID, "s_vPosition");
//Get the color attribute from the shader
colorID = glGetAttribLocation(shaderProgramID, "s_vColor");
//Tell the variables where they can find its info in the buffer
glVertexAttribPointer(positionID, 3, GL_FLOAT, GL_FALSE, 0, 0);
glVertexAttribPointer(colorID, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(sizeof(vertices_0)));
//Tell the program to use the shader program
glUseProgram(shaderProgramID);
//Turn on the position variable in the shader
glEnableVertexAttribArray(positionID);
//Turn on the color variable in the shader
glEnableVertexAttribArray(colorID);
glutMainLoop();
return 0;
}
答案 0 :(得分：3)
如评论中所述，顶点定义中存在错误（0.05f应为0.5f）。
但是，你只看到一个三角形而不是2个的原因是这一行：
glDrawElements(GL_TRIANGLES, 4, GL_UNSIGNED_INT, NULL);
第二个参数在OpenGL documentation中描述为
count - 指定要渲染的元素数。
这是索引计数（每个元素都由索引数组中的条目引用），因此应该是索引数组中的条目数，而不是唯一索引的数量。所以将4改为6。
请注意，如果您使用GL_TRIANGLE_STRIP或GL_TRIANGLE_FAN代替GL_TRIANGLES，那么您只需要4个{0,1,2,3}索引，并且可以在执行时传递4个，因为第一个三角形之后的每个三角形与前一个三角形共享其2个索引，因此总指数计数为（三角形计数+2）。但是，当使用GL_TRIANGLES时，每个三角形分别指定，每个三角形有三个索引（三角形数* 3）。