C4保存图像的部分

Question

嗨，我看了一下保存图像的例子，然后我只想保存屏幕的一部分。我设法保存了从图像左上角开始的部分，但实际上我想保存屏幕的中心。只保存图像一部分的神奇之处是设置具有一定大小的Graphics上下文，如下所示：

 UIGraphicsBeginImageContextWithOptions(CGSizeMake(300, 300), YES, 5.0f);

我认为可能有一种方法可以使用CGRect而不是大小，但这给了我一个错误。还有其他的企图或想法吗？我是否需要查看屏幕截图中的像素，获取所需的像素并从中生成新的图像(这将是我所能想到的那种复杂的方式，但也许有一个更简单的方法)？

Answer 1

要对C4Image对象执行此操作，您可以修改incmiko的答案，如下所示：

#import "C4Workspace.h"

@implementation C4WorkSpace{
    C4Image *image;
    C4Image *croppedImage;
}

-(void)setup {
    image=[C4Image imageNamed:@"C4Sky.png"];
    image.origin=CGPointMake(0, 20);

    croppedImage = [self cropImage:image toArea:CGRectMake(150,50,100,100)];
    croppedImage.origin = CGPointMake(20, 360);
    [self.canvas addObjects:@[image,croppedImage]];
}

-(C4Image *)cropImage:(C4Image *)originalImage toArea:(CGRect)rect{
    //grab the image scale
    CGFloat scale = originalImage.UIImage.scale;

    //begin an image context
    UIGraphicsBeginImageContextWithOptions(rect.size, NO, scale);

    //create a new context ref
    CGContextRef c = UIGraphicsGetCurrentContext();

    //shift BACKWARDS in both directions because this moves the image
    //the area to crop shifts INTO: (0, 0, rect.size.width, rect.size.height)
    CGContextTranslateCTM(c, -rect.origin.x, -rect.origin.y);

    //render the original image into the context
    [originalImage renderInContext:c];

    //grab a UIImage from the context
    UIImage *newUIImage = UIGraphicsGetImageFromCurrentImageContext();

    //end the image context
    UIGraphicsEndImageContext();

    //create a new C4Image
    C4Image *newImage = [C4Image imageWithUIImage:newUIImage];

    //return the new image
    return newImage;
}
@end

除了代码中的注释之外，还有其他几件事情需要注意：

1. 您正在裁剪的“区域”将始终引用您正在裁剪的“图像”。因此，如果您想从图像中裁剪{150,50}，并且图像的起源在{20,20}，那么它看起来就像是在从画布中裁剪{170,70}。
2. C4Image对象实际上有一个renderInContext:方法，因此您不必从图像的层执行此操作。
3. C4Image对象包装了UIImage对象，这就是为什么我们使用从当前上下文获得的UIImage构建一个新对象的原因。

Answer 2

我为此编写的这个方法非常有效：

+ (UIImage*) getTheArea:(CGRect)area inView:(UIView*)view{

    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)])
        UIGraphicsBeginImageContextWithOptions(CGSizeMake(area.size.width, area.size.height), NO, [UIScreen mainScreen].scale);
    else
        UIGraphicsBeginImageContext(view.bounds.size);

    CGContextRef c = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(c, -area.origin.x, -area.origin.y);    // <-- shift everything up by 40px when drawing.
    [view.layer renderInContext:c];
    UIImage* thePrintScreen = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return thePrintScreen;
}

例如，如果您想要在(100,50,100,100)中创建主视图的printscreen

UIImage* image = [self getTheArea:CGRectMake(100,50,100,100) inView:view];

Answer 3

稍微修改一下Travis的答案，就可以使其独立于图像，但取决于画布的来源：

-(C4Image *)cropImage:(C4Image *)originalImage withOrigin:(CGPoint)origin toArea:(CGRect)rect{
    //grab the image scale
    CGFloat scale = originalImage.UIImage.scale;

    //begin an image context
    UIGraphicsBeginImageContextWithOptions(rect.size, NO, scale);

    //create a new context ref
    CGContextRef c = UIGraphicsGetCurrentContext();

    //shift BACKWARDS in both directions because this moves the image
    //the area to crop shifts INTO: (0, 0, rect.size.width, rect.size.height)
    CGContextTranslateCTM(c, origin.x-rect.origin.x, origin.y-rect.origin.y);

    //render the original image into the context
    [originalImage renderInContext:c];

    //grab a UIImage from the context
    UIImage *newUIImage = UIGraphicsGetImageFromCurrentImageContext();

    //end the image context
    UIGraphicsEndImageContext();

    //create a new C4Image
    C4Image *newImage = [C4Image imageWithUIImage:newUIImage];

    //return the new image
    return newImage;
}