旋转图层的图形上下文忽略任何变换

Question

我正在由几个层组成一个视图，其中一些层是旋转的。这些层的显示效果很好，但当我尝试制作图像时，所有的旋转/变换都会被忽略。

例如，如果我有一个CALayer，其中包含指向左侧的箭头图像，则在绕z轴旋转（使用CATransform3DMakeRotation）后，它将指向顶部，并且该层将正确显示。但是，如果我获得该层的图像（使用renderInContext），箭头仍然指向左侧，忽略任何变换。

有人知道为什么吗？我必须做什么才能得到我想要的结果？：-）

示例代码：

//  ViewController.m
#import "ViewController.h"
#import <QuartzCore/QuartzCore.h>
// Don't forget to add the QuartzCore framework to your target.
@interface ViewController ()
@end
@implementation ViewController
- (void)loadView {
    self.view = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.view.backgroundColor = [UIColor lightGrayColor];
}
- (void)viewDidLoad {
    [super viewDidLoad];
    // Get the arrow image (tip to the left, 50 x 50 pixels).
    // (Download the image from here: http://www.4shared.com/photo/uQfor7vC/arrow.html )
    UIImage *arrowImage = [UIImage imageNamed:@"arrow"];
    // The arrow layer in it's starting orientation (tip to the left).
    CGRect layerFrame = CGRectMake(50.0, 50.0, arrowImage.size.width, arrowImage.size.height);
    CALayer *arrowLayerStart = [CALayer new];
    arrowLayerStart.frame = layerFrame;
    [arrowLayerStart setContents:(id)[arrowImage CGImage]];
    [self.view.layer addSublayer:arrowLayerStart];
    // The arrow layer rotated around the z axis by 90 degrees.
    layerFrame.origin = CGPointMake(layerFrame.origin.x + layerFrame.size.width + 25.0, layerFrame.origin.y);
    CALayer *arrowLayerZ90 = [CALayer new];
    arrowLayerZ90.frame = layerFrame;
    [arrowLayerZ90 setContents:(id)[arrowImage CGImage]];
    arrowLayerZ90.transform = CATransform3DMakeRotation(M_PI/2.0, 0.0, 0.0, 1.0);
    [self.view.layer addSublayer:arrowLayerZ90];
    // Now make images of each of these layers and display them in a second row.
    // The starting layer without any rotation.
    UIGraphicsBeginImageContext(arrowLayerStart.frame.size);
    [arrowLayerStart renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *arrowStartImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    layerFrame.origin = CGPointMake(arrowLayerStart.frame.origin.x,
                                    arrowLayerStart.frame.origin.y + arrowLayerStart.frame.size.height + 25.0);
    CALayer *arrowLayerStartCaptured = [CALayer new];
    arrowLayerStartCaptured.frame = layerFrame;
    [arrowLayerStartCaptured setContents:(id)[arrowStartImage CGImage]];
    [self.view.layer addSublayer:arrowLayerStartCaptured];
    // The second layer, rotated around the z axis by 90 degrees.
    UIGraphicsBeginImageContext(arrowLayerZ90.frame.size);
    [arrowLayerZ90 renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *arrowZ90Image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    layerFrame.origin = CGPointMake(arrowLayerZ90.frame.origin.x,
                                    arrowLayerZ90.frame.origin.y + arrowLayerZ90.frame.size.height + 25.0);
    CALayer *arrowLayerZ90Captured = [CALayer new];
    arrowLayerZ90Captured.frame = layerFrame;
    [arrowLayerZ90Captured setContents:(id)[arrowZ90Image CGImage]];
    [self.view.layer addSublayer:arrowLayerZ90Captured];    
}
@end

//  ViewController.h
#import <UIKit/UIKit.h>
@interface ViewController : UIViewController
@end

//  AppDelegate.h
#import <UIKit/UIKit.h>
@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@end

//  AppDelegate.m
#import "AppDelegate.h"
#import "ViewController.h"
@implementation AppDelegate
@synthesize window = _window;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.rootViewController = [[ViewController alloc] init];
    [self.window makeKeyAndVisible];
    return YES;
}
@end

Answer 1

-[CALayer renderInContext:]的文档中写道：

此方法的Mac OS X v10.5实现不会支持整个核心动画合成模型。QCCompositionLayer、CAOpenGLLayer和QTMovieLayer层不是渲染。此外，使用3D变换的层不是渲染，指定backgroundFilters、filter、，compositingFilter或遮罩值。未来版本的Mac OS X可能添加对渲染这些层和属性的支持。

（iOS上也存在此限制。）

标题CALayer.h还表示：

 * WARNING: currently this method does not implement the full
 * CoreAnimation composition model, use with caution. */

从本质上讲，-renderInContext:在一些简单的情况下是有用的，但在更复杂的情况下不会像预期的那样工作。你需要找到一些其他的方法来绘制。Quartz（核心图形）将适用于2D，但对于3D，您只能靠自己。

Answer 2

您可以使用Core Graphic而不是CATransform3DMakeRotation：）

CGAffineTransform flip = CGAffineTransformMakeScale(1.0, -1.0);