本文介绍通过基于OpenGL ES的GLKView简单实现将一张图片绘制到屏幕的方法，仅用于入门学习。
ViewController.h
#import <UIKit/UIKit.h>
#import <GLKit/GLKit.h>
@interface ViewController : GLKViewController<GLKViewDelegate>
@end
ViewController.m
#import "ViewController.h"
#import <CoreImage/CoreImage.h>
@interface ViewController ()
@property (nonatomic,strong)EAGLContext *eaglContext;//上下文
@property (nonatomic,strong)GLKBaseEffect *effect;//着色器
@property (nonatomic,assign)int count;//顶点个数
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
/****1、新建OpenGLES上下文****/
//新建OpenGLES上下文
self.eaglContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];//2.0
GLKView *glkView = (GLKView *)self.view;//这里需要在storyboard里面把view的类设置成GLKView
glkView.delegate = self;
glkView.context = self.eaglContext;
glkView.drawableColorFormat = GLKViewDrawableColorFormatRGBA8888;//颜色缓冲区格式
[EAGLContext setCurrentContext:self.eaglContext];
/****2、顶点数组和索引数组****/
//顶点数据，前三个是顶点坐标，后面两个是纹理坐标
GLfloat squareVertexData[] =
{
// 0.5,-0.5,0.0f, 1.0f,0.0f,//右下 0
// 0.5,0.5,0.0f, 1.0f,1.0f,//右上 1
// -0.5,0.5,0.0f, 0.0f,1.0f,//左上 2
// -0.5,-0.5,0.0f, 0.0f,0.0f,//左下 3
0.0,-0.5,0.0f, 1.0f,0.0f,//右下 0
0.0,0.5,0.0f, 1.0f,1.0f,//右上 1
-1.0,0.5,0.0f, 0.0f,1.0f,//左上 2
-1.0,-0.5,0.0f, 0.0f,0.0f,//左下 3
1.0,-0.5,0.0f, 0.0f,0.0f,//右下 4（纹理左下,左右翻转）
1.0,0.5,0.0f, 0.0f,1.0f,//右上 5（纹理左上）
-0.0,0.5,0.0f, 1.0f,1.0f,//左上 6（纹理右上）
-0.0,-0.5,0.0f, 1.0f,0.0f,//左下 7（纹理右下）
};
//顶点索引
GLuint indices[] =
{
// 0,1,2,
// 0,2,3,
0,1,2,
0,2,3,
4,5,6,
4,6,7,
};
self.count = sizeof(indices)/sizeof(GLuint);
/*
顶点坐标：范围是[-1,1]，[0,0]原点在屏幕中间，[x,y,z]
纹理坐标：范围是[0,1]，[0,0]原点在左下角，[1,1]在右上角
索引数组是顶点数组的索引，把squareVertexData数组看成4个顶点，每个顶点会有5个GLfloat数据，索引从0开始。
*/
/****3、顶点数据缓存****/
//创建顶点缓存对象（VBO）
/*
1.使用glGenBuffers()生成新缓存对象。
2.使用glBindBuffer()绑定缓存对象。
3.使用glBufferData()将顶点数据拷贝到缓存对象中。
*/
GLuint buffer;
glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(squareVertexData), squareVertexData, GL_STATIC_DRAW);
GLuint index;
glGenBuffers(1, &index);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, index);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
glEnableVertexAttribArray(GLKVertexAttribPosition);//顶点数据缓存
glVertexAttribPointer(GLKVertexAttribPosition,
3,
GL_FLOAT,
GL_FALSE,
sizeof(GLfloat)*5,
(GLfloat *)NULL+0);
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);//纹理数据缓存
glVertexAttribPointer(GLKVertexAttribTexCoord0,
2,
GL_FLOAT,
GL_FALSE,
sizeof(GLfloat)*5,
(GLfloat *)NULL+3);
/*
核心内容：
glGenBuffers申请一个标识符
glBindBuffer把标识符绑定到GL_ARRAY_BUFFER上
glBufferData把顶点数据从cpu内存复制到gpu内存
glEnableVertexAttribArray 是开启对应的顶点属性
glVertexAttribPointer设置合适的格式从buffer里面读取数据
*/
/****4、纹理贴图****/
//纹理贴图
/*
我们把一张图片加载成为要渲染的纹理，由于纹理坐标系是跟手机显示的Quartz 2D坐标系的y轴正好相反，纹理坐标系使用左下角为原点，往上为y轴的正值，往右是x轴的正值，所以需要设置一下GLKTextureLoaderOriginBottomLeft。
GLKit中使用GLKTextureInfo表示纹理对象。
*/
NSString *filePath = [[NSBundle mainBundle] pathForResource:@"for_test" ofType:@"jpg"];
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:@(1),GLKTextureLoaderOriginBottomLeft, nil];
GLKTextureInfo *textureInfo = [GLKTextureLoader textureWithContentsOfFile:filePath options:options error:nil];
//GLKTextureLoader读取图片，创建纹理GLKTextureInfo
//着色器
self.effect = [[GLKBaseEffect alloc] init];
self.effect.texture2d0.enabled = GL_TRUE;
self.effect.texture2d0.name = textureInfo.name;
//创建着色器GLKBaseEffect，把纹理赋值给着色器
}
#pragma mark - GLKViewDelegate
/**
* 渲染场景代码
*/
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect {
// //清除颜色缓冲区
// glClearColor(0.3f, 0.6f, 1.0f, 1.0f);
// glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//启动着色器
[self.effect prepareToDraw];
glDrawElements(GL_TRIANGLES, self.count, GL_UNSIGNED_INT, 0);
}
@end
