2.给三角形贴纹理

只有一个单色的三角形，总归是不好看，下面就往上面粘贴一个图片。纹理贴图会引入映射、UV坐标等概念。

#import "GameViewController.h"
#import 
//将顶点坐标和UV纹理坐标放入同一个数据中
typedef struct {
    GLKVector3 positionCoords;
    GLKVector2 textureCoords;
} VertexType;

//三角形顶点数据
static const VertexType vertices[] = {
    {{-0.5f, -0.5f, 0.0f}, {0.0f, 0.0f}},
    {{0.5f, -0.5f, 0.0f}, {1.0f, 0.0f}},
    {{-0.5f, 0.5f, 0.0f}, {0.0f, 1.0f}}
};

@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);
    
    //生成纹理，由于UIKit的坐标系Y轴与OpenGL ES的Y轴刚好上下颠倒，因此如果图片不加任何处理的话，在贴图后将是颠倒显示
    CGImageRef imageRef = [UIImage imageNamed:@"1.jpg"].CGImage;
    GLKTextureInfo *textureInfo = [GLKTextureLoader textureWithCGImage:imageRef options:NULL error:nil];
    self.baseEffect.texture2d0.name = textureInfo.name;
    self.baseEffect.texture2d0.target = textureInfo.target;
    
    //设置背景色
    glClearColor(0.0f, 1.0f, 0.0f, 1.0f);
    
    //1. 生成缓存ID
    glGenBuffers(1, &vertexBufferID);
    //2. 
    glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID);
    //3. 
    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];
    
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    //4. 顶点坐标（x, y, z）
    glEnableVertexAttribArray(GLKVertexAttribPosition);
    //5. 类型，成员个数，类型，归一化，间隔，偏移
    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(VertexType), NULL + offsetof(VertexType, positionCoords));
    //4.1 纹理坐标(u, v)
    glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
    //5.1
    glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(VertexType), NULL + offsetof(VertexType, textureCoords));

    //6. 绘制
    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;
        
        glDeleteBuffers(1, &vertexBufferID);
        vertexBufferID = 0;
    }
}
@end

UIImage获取的CGImageRefY坐标方向与OpenGL ES的Y坐标相反，因此看到的图片是反的。

难点是glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(VertexType), NULL + offsetof(VertexType, positionCoords));的参数含义：

1. 顶点属性类型
2. 该类型每个顶点所含数据个数
3. 数据类型
4. 是否归一化
5. 每个顶点数组间的间隔（即每个顶点的数据所占空间大小）
6. 该类型的数据起始位置相对于起点的偏移量

offsetof用来获取结构体的成员positionCoords在整个结构体VertexType中的偏移量。