Opengl ES 实现六个月球绕地球旋转

1. 配置环境

引入#import <GLKit/GLKit.h>；
将视图控制器设置继承GLKViewController；

#import <UIKit/UIKit.h>
#import <GLKit/GLKit.h>

@interface ViewController : GLKViewController

@end

2. 编程代码

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //1.新建OpenGL ES 上下文
    [self setupContext];
    
    //开启深度测试
    glEnable(GL_DEPTH_TEST);
    
    //3.创建GLKBaseEffect 只能有3个光照、2个纹理
    [self setupBaseEffect];
    
    //6.设置清屏颜色
    GLKVector4 colorVector4 = GLKVector4Make(0.0f, 0.0f, 0.0f, 1.0f);
    [self setClearColor:colorVector4];
    
    //7.顶点数组 从cpu 复制data到gpu
    [self bufferData];
}

这一部分主要初始化pengl es的，创建数组缓存区、顶点缓存区、法线缓冲区、纹理缓存区，并将相应的数据从CPU复制到GPU。创建一个GLKBaseEffect对象，用来绘制几何图形。只需要复制一次就可以。

由于继承了GLKViewController，所以屏幕在每一帧刷新时，会自动调用GLKViewDelegate代理方法；

-(void)glkView:(GLKView *)view drawInRect:(CGRect)rect;

我们的绘制操作、清屏操作等，都在这里执行；

-(void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
    //设置清屏颜色
    glClearColor(0.3f, 0.3f, 0.3f, 1.0f);
    
    //清空颜色缓存区和深度缓存区
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    
    //地球旋转角度
    _earthRotationAngleDegress += 360.0f/60.0f / SceneDaysPerMoonOrbit * 2;
    //月球旋转角度
    _moonRotationAngleDegress += (360.0f/60.0f)/SceneDaysPerMoonOrbit * 2;
    
    //2、准备绘制
    [self.vertexPositionBuffer prepareToDrawWithAttrib:GLKVertexAttribPosition numberOfCoordinates:3 attribOffset:0 shouldEnable:YES];
    [self.vertexNormalBuffer prepareToDrawWithAttrib:GLKVertexAttribNormal numberOfCoordinates:3 attribOffset:0 shouldEnable:YES];
    [self.vertextTextureCoordBuffer prepareToDrawWithAttrib:GLKVertexAttribTexCoord0 numberOfCoordinates:2 attribOffset:0 shouldEnable:YES];
    
    //3.开始绘制
    [self drawEarth];
    [self drawMoon];
    
}

3. 绘制几何图形

设置清屏颜色，并清空渲染缓存区

    //设置清屏颜色
    glClearColor(0.3f, 0.3f, 0.3f, 1.0f);
    
    //清空颜色缓存区和深度缓存区
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

使用 GLKTextureInfo 对象，来加载纹理图片；
使用 AGLKVertexAttribArrayBuffer 来设置如何读取数据；

    [self.vertexPositionBuffer prepareToDrawWithAttrib:GLKVertexAttribPosition numberOfCoordinates:3 attribOffset:0 shouldEnable:YES];
    [self.vertexNormalBuffer prepareToDrawWithAttrib:GLKVertexAttribNormal numberOfCoordinates:3 attribOffset:0 shouldEnable:YES];
    [self.vertextTextureCoordBuffer prepareToDrawWithAttrib:GLKVertexAttribTexCoord0 numberOfCoordinates:2 attribOffset:0 shouldEnable:YES];

由于使用累两个不同的纹理，和几何图形，所以需要使用到 GLKMatrixStack 对模型试图坐标进行出栈或入栈操作；不然会导致模型坐标混乱，显示不出效果；

// 绘制地球
-(void)drawEarth
{
    //获取纹理的name、target
    self.baseEffect.texture2d0.name = self.earchTextureInfo.name;
    self.baseEffect.texture2d0.target = self.earchTextureInfo.target;
    
    
    //将当前的modelViewMatrixStack 压栈
    GLKMatrixStackPush(self.modelViewMatrixStack);
    
    //在指定的轴上旋转最上面的矩阵。
    GLKMatrixStackRotate(self.modelViewMatrixStack, GLKMathDegreesToRadians(SceneEarthAxialTiltDeg), 1.0f, 0.0f, 0.0f);
    GLKMatrixStackRotate(self.modelViewMatrixStack, GLKMathDegreesToRadians(_earthRotationAngleDegress), 0.0f, 1.0f, 0.0f);
   
    self.baseEffect.transform.modelviewMatrix = GLKMatrixStackGetMatrix4(self.modelViewMatrixStack);
    
    //准备绘制
    [self.baseEffect prepareToDraw];
    

    // 设置顶点
    [AGLKVertexAttribArrayBuffer drawPreparedArraysWithMode:GL_TRIANGLES startVertexIndex:0 numberOfVertices:sphereNumVerts];
    
    //绘制完毕，则出栈
    GLKMatrixStackPop(self.modelViewMatrixStack);
    
    self.baseEffect.transform.modelviewMatrix = GLKMatrixStackGetMatrix4(self.modelViewMatrixStack);
    
}

// 绘制6个月球
-(void)drawMoon {
    // 绘制6个月球
    for (NSInteger i=0; i<6; i++) {
        
        //获取纹理的name、target
        self.baseEffect.texture2d0.name = self.moomTextureInfo.name;
        self.baseEffect.texture2d0.target = self.moomTextureInfo.target;
        
        //压栈
        GLKMatrixStackPush(self.modelViewMatrixStack);
        
        
        //围绕Y轴旋转moonRotationAngleDegress角度
        //自转
        GLKMatrixStackRotate(self.modelViewMatrixStack, GLKMathDegreesToRadians(self.moonRotationAngleDegress), 0.0f, 1.0f, 0.0f);
        
        // 设置不同月球的间距
        float xDistance = SceneMoonDistanceFromEarth * cos(i * GLKMathDegreesToRadians(60));
        float zDistance = SceneMoonDistanceFromEarth * sin(i * GLKMathDegreesToRadians(60));
        
        //平移 -月球距离地球的距离
        GLKMatrixStackTranslate(self.modelViewMatrixStack, xDistance, 0.0f, zDistance);
        
        //缩放，把月球缩放
        GLKMatrixStackScale(self.modelViewMatrixStack, SceneMoonRadiusFractionOfEarth, SceneMoonRadiusFractionOfEarth, SceneMoonRadiusFractionOfEarth);
        
        
        self.baseEffect.transform.modelviewMatrix = GLKMatrixStackGetMatrix4(self.modelViewMatrixStack);
        
        [self.baseEffect prepareToDraw];
        
        [AGLKVertexAttribArrayBuffer drawPreparedArraysWithMode:GL_TRIANGLES startVertexIndex:0 numberOfVertices:sphereNumVerts];
        
        GLKMatrixStackPop(self.modelViewMatrixStack);
    }
    
    self.baseEffect.transform.modelviewMatrix = GLKMatrixStackGetMatrix4(self.modelViewMatrixStack);
}

demo代码EarthMoon.