ios - 如何在转换图像图形上下文后使用 drawAtPoint 绘制 UIImage

Question

我有一个时钟指针的图像。我正在尝试创建此图像的旋转版本，以便我可以使用动画播放它(使用 UIImageView 的 animatedImages 方法)。

在下面的代码中，我看到如果我将 CGContextTranslateCTM 应用于图像图形上下文，则不会绘制图像，除非我增加我给 UIGraphicsBeginImageContextWithOptions 的大小以使其大于图片大小。

为什么需要这个？我认为所有翻译所做的就是移动 drawAtPoint: 解释 0,0 在哪里的定义？

**显示更多细节的编辑代码*

-(void) animatedClock
{
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(20, 100), NO, 0);

    UIBezierPath *clockHand = [[UIBezierPath alloc] init];
    [[UIColor yellowColor] setFill];
    [clockHand moveToPoint:CGPointMake(0, 20)];
    [clockHand addLineToPoint:CGPointMake(10, 0)];
    [clockHand addLineToPoint:CGPointMake(20, 20)];
    [clockHand fill];

    [clockHand removeAllPoints];
    [clockHand moveToPoint:CGPointMake(5, 100)];
    [clockHand addLineToPoint:CGPointMake(5, 20)];
    [clockHand addLineToPoint:CGPointMake(15, 20)];
    [clockHand addLineToPoint:CGPointMake(15, 100)];
    [clockHand closePath];
    [clockHand fill];


    UIImage *baseImage = UIGraphicsGetImageFromCurrentImageContext();
    NSLog(@"Image size is %@", NSStringFromCGSize(baseImage.size));
    UIGraphicsEndImageContext();

    //The screenshot will show baseimage, an arrow that is pointed upwards and at the top left of the screen
    UIImageView *imageView = [[UIImageView alloc] initWithImage:baseImage];
    UIGraphicsEndImageContext();
    [self.view addSubview:imageView];

    //This code is just to see where 100, 100 is on screen
    UIView *redView = [[UIView alloc] initWithFrame:CGRectMake(100,100, 5, 5)];
    redView.backgroundColor = [UIColor redColor];
    [self.view addSubview:redView];

    for (NSUInteger index = 0; index <= 8; index++)
    {
        //UIGraphicsBeginImageContextWithOptions(baseImage.size, NO, 0); //If I keep this line then nothing is shown
        UIGraphicsBeginImageContextWithOptions(CGSizeMake(500, 600), NO, 0); // If I keep this line I can see some of teh rotations
        CGContextRef con = UIGraphicsGetCurrentContext();
        CGContextTranslateCTM(con, 100, 100); //I want the clock hands to be centered around 100, 100
        CGContextRotateCTM(con, 45*index*M_PI/180);
        [baseImage drawAtPoint:CGPointMake(0, 0)];
        UIImageView *imageView = [[UIImageView alloc] initWithImage:UIGraphicsGetImageFromCurrentImageContext()];
        UIGraphicsEndImageContext();
        [self.view addSubview:imageView];
        imageView.opaque = YES;
        imageView.backgroundColor = [UIColor clearColor];
        [self.view addSubview:imageView];
    }
}

Answer 1

I am seeing that if I apply a CGContextTranslateCTM to the image graphics context, the image is not drawn

当然。您在 0,0 绘制。但首先您应用 100,100 的翻译 CTM。所以你实际上画了 100,100。但是上下文只有 20 点宽。因此，您完全在上下文之外进行绘制。上下文之外没有上下文，所以什么也画不出来。