IOS應用開發－圖片處理

在做項目時我們經常要對圖片進行一些處理，以達到效能最佳化或滿足需求。常見的情形有以下幾種

展開圖片

項目中使用的圖片素材如果能通過展開獲得就盡量這樣去做。這樣做有兩個顯而易見的好處，一是能夠減少App安裝包的大小，另外一個則是減少App運行時佔據的記憶體空間大小。畢竟App的UI基本上來說是建立在大量的精緻的圖片上，如果這些圖片都一概使用螢幕等大小的圖片，那麼對App的效能及安裝量都是有一定的負面影響的。

對於展開圖片，適配ios 5及之後可以使用

- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets

這個方法只接收一個UIEdgeInsets類型的參數，可以通過設定UIEdgeInsets的top、left、bottom、right來分別指定上端蓋高度、左端蓋寬度、下端蓋高度、右端蓋寬度。這個端蓋的距離值是用單位pt（點，point）衡量的，在普通顯示屏中，1pt = 1pix；在retina顯示屏中，1pt ＝ 2pix。還有一個需要注意的地方，如果端蓋距離值不是整數的話，展開後的圖片會有白條細線。

建立縮圖

如果有一張大圖，我們只想要顯示它的指定大小的縮圖內容，可以這樣做:在UIImage的類別中實現如下方法，調用方法建立縮圖

- (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();    return newImage;}

解決圖片旋轉的問題

ios程式中使用系統相機拍照和從相簿選取圖片，直接上傳後在非mac系統下看到的圖片會發生旋轉的現象，那是因為我們沒有通過圖片的旋轉屬性修改圖片轉向。可以用下面的方法解決這個問題:

@interface UIImage (fixOrientation)   - (UIImage *)fixOrientation;   @end         @implementation UIImage (fixOrientation)   - (UIImage *)fixOrientation {       // No-op if the orientation is already correct    if (self.imageOrientation == UIImageOrientationUp) return self;       // 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) {        case UIImageOrientationDown:        case UIImageOrientationDownMirrored:            transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height);            transform = CGAffineTransformRotate(transform, M_PI);            break;           case UIImageOrientationLeft:        case UIImageOrientationLeftMirrored:            transform = CGAffineTransformTranslate(transform, self.size.width, 0);            transform = CGAffineTransformRotate(transform, M_PI_2);            break;           case UIImageOrientationRight:        case UIImageOrientationRightMirrored:            transform = CGAffineTransformTranslate(transform, 0, self.size.height);            transform = CGAffineTransformRotate(transform, -M_PI_2);            break;    }       switch (self.imageOrientation) {        case UIImageOrientationUpMirrored:        case UIImageOrientationDownMirrored:            transform = CGAffineTransformTranslate(transform, self.size.width, 0);            transform = CGAffineTransformScale(transform, -1, 1);            break;           case UIImageOrientationLeftMirrored:        case UIImageOrientationRightMirrored:            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) {        case UIImageOrientationLeft:        case UIImageOrientationLeftMirrored:        case UIImageOrientationRight:        case UIImageOrientationRightMirrored:            // 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);    return img;}   @end

圖片編碼及上傳

有時候我們會需要將圖片資料以字串的形式上傳到伺服器。在將UIImage對象轉化為NSData再轉化為NSString的時候，NSString對象中會出現有亂碼的情況，這個時候再將NSData轉化為NSString之前要編碼NSData對象

#import "UIImage+Ext.h"#import "GTMBase64.h"@interface UIImage (Ext)- (NSString *)convertToString; @end@implementation UIImage (Ext)- (NSString *)convertToString{    if (!self) {        return nil;    }        NSData *imgData = UIImageJPEGRepresentation(self,0.5);    NSData *encode = [GTMBase64 encodeData:imgData]; // base64編碼NSData(解決亂碼問題)    NSString *imgStr = [[NSString alloc] initWithData:encode encoding:NSUTF8StringEncoding];    return imgStr;}@end

將圖片寫入磁碟

要將圖片儲存到本地磁碟中，需要先把圖片對象轉化為NSData對象，然後調用writeToFile:介面寫入

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