IOS图像处理（7）绘制位图

UIImage提供了多种方法绘制位图，除了之前用过的drawInRect，我们还可以使用以下方法

drawAtPoint:需要提供CGPoint指定位图在CGContextRef中的起点

drawAtPoint:blendMode:alpha:后两个参数指定图片的叠加模式以及透明度

drawInRect:blendMode:alpha:后两个参数指定图片的叠加模式以及透明度

此外我们还可以借助Core Graphics中的方法绘制位图

void CGContextDrawImage(CGContextRef,CGRect,CGImageRef);

void CGContextDrawTiledImage(CGContextRef,CGRect,CGImageRef);此方法采用平铺模式将图片绘制到rect区域中

如果需要获得已有图片的全部或者部分，可以使用以下方法

CGImageRef CGImageCreateCopy(CGImageRef);

CGImageRef CGImageCreateWithImageInRect(CGImageRef,CGRect);

我们可以通过绘制位图给图片添加水印

@implementation ZLTView {
    UIImage *_image;
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        [self drawImage];

    }
    return self;
}

- (void)drawImage {
    UIGraphicsBeginImageContext(self.frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSaveGState(context);

    //Core Graphics中坐标系的坐标原点在屏幕左下角，正方形沿y轴向上，与UIKit相反，因此使用Core Graphic的函数直接绘制会得到y轴相反的图像
    //首先y轴缩放-1，相当于沿着x张旋转180
    CGContextScaleCTM(context, 1, -1);
    //y轴进行平移，使原点移动到左上角
    CGContextTranslateCTM(context, 0, -self.frame.size.height);
    CGContextDrawImage(context, self.frame, [UIImage imageNamed:@"girl.jpg"].CGImage);

    //回复之前的绘图环境
    CGContextRestoreGState(context);

    //使用UIKit进行绘图不需要变换举证，因为UIKit进行了处理
    [@"made by zlt" drawAtPoint:CGPointMake(0, self.frame.size.height - 40) withAttributes:@{NSForegroundColorAttributeName:[UIColor greenColor],NSFontAttributeName:[UIFont fontWithName:@"Arial" size:25]}];

    _image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

}

- (void)drawRect:(CGRect)rect{
    [_image drawAtPoint:CGPointZero];
}

下面例子为UIImage创建了一个分类，具体功能如下

+(UIImage *)captureView:(UIView *)targetView;　　　　　　//获得UIView的截屏

+(UIImage *)captureScreent;　　　　　　　　　　　　　　　  //获得屏幕截屏

-(UIImage *)imageAtRect:(CGRect)rect;　　　　　　　　　　 //获取已有的UIImage的一部分

-(UIImage *)imageAspectByMinSize:(CGSize)targerSize;　　//根据提供的区域短边对UIImage按比例缩放

-(UIImage *)imageAspectByMaxSize:(CGSize)targerSize;　  //根据提供区域的长边对UIImage按比例缩放

-(UIImage *)imageRotate:(CGFloat)radians;　　　　　　　　 //对UIImage进行旋转

+(UIImage *)captureView:(UIView *)targetView {
    UIGraphicsBeginImageContext(targetView.frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [targetView.layer renderInContext:context];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

+(UIImage *)captureScreent {
    //私有函数，也可以截取根UIView
    extern CGImageRef UIGetScreenImage();
    UIImage *image = [UIImage imageWithCGImage:UIGetScreenImage()];
    return image;
}

-(UIImage *)imageAtRect:(CGRect)rect {
    CGImageRef imgRef = CGImageCreateWithImageInRect([self CGImage], rect);
    UIImage *image = [UIImage imageWithCGImage:imgRef];
    return image;
}

-(UIImage *)imageAspectByMinSize:(CGSize)targerSize {

    if (!CGSizeEqualToSize(self.size, targerSize)) {
        CGFloat xfactor = targerSize.width / self.size.width;
        CGFloat yfacotr = targerSize.height / self.size.height;
        CGFloat factor = xfactor < yfacotr ? xfactor : yfacotr;

        CGFloat nWidth = self.size.width * factor;
        CGFloat nHeight = self.size.height * factor;

        CGRect rect;
        if (xfactor < yfacotr) {

            rect = CGRectMake(0, (targerSize.height - nHeight) / 2, nWidth, nHeight);
        } else {
            rect = CGRectMake((targerSize.width - nWidth) / 2, 0, nWidth, nHeight);
        }

        UIGraphicsBeginImageContext(targerSize);
        [self drawInRect:rect];
        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return image;

    }

    return self;
}

-(UIImage *)imageAspectByMaxSize:(CGSize)targerSize {

    if (!CGSizeEqualToSize(self.size,targerSize)) {
        CGFloat xfactor = targerSize.width / self.size.width;
        CGFloat yfactor = targerSize.height / self.size.height;

        CGFloat factor = xfactor > yfactor ? xfactor : yfactor;

        CGFloat nWidth = self.size.width * factor;
        CGFloat nHeight = self.size.height * factor;

        CGRect rect = CGRectZero;
        if (xfactor > yfactor) {
            rect = CGRectMake(0, -(nHeight - targerSize.height)/2, nWidth, nHeight);

        } else {
            rect = CGRectMake(-(nWidth - targerSize.width)/2, 0, nWidth, nHeight);
        }

        UIGraphicsBeginImageContext(targerSize);
        [self drawInRect:rect];
        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return image;
    }

    return self;
}

-(UIImage *)imageRotate:(CGFloat)radians {

    //获取旋转后的矩形区域
    CGRect rect = CGRectApplyAffineTransform(CGRectMake(0, 0, self.size.width, self.size.height), CGAffineTransformMakeRotation(radians));

    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();

    //将坐标中心移到图片中心，是图片绕中心旋转
    CGContextTranslateCTM(context, rect.size.width/2, rect.size.height/2);
    CGContextRotateCTM(context, radians);
    CGContextScaleCTM(context, 1, -1);
    CGContextDrawImage(context, CGRectMake(-rect.size.width/2, -rect.size.height/2, rect.size.width, rect.size.height), self.CGImage);

    UIImage *nImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return nImage;
}