前言：還是這個圖片瀏覽的小demo，在里面實現了對單張圖片的放大縮小。現在把我已知的圖片縮放的各種方法記錄下來，方便以后查閱，畢竟圖片縮放還是經常用到的。

正文：構建圖像縮略圖，通常有3種形式：保持圖像比例不變調整大小；裁剪部分圖像來匹配可用空間；通過匹配可用空間的長度和寬度來填充圖像。

一、最簡單的方法
UIGraphicsBeginImageContext(size); //size 為CGSize類型，即你所需要的圖片尺寸
[sourceImage drawInRect:newImageRect]; //sourceImage為原圖，newImageRect指定了圖片繪制區域
newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

OK，newImage 就是你要的圖。缺點：回丟失很多圖片信息

二、利用UISrcollView的方法

_srcollView.minimumZoomScale=0.5f;//最小縮小倍數

_srcollView.maximumZoomScale=2.0f;//最大方法倍數

// return a view that will be scaled. if delegate returns nil, nothing happen
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    NSLog(@"viewForZoomingInScrollView");
    for (id view in [_srcollView subviews]) {
        if ([view isKindOfClass:[UIImageView class]]) {
            return view;
        }
    }
    return nil;
}

三、自己寫縮放方法
我們一般會寫一個ImageHelper類，在這個類中實現圖片的尺寸調整等所有功能。先附上代碼：

 1 #import <Foundation/Foundation.h>
 2 
 3 #define CGAutorelease(x) (__typeof(x))[NSMakeCollectable(x) autorelease]
 4 
 5 #define SUPPPORTS_UNDOCUMENTED_APIS    1
 6 
 7 @interface ImageHelper : NSObject
 8 // Create image
 9 + (UIImage *) imageFromView: (UIView *) theView;
10 
11 // Base Image Fitting
12 + (CGSize) fitSize: (CGSize)thisSize inSize: (CGSize) aSize;
13 + (UIImage *) unrotateImage: (UIImage *) image;
14 
15 + (UIImage *) image: (UIImage *) image fitInSize: (CGSize) size; // retain proportions, fit in size
16 + (UIImage *) image: (UIImage *) image fitInView: (UIView *) view;
17 
18 + (UIImage *) image: (UIImage *) image centerInSize: (CGSize) size; // center, no resize
19 + (UIImage *) image: (UIImage *) image centerInView: (UIView *) view;
20 
21 + (UIImage *) image: (UIImage *) image fillSize: (CGSize) size; // fill all pixels
22 + (UIImage *) image: (UIImage *) image fillView: (UIView *) view;
23 
24 #if SUPPPORTS_UNDOCUMENTED_APIS
25 + (UIImage *) image: (UIImage *) image withOrientation: (UIImageOrientation) orientation;
26 #endif
27 
28 //截取view部分圖片
29 + (UIImage *) imageFromViewWithRect:(UIView *)theView rect:(CGRect)rect;
30 @end

ImageHelper.h

  1 #import "ImageHelper.h"
  2 #import <QuartzCore/QuartzCore.h>
  3 
  4 #if SUPPPORTS_UNDOCUMENTED_APIS
  5 @interface UIImage (privateAPISForOrientation)
  6 - (id)initWithCGImage:(struct CGImage *)fp8 imageOrientation:(int)fp12;
  7 @end
  8 #endif
  9 
 10 @implementation ImageHelper
 11 
 12 #pragma mark Create Image
 13 
 14 // Screen shot the view
 15 + (UIImage *) imageFromView: (UIView *) theView
 16 {
 17     UIGraphicsBeginImageContext(theView.frame.size);
 18     CGContextRef context = UIGraphicsGetCurrentContext();
 19     [theView.layer renderInContext:context];
 20     UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
 21     UIGraphicsEndImageContext();
 22     return theImage;
 23 }
 24 
 25 #pragma mark Base Image Utility
 26 + (CGSize) fitSize: (CGSize)thisSize inSize: (CGSize) aSize
 27 {
 28     CGFloat scale;
 29     CGSize newsize = thisSize;
 30     
 31     
 32     if (newsize.height && (newsize.height > aSize.height))//如果新圖的高度不為0,且大于原圖高度
 33     {
 34         scale = aSize.height / newsize.height;//比例為 原高／新高
 35         newsize.width *= scale;//新圖的寬度等于 ＝ 
 36         newsize.height *= scale;
 37     }
 38     
 39     if (newsize.width && (newsize.width >= aSize.width))
 40     {
 41         scale = aSize.width / newsize.width;
 42         newsize.width *= scale;
 43         newsize.height *= scale;
 44     }
 45     
 46     return newsize;
 47 }
 48 
 49 #define MIRRORED ((image.imageOrientation == UIImageOrientationUpMirrored) || (image.imageOrientation == UIImageOrientationLeftMirrored) || (image.imageOrientation == UIImageOrientationRightMirrored) || (image.imageOrientation == UIImageOrientationDownMirrored))
 50 #define ROTATED90    ((image.imageOrientation == UIImageOrientationLeft) || (image.imageOrientation == UIImageOrientationLeftMirrored) || (image.imageOrientation == UIImageOrientationRight) || (image.imageOrientation == UIImageOrientationRightMirrored))
 51 
 52 + (UIImage *) doUnrotateImage: (UIImage *) image
 53 {
 54     CGSize size = image.size;
 55     if (ROTATED90) size = CGSizeMake(image.size.height, image.size.width);
 56     
 57     UIGraphicsBeginImageContext(size);
 58     CGContextRef context = UIGraphicsGetCurrentContext();
 59     CGAffineTransform transform = CGAffineTransformIdentity;
 60     
 61     // Rotate as needed
 62     switch(image.imageOrientation)
 63     {
 64         case UIImageOrientationLeft:
 65         case UIImageOrientationRightMirrored:
 66             transform = CGAffineTransformRotate(transform, M_PI / 2.0f);
 67             transform = CGAffineTransformTranslate(transform, 0.0f, -size.width);
 68             size = CGSizeMake(size.height, size.width);
 69             CGContextConcatCTM(context, transform);
 70             break;
 71         case UIImageOrientationRight:
 72         case UIImageOrientationLeftMirrored:
 73             transform = CGAffineTransformRotate(transform, -M_PI / 2.0f);
 74             transform = CGAffineTransformTranslate(transform, -size.height, 0.0f);
 75             size = CGSizeMake(size.height, size.width);
 76             CGContextConcatCTM(context, transform);
 77             break;
 78         case UIImageOrientationDown:
 79         case UIImageOrientationDownMirrored:
 80             transform = CGAffineTransformRotate(transform, M_PI);
 81             transform = CGAffineTransformTranslate(transform, -size.width, -size.height);
 82             CGContextConcatCTM(context, transform);
 83             break;
 84         default:
 85             break;
 86     }
 87     
 88     
 89     if (MIRRORED)
 90     {
 91         // de-mirror
 92         transform = CGAffineTransformMakeTranslation(size.width, 0.0f);
 93         transform = CGAffineTransformScale(transform, -1.0f, 1.0f);
 94         CGContextConcatCTM(context, transform);
 95     }
 96     
 97     // Draw the image into the transformed context and return the image
 98     [image drawAtPoint:CGPointMake(0.0f, 0.0f)];
 99     UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
100     UIGraphicsEndImageContext();
101     return newimg;
102 }
103 
104 + (UIImage *) unrotateImage: (UIImage *) image
105 {
106     if (image.imageOrientation == UIImageOrientationUp) return image;
107     return [ImageHelper doUnrotateImage:image];
108 }
109 
110 
111 
112 // Proportionately resize, completely fit in view, no cropping
113 + (UIImage *) image: (UIImage *) image fitInSize: (CGSize) viewsize
114 {
115     // calculate the fitted size
116     CGSize size = [ImageHelper fitSize:image.size inSize:viewsize];
117     
118     UIGraphicsBeginImageContext(viewsize);
119     
120     float dwidth = (viewsize.width - size.width) / 2.0f;
121     float dheight = (viewsize.height - size.height) / 2.0f;
122     
123     CGRect rect = CGRectMake(dwidth, dheight, size.width, size.height);
124     [image drawInRect:rect];
125     
126     UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
127     UIGraphicsEndImageContext();
128     
129     return newimg;
130 }
131 
132 + (UIImage *) image: (UIImage *) image fitInView: (UIView *) view
133 {
134     return [self image:image fitInSize:view.frame.size];
135 }
136 
137 // No resize, may crop
138 + (UIImage *) image: (UIImage *) image centerInSize: (CGSize) viewsize
139 {
140     CGSize size = image.size;
141     
142     UIGraphicsBeginImageContext(viewsize);
143     float dwidth = (viewsize.width - size.width) / 2.0f;
144     float dheight = (viewsize.height - size.height) / 2.0f;
145     
146     CGRect rect = CGRectMake(dwidth, dheight, size.width, size.height);
147     [image drawInRect:rect];
148     
149     UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
150     UIGraphicsEndImageContext();
151     
152     return newimg;
153 }
154 
155 + (UIImage *) image: (UIImage *) image centerInView: (UIView *) view
156 {
157     return [self image:image centerInSize:view.frame.size];
158 }
159 
160 // Fill every view pixel with no black borders, resize and crop if needed
161 + (UIImage *) image: (UIImage *) image fillSize: (CGSize) viewsize
162 
163 {
164     CGSize size = image.size;
165     
166     CGFloat scalex = viewsize.width / size.width;
167     CGFloat scaley = viewsize.height / size.height;
168     CGFloat scale = MAX(scalex, scaley);
169     
170     UIGraphicsBeginImageContext(viewsize);
171     
172     CGFloat width = size.width * scale;
173     CGFloat height = size.height * scale;
174     
175     float dwidth = ((viewsize.width - width) / 2.0f);
176     float dheight = ((viewsize.height - height) / 2.0f);
177     
178     CGRect rect = CGRectMake(dwidth, dheight, size.width * scale, size.height * scale);
179     [image drawInRect:rect];
180     
181     UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
182     UIGraphicsEndImageContext();
183     
184     return newimg;
185 }
186 
187 + (UIImage *) image: (UIImage *) image fillView: (UIView *) view
188 {
189     return [self image:image fillSize:view.frame.size];
190 }
191 
192 #if SUPPPORTS_UNDOCUMENTED_APIS
193 + (UIImage *) image: (UIImage *) image withOrientation: (UIImageOrientation) orientation
194 {
195     UIImage *newimg = [[UIImage alloc] initWithCGImage:[image CGImage] imageOrientation:orientation];
196     return [newimg autorelease];
197 }
198 #endif
199 
200 //截取view部分圖片
201 + (UIImage *) imageFromViewWithRect:(UIView *)theView rect:(CGRect)rect
202 {
203     CGFloat scale = 1.0f;
204     if (UIGraphicsBeginImageContextWithOptions != NULL)
205     {
206         UIGraphicsBeginImageContextWithOptions(theView.bounds.size, NO, theView.layer.contentsScale);
207         scale = theView.layer.contentsScale;
208     }
209     else
210     {
211         UIGraphicsBeginImageContext(theView.bounds.size);
212     }
213     
214     CALayer * curLayer = [theView layer];
215     CGContextRef context = UIGraphicsGetCurrentContext();
216     [curLayer renderInContext:context];
217     
218     UIImage * pImage = UIGraphicsGetImageFromCurrentImageContext();
219     CGImageRef imageRef = pImage.CGImage;
220     CGRect rectSubImage = {rect.origin.x*scale,rect.origin.y*scale,rect.size.width*scale,rect.size.height*scale};
221     CGImageRef subImageRef = CGImageCreateWithImageInRect(imageRef, rectSubImage);
222     rect.size.width  *=scale;
223     rect.size.height *=scale;
224     CGSize size = rect.size;
225     UIGraphicsBeginImageContext(size);
226     CGContextRef subcontext = UIGraphicsGetCurrentContext();
227     CGContextDrawImage(subcontext, rect, subImageRef);
228     
229     UIImage * image = [UIImage imageWithCGImage:subImageRef scale:scale orientation:UIImageOrientationUp];
230     UIGraphicsEndImageContext();
231     
232     CGImageRelease(subImageRef);   //這里不必釋放
233     UIGraphicsEndImageContext();
234     
235     return image;
236 }
237 @end

ImageHelper.m

這份代碼iphone開發秘籍這本書中給的，實現了開頭介紹的3種縮放方法。下面會詳細的分析這份代碼。
+(CGSize)fitSize:(CGSize)thisSizeinSize:(CGSize)aSize; 計算出合適的尺寸

+(UIImage*)image:(UIImage*)imagefitInSize:(CGSize)size;成比例縮放，不裁剪，需重新測量尺寸

+(UIImage*)image:(UIImage*)imagecenterInSize:(CGSize)size;不需要再重新測量，可能裁剪

+(UIImage*)image:(UIImage*)imagecenterInSize:(CGSize)size;可能需要裁剪和重新測量，每個像素都會被使用

開始逐個介紹每一種縮放方法究竟是怎么實現的：

1、fitSize: (CGSize)thisSize inSize: (CGSize) aSize;第一個參數是原圖尺寸，第二個是新View的尺寸

根據原圖的尺寸，成比例的計算出新圖的尺寸。

新圖尺寸初始化：CGSize newSize = thisSize;先令新圖的尺寸等于原圖的尺寸

計算縮放比例：原W/原H = 新W/新H

1、fitInSize的實現

image: (UIImage *) image fitInSize: (CGSize) size; 第一個參數是原圖，第二個參數是要構建的新圖的尺寸

成比例的縮放自然要先

待寫。。。

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