iOS 简单的图片缩放

前言：还是这个图片浏览的小demo，在里面实现了对单张图片的放大缩小。现在把我已知的图片缩放的各种方法记录下来，方便以后查阅，毕竟图片缩放还是经常用到的。

正文：构建图像缩略图，通常有3种形式：保持图像比例不变调整大小；裁剪部分图像来匹配可用空间；通过匹配可用空间的长度和宽度来填充图像。

一、最简单的方法

UIGraphicsBeginImageContext(size); //size 为CGSize类型，即你所需要的图片尺寸
[sourceImage drawInRect:newImageRect]; //sourceImage为原图，newImageRect指定了图片绘制区域
newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

OK，newImage 就是你要的图。缺点：回丢失很多图片信息

 

二、利用UISrcollView的方法 

  _srcollView.minimumZoomScale=0.5f;//最小缩小倍数

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

// 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、自己写缩放方法

我们一般会写一个ImageHelper类，在这个类中实现图片的尺寸调整等所有功能。先附上代码：

#import <Foundation/Foundation.h>

#define CGAutorelease(x) (__typeof(x))[NSMakeCollectable(x) autorelease]

#define SUPPPORTS_UNDOCUMENTED_APIS    1

@interface ImageHelper : NSObject
// Create image
+ (UIImage *) imageFromView: (UIView *) theView;

// Base Image Fitting
+ (CGSize) fitSize: (CGSize)thisSize inSize: (CGSize) aSize;
+ (UIImage *) unrotateImage: (UIImage *) image;

+ (UIImage *) image: (UIImage *) image fitInSize: (CGSize) size; // retain proportions, fit in size
+ (UIImage *) image: (UIImage *) image fitInView: (UIView *) view;

+ (UIImage *) image: (UIImage *) image centerInSize: (CGSize) size; // center, no resize
+ (UIImage *) image: (UIImage *) image centerInView: (UIView *) view;

+ (UIImage *) image: (UIImage *) image fillSize: (CGSize) size; // fill all pixels
+ (UIImage *) image: (UIImage *) image fillView: (UIView *) view;

#if SUPPPORTS_UNDOCUMENTED_APIS
+ (UIImage *) image: (UIImage *) image withOrientation: (UIImageOrientation) orientation;
#endif

//截取view部分图片
+ (UIImage *) imageFromViewWithRect:(UIView *)theView rect:(CGRect)rect;
@end

#import "ImageHelper.h"
#import <QuartzCore/QuartzCore.h>

#if SUPPPORTS_UNDOCUMENTED_APIS
@interface UIImage (privateAPISForOrientation)
- (id)initWithCGImage:(struct CGImage *)fp8 imageOrientation:(int)fp12;
@end
#endif

@implementation ImageHelper

#pragma mark Create Image

// Screen shot the view
+ (UIImage *) imageFromView: (UIView *) theView
{
    UIGraphicsBeginImageContext(theView.frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [theView.layer renderInContext:context];
    UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return theImage;
}

#pragma mark Base Image Utility
+ (CGSize) fitSize: (CGSize)thisSize inSize: (CGSize) aSize
{
    CGFloat scale;
    CGSize newsize = thisSize;
    
    
    if (newsize.height && (newsize.height > aSize.height))//如果新图的高度不为0,且大于原图高度
    {
        scale = aSize.height / newsize.height;//比例为 原高／新高
        newsize.width *= scale;//新图的宽度等于 ＝ 
        newsize.height *= scale;
    }
    
    if (newsize.width && (newsize.width >= aSize.width))
    {
        scale = aSize.width / newsize.width;
        newsize.width *= scale;
        newsize.height *= scale;
    }
    
    return newsize;
}

#define MIRRORED ((image.imageOrientation == UIImageOrientationUpMirrored) || (image.imageOrientation == UIImageOrientationLeftMirrored) || (image.imageOrientation == UIImageOrientationRightMirrored) || (image.imageOrientation == UIImageOrientationDownMirrored))
#define ROTATED90    ((image.imageOrientation == UIImageOrientationLeft) || (image.imageOrientation == UIImageOrientationLeftMirrored) || (image.imageOrientation == UIImageOrientationRight) || (image.imageOrientation == UIImageOrientationRightMirrored))

+ (UIImage *) doUnrotateImage: (UIImage *) image
{
    CGSize size = image.size;
    if (ROTATED90) size = CGSizeMake(image.size.height, image.size.width);
    
    UIGraphicsBeginImageContext(size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGAffineTransform transform = CGAffineTransformIdentity;
    
    // Rotate as needed
    switch(image.imageOrientation)
    {
        case UIImageOrientationLeft:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformRotate(transform, M_PI / 2.0f);
            transform = CGAffineTransformTranslate(transform, 0.0f, -size.width);
            size = CGSizeMake(size.height, size.width);
            CGContextConcatCTM(context, transform);
            break;
        case UIImageOrientationRight:
        case UIImageOrientationLeftMirrored:
            transform = CGAffineTransformRotate(transform, -M_PI / 2.0f);
            transform = CGAffineTransformTranslate(transform, -size.height, 0.0f);
            size = CGSizeMake(size.height, size.width);
            CGContextConcatCTM(context, transform);
            break;
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformRotate(transform, M_PI);
            transform = CGAffineTransformTranslate(transform, -size.width, -size.height);
            CGContextConcatCTM(context, transform);
            break;
        default:
            break;
    }
    
    
    if (MIRRORED)
    {
        // de-mirror
        transform = CGAffineTransformMakeTranslation(size.width, 0.0f);
        transform = CGAffineTransformScale(transform, -1.0f, 1.0f);
        CGContextConcatCTM(context, transform);
    }
    
    // Draw the image into the transformed context and return the image
    [image drawAtPoint:CGPointMake(0.0f, 0.0f)];
    UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newimg;
}

+ (UIImage *) unrotateImage: (UIImage *) image
{
    if (image.imageOrientation == UIImageOrientationUp) return image;
    return [ImageHelper doUnrotateImage:image];
}



// Proportionately resize, completely fit in view, no cropping
+ (UIImage *) image: (UIImage *) image fitInSize: (CGSize) viewsize
{
    // calculate the fitted size
    CGSize size = [ImageHelper fitSize:image.size inSize:viewsize];
    
    UIGraphicsBeginImageContext(viewsize);
    
    float dwidth = (viewsize.width - size.width) / 2.0f;
    float dheight = (viewsize.height - size.height) / 2.0f;
    
    CGRect rect = CGRectMake(dwidth, dheight, size.width, size.height);
    [image drawInRect:rect];
    
    UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return newimg;
}

+ (UIImage *) image: (UIImage *) image fitInView: (UIView *) view
{
    return [self image:image fitInSize:view.frame.size];
}

// No resize, may crop
+ (UIImage *) image: (UIImage *) image centerInSize: (CGSize) viewsize
{
    CGSize size = image.size;
    
    UIGraphicsBeginImageContext(viewsize);
    float dwidth = (viewsize.width - size.width) / 2.0f;
    float dheight = (viewsize.height - size.height) / 2.0f;
    
    CGRect rect = CGRectMake(dwidth, dheight, size.width, size.height);
    [image drawInRect:rect];
    
    UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return newimg;
}

+ (UIImage *) image: (UIImage *) image centerInView: (UIView *) view
{
    return [self image:image centerInSize:view.frame.size];
}

// Fill every view pixel with no black borders, resize and crop if needed
+ (UIImage *) image: (UIImage *) image fillSize: (CGSize) viewsize

{
    CGSize size = image.size;
    
    CGFloat scalex = viewsize.width / size.width;
    CGFloat scaley = viewsize.height / size.height;
    CGFloat scale = MAX(scalex, scaley);
    
    UIGraphicsBeginImageContext(viewsize);
    
    CGFloat width = size.width * scale;
    CGFloat height = size.height * scale;
    
    float dwidth = ((viewsize.width - width) / 2.0f);
    float dheight = ((viewsize.height - height) / 2.0f);
    
    CGRect rect = CGRectMake(dwidth, dheight, size.width * scale, size.height * scale);
    [image drawInRect:rect];
    
    UIImage *newimg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return newimg;
}

+ (UIImage *) image: (UIImage *) image fillView: (UIView *) view
{
    return [self image:image fillSize:view.frame.size];
}

#if SUPPPORTS_UNDOCUMENTED_APIS
+ (UIImage *) image: (UIImage *) image withOrientation: (UIImageOrientation) orientation
{
    UIImage *newimg = [[UIImage alloc] initWithCGImage:[image CGImage] imageOrientation:orientation];
    return [newimg autorelease];
}
#endif

//截取view部分图片
+ (UIImage *) imageFromViewWithRect:(UIView *)theView rect:(CGRect)rect
{
    CGFloat scale = 1.0f;
    if (UIGraphicsBeginImageContextWithOptions != NULL)
    {
        UIGraphicsBeginImageContextWithOptions(theView.bounds.size, NO, theView.layer.contentsScale);
        scale = theView.layer.contentsScale;
    }
    else
    {
        UIGraphicsBeginImageContext(theView.bounds.size);
    }
    
    CALayer * curLayer = [theView layer];
    CGContextRef context = UIGraphicsGetCurrentContext();
    [curLayer renderInContext:context];
    
    UIImage * pImage = UIGraphicsGetImageFromCurrentImageContext();
    CGImageRef imageRef = pImage.CGImage;
    CGRect rectSubImage = {rect.origin.x*scale,rect.origin.y*scale,rect.size.width*scale,rect.size.height*scale};
    CGImageRef subImageRef = CGImageCreateWithImageInRect(imageRef, rectSubImage);
    rect.size.width  *=scale;
    rect.size.height *=scale;
    CGSize size = rect.size;
    UIGraphicsBeginImageContext(size);
    CGContextRef subcontext = UIGraphicsGetCurrentContext();
    CGContextDrawImage(subcontext, rect, subImageRef);
    
    UIImage * image = [UIImage imageWithCGImage:subImageRef scale:scale orientation:UIImageOrientationUp];
    UIGraphicsEndImageContext();
    
    CGImageRelease(subImageRef);   //这里不必释放
    UIGraphicsEndImageContext();
    
    return image;
}
@end

这份代码iphone开发秘籍这本书中给的，实现了开头介绍的3种缩放方法。下面会详细的分析这份代码。
+ (CGSize) fitSize: (CGSize)thisSize inSize: (CGSize) aSize; 计算出合适的尺寸

+ (UIImage *) image: (UIImage *) image fitInSize: (CGSize) size;成比例缩放，不裁剪，需重新测量尺寸 

+ (UIImage *) image: (UIImage *) image centerInSize: (CGSize) size;不需要再重新测量，可能裁剪

+ (UIImage *) image: (UIImage *) image centerInSize: (CGSize) size;可能需要裁剪和重新测量，每个像素都会被使用

开始逐个介绍每一种缩放方法究竟是怎么实现的：

1、fitSize: (CGSize)thisSize inSize: (CGSize) aSize;第一个参数是原图尺寸，第二个是新View的尺寸

    根据原图的尺寸，成比例的计算出新图的尺寸。

    新图尺寸初始化：CGSize newSize = thisSize;先令新图的尺寸等于原图的尺寸

    计算缩放比例：原W/原H = 新W/新H

1、fitInSize的实现

   image: (UIImage *) image fitInSize: (CGSize) size; 第一个参数是原图，第二个参数是要构建的新图的尺寸

成比例的缩放自然要先

待写。。。