默认情况下，OpenGL ES使用正交的笛卡尔坐标系，其中x轴水平向右，y轴垂直向上，而z轴指向屏幕外面，因此每个点用(x, y, z)三个值表示。这个坐标系的原点默认在屏幕中央，并且屏幕的四个角的坐标从左上角顺时针分别为：(-1.0, 1.0, 0)、(1.0, 1.0, 0.0)、(1.0, -1.0, 0.0)、(-1.0, -1.0, 0.0)。

#import "GameViewController.h"
#import <OpenGLES/ES2/glext.h>

//三角形顶点数据
static const GLKVector3 vertices[] = {
    {-0.5f, -0.5f, 0.0f},
    {0.5f, -0.5f, 0.0f},
    {-0.5f, 0.5f, 0.0f}
};

@interface GameViewController () {
    GLuint vertexBufferID;
}
//用于设置通用的OpenGL ES环境
@property (strong, nonatomic) GLKBaseEffect *baseEffect;
//OpenGL ES上下文
@property (strong, nonatomic) EAGLContext *context;
@end

@implementation GameViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    //使用支持OpenGL ES2的上下文
    self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];

    if (!self.context) {
        NSLog(@"Failed to create ES context");
    }
    
    GLKView *view = (GLKView *)self.view;
    view.context = self.context;
    view.drawableDepthFormat = GLKViewDrawableDepthFormat24;

    [EAGLContext setCurrentContext:self.context];
    
    self.baseEffect = [[GLKBaseEffect alloc] init];
    //启用元素的默认颜色
    self.baseEffect.useConstantColor = GL_TRUE;
    //设置默认颜色
    self.baseEffect.constantColor = GLKVector4Make(0.0f, 1.0f, 1.0f, 1.0f);
    //设置背景色
    glClearColor(0.0f, 1.0f, 0.0f, 1.0f);
    
    //1. 生成缓存ID
    glGenBuffers(1, &vertexBufferID);
    //2. 绑定缓存ID对应的缓存类型
    glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID);
    //3. 分配缓存空间
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
}

- (BOOL)prefersStatusBarHidden {
    return YES;
}

#pragma mark - GLKView and GLKViewController delegate methods

- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
    [self.baseEffect prepareToDraw];
    //使用背景色清屏
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    //4. 启用顶点位置数组
    glEnableVertexAttribArray(GLKVertexAttribPosition);
    //5. 设置顶点位置指针
    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(GLKVector3), NULL);
    //6. 绘制三角形
    glDrawArrays(GL_TRIANGLES, 0, 3);
}

- (void)dealloc
{
    if ([EAGLContext currentContext] == self.context) {
        [EAGLContext setCurrentContext:nil];
    }
    
    glDeleteBuffers(1, &vertexBufferID);
    vertexBufferID = 0;
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    
    if ([self isViewLoaded] && ([[self view] window] == nil)) {
        self.view = nil;
        
        if ([EAGLContext currentContext] == self.context) {
            [EAGLContext setCurrentContext:nil];
        }
        self.context = nil;
        //释放缓存
        glDeleteBuffers(1, &vertexBufferID);
        vertexBufferID = 0;
    }
}

@end

上面的代码是基于Xcode的iOS应用中OpenGL模板创建的项目。直接替换GameViewController.m中的内容就行。