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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;}
IOS图像处理（7）绘制位图
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