IOS应用开发－图片处理（拉伸图片 创建缩略图 解决图片旋转的问题 图片编码及上传 将图片写入磁盘）

在做项目时我们经常要对图片进行一些处理，以达到性能优化或满足需求。常见的情形有以下几种

//http://stackoverflow.com/questions/5427656/ios-uiimagepickercontroller-result-image-orientation-after-upload

拉伸图片

项目中使用的图片素材如果能通过拉伸获得就尽量这样去做。这样做有两个显而易见的好处，一是能够减少App安装包的大小，另外一个则是减少App运行时占据的内存空间大小。毕竟App的UI基本上来说是建立在大量的精致的图片上，如果这些图片都一概使用屏幕等大小的图片，那么对App的性能及安装量都是有一定的负面影响的。

对于拉伸图片，适配ios 5及之后可以使用

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- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets

这个方法只接收一个UIEdgeInsets类型的参数，可以通过设置UIEdgeInsets的top、left、bottom、right来分别指定上端盖高度、左端盖宽度、下端盖高度、右端盖宽度。这个端盖的距离值是用单位pt（点，point）衡量的，在普通显示屏中，1pt = 1pix；在retina显示屏中，1pt ＝ 2pix。还有一个需要注意的地方，如果端盖距离值不是整数的话，拉伸后的图片会有白条细线。

创建缩略图

如果有一张大图，我们只想要显示它的指定大小的缩略图内容，可以这样做:在UIImage的类别中实现如下方法，调用方法创建缩略图

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- (UIImage *)imageByScalingAndCroppingForSize:(CGSize)targetSize

{

    UIImage *sourceImage = self;

    UIImage *newImage = nil;

    CGSize imageSize = sourceImage.size;

    CGFloat width = imageSize.width;

    CGFloat height = imageSize.height;

    CGFloat targetWidth = targetSize.width;

    CGFloat targetHeight = targetSize.height;

    CGFloat scaleFactor =0.0;

    CGFloat scaledWidth = targetWidth;

    CGFloat scaledHeight = targetHeight;

    CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

     

    if(CGSizeEqualToSize(imageSize, targetSize) == NO)

    {

        CGFloat widthFactor = targetWidth / width;

        CGFloat heightFactor = targetHeight / height;

         

        if(widthFactor > heightFactor)

            scaleFactor = widthFactor;// scale to fit height

        else

            scaleFactor = heightFactor;// scale to fit width

        scaledWidth  = width * scaleFactor;

        scaledHeight = height * scaleFactor;

         

        // center the image

        if(widthFactor > heightFactor)

        {

            thumbnailPoint.y = (targetHeight - scaledHeight) *0.5;

        }

        else

            if(widthFactor < heightFactor)

            {

                thumbnailPoint.x = (targetWidth - scaledWidth) *0.5;

            }

    }

     

    UIGraphicsBeginImageContext(targetSize);// this will crop

     

    CGRect thumbnailRect = CGRectZero;

    thumbnailRect.origin = thumbnailPoint;

    thumbnailRect.size.width  = scaledWidth;

    thumbnailRect.size.height = scaledHeight;

     

    [sourceImage drawInRect:thumbnailRect];

     

    newImage = UIGraphicsGetImageFromCurrentImageContext();

    if(newImage == nil)

        NSLog(@"could not scale image");

     

    //pop the context to get back to the default

    UIGraphicsEndImageContext();

    returnnewImage;

}

解决图片旋转的问题

ios程序中使用系统相机拍照和从相册选取图片，直接上传后在非mac系统下看到的图片会发生旋转的现象，那是因为我们没有通过图片的旋转属性修改图片转向。可以用下面的方法解决这个问题:

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@interfaceUIImage (fixOrientation)

    

- (UIImage *)fixOrientation;

    

@end

    

    

    

@implementationUIImage (fixOrientation)

    

- (UIImage *)fixOrientation {

    

    // No-op if the orientation is already correct

    if(self.imageOrientation == UIImageOrientationUp) returnself;

    

    // We need to calculate the proper transformation to make the image upright.

    // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.

    CGAffineTransform transform = CGAffineTransformIdentity;

    

    switch(self.imageOrientation) {

        caseUIImageOrientationDown:

        caseUIImageOrientationDownMirrored:

            transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height);

            transform = CGAffineTransformRotate(transform, M_PI);

            break;

    

        caseUIImageOrientationLeft:

        caseUIImageOrientationLeftMirrored:

            transform = CGAffineTransformTranslate(transform, self.size.width,0);

            transform = CGAffineTransformRotate(transform, M_PI_2);

            break;

    

        caseUIImageOrientationRight:

        caseUIImageOrientationRightMirrored:

            transform = CGAffineTransformTranslate(transform,0, self.size.height);

            transform = CGAffineTransformRotate(transform, -M_PI_2);

            break;

    }

    

    switch(self.imageOrientation) {

        caseUIImageOrientationUpMirrored:

        caseUIImageOrientationDownMirrored:

            transform = CGAffineTransformTranslate(transform, self.size.width,0);

            transform = CGAffineTransformScale(transform, -1,1);

            break;

    

        caseUIImageOrientationLeftMirrored:

        caseUIImageOrientationRightMirrored:

            transform = CGAffineTransformTranslate(transform, self.size.height,0);

            transform = CGAffineTransformScale(transform, -1,1);

            break;

    }

    

    // Now we draw the underlying CGImage into a new context, applying the transform

    // calculated above.

    CGContextRef ctx = CGBitmapContextCreate(NULL, self.size.width, self.size.height,

                                             CGImageGetBitsPerComponent(self.CGImage),0,

                                             CGImageGetColorSpace(self.CGImage),

                                             CGImageGetBitmapInfo(self.CGImage));

    CGContextConcatCTM(ctx, transform);

    switch(self.imageOrientation) {

        caseUIImageOrientationLeft:

        caseUIImageOrientationLeftMirrored:

        caseUIImageOrientationRight:

        caseUIImageOrientationRightMirrored:

            // Grr...

            CGContextDrawImage(ctx, CGRectMake(0,0,self.size.height,self.size.width), self.CGImage);

            break;

    

        default:

            CGContextDrawImage(ctx, CGRectMake(0,0,self.size.width,self.size.height), self.CGImage);

            break;

    }

    

    // And now we just create a new UIImage from the drawing context

    CGImageRef cgimg = CGBitmapContextCreateImage(ctx);

    UIImage *img = [UIImage imageWithCGImage:cgimg];

    CGContextRelease(ctx);

    CGImageRelease(cgimg);

    returnimg;

}

    

@end

图片编码及上传

有时候我们会需要将图片数据以字符串的形式上传到服务器。在将UIImage对象转化为NSData再转化为NSString的时候，NSString对象中会出现有乱码的情况，这个时候再将NSData转化为NSString之前要编码NSData对象

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#import"UIImage+Ext.h"

#import"GTMBase64.h"

 

@interfaceUIImage (Ext)

 

- (NSString *)convertToString;

 

@end

 

@implementationUIImage (Ext)

 

- (NSString *)convertToString

{

    if(!self) {

        returnnil;

    }

     

    NSData *imgData = UIImageJPEGRepresentation(self,0.5);

    NSData *encode = [GTMBase64 encodeData:imgData];// base64编码NSData(解决乱码问题)

    NSString *imgStr = [[NSString alloc] initWithData:encode encoding:NSUTF8StringEncoding];

    returnimgStr;

}

 

@end

将图片写入磁盘

要将图片存储到本地磁盘中，需要先把图片对象转化为NSData对象，然后调用writeToFile:接口写入

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- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;