iOS 读取图片的各种方法

出处：http://blog.sina.com.cn/s/blog_a843a8850101flo3.html

一.读取图片

1.从资源(resource)读取

[cpp] view plaincopyprint?

UIImage* image=[UIImage imageNamed:@"1.jpg"];  

2.从网络读取

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NSURL *url=[NSURL URLWithString:@"http://www.sinaimg.cn/qc/photo_auto/chezhan/2012/50/00/15/80046_950.jpg"];  

UIImage *imgFromUrl =[[UIImage alloc]initWithData:[NSData dataWithContentsOfURL:url]];  

3.从手机本地读取

[cpp] view plaincopyprint?

//读取本地图片非resource  

NSString *aPath3=[NSString stringWithFormat:@"%@/Documents/%@.jpg",NSHomeDirectory(),@"test"];  

UIImage *imgFromUrl3=[[UIImage alloc]initWithContentsOfFile:aPath3];  

UIImageView* imageView3=[[UIImageView alloc]initWithImage:imgFromUrl3];  

4.从现有的context中获得图像

[cpp] view plaincopyprint?

//add ImageIO.framework and #import     

CGImageSourceRef source = CGImageSourceCreateWithURL((CFURLRef)url, NULL);  

CGImageRef img= CGImageSourceCreateImageAtIndex(source,0,NULL);  

CGContextRef ctx=UIGraphicsGetCurrentContext();  

CGContextSaveGState(ctx);  

//transformCTM的2种方式  

//CGContextConcatCTM(ctx, CGAffineTransformMakeScale(.2, -0.2));  

//CGContextScaleCTM(ctx,1,-1);  

//注意坐标要反下,用ctx来作为图片源   

CGImageRef capture=CGBitmapContextCreateImage(ctx);  

CGContextDrawImage(ctx, CGRectMake(160, 0, 160, 230), [image CGImage]);  

CGContextDrawImage(ctx, CGRectMake(160, 230, 160, 230), img);  

CGImageRef capture2=CGBitmapContextCreateImage(ctx);  

5.用Quartz的CGImageSourceRef来读取图片

[cpp] view plaincopyprint?

CGImageSourceRef source = CGImageSourceCreateWithURL((CFURLRef)url, NULL);  

CGImageRef img= CGImageSourceCreateImageAtIndex(source,0,NULL);  

二.保存图片

1.转换成NSData来保存图片（imgFromUrl是UIImage）

[cpp] view plaincopyprint?

//保存图片 2种获取路径都可以  

//NSArray*paths=NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);  

//NSString*documentsDirectory=[paths objectAtIndex:0];    

//NSString*aPath=[documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.jpg",@"test"]];   

NSString *aPath=[NSString stringWithFormat:@"%@/Documents/%@.jpg",NSHomeDirectory(),@"test"];  

NSData *imgData = UIImageJPEGRepresentation(imgFromUrl,0);      

[imgData writeToFile:aPath atomically:YES];     

2.用Quartz的CGImageDestinationRef来输出图片，这个方式不常见，所以不做介绍，详细可以看apple文档Quartz 2D Programming Guide

三.绘制图(draw|painting)

1.UIImageView方式加入到UIView层

[cpp] view plaincopyprint?

UIImageView* imageView=[[UIImageView alloc]initWithImage:image];  

imageView.frame=CGRectMake(0, 0, 320, 480);  

[self addSubview:imageView];  

[imageView release];  

2.[img drawAtPoint]系列方法

[cpp] view plaincopyprint?

[image4 drawAtPoint:CGPointMake(100, 0)];    

3.CGContextDrawImage

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CGContextDrawImage(ctx, CGRectMake(160, 0, 160, 230), [image CGImage]);  

4.CGLayer

这个是apple推荐的一种offscreen的绘制方法，相比bitmapContext更好，因为它似乎会利用iphone硬件(drawing-card)加速

[cpp] view plaincopyprint?

CGLayerRef cg=CGLayerCreateWithContext(ctx, CGSizeMake(320, 480), NULL);  

//需要将CGLayerContext来作为缓存context，这个是必须的  

CGContextRef layerContext=CGLayerGetContext(cg);  

CGContextDrawImage(layerContext, CGRectMake(160, 230, 160, 230), img);   

CGContextDrawLayerAtPoint(ctx, CGPointMake(0, 0), cg);  

5.CALayer的contents

[cpp] view plaincopyprint?

UIImage* image=[UIImage imageNamed:@"1.jpg"];  

CALayer *ly=[CALayer layer];  

ly.frame=CGRectMake(0, 0, 320, 460);  

ly.contents=[image CGImage];  

[self.layer addSublayer:ly];  

四.其它

1.CGImage和UIImage互换

这样就可以随时切换UIKit和Quartz之间类型，并且选择您熟悉的方式来处理图片.

CGImage cgImage=[uiImage CGImage];

UIImage* uiImage=[UIImage imageWithCGImage:cgImage];