IOS應用開發－圖片處理（拉伸圖片 建立縮圖 解決圖片旋轉的問題 圖片編碼及上傳 將圖片寫入磁碟）
阿新 • • 發佈：2019-01-11
在做專案時我們經常要對圖片進行一些處理，以達到效能優化或滿足需求。常見的情形有以下幾種
//http://stackoverflow.com/questions/5427656/ios-uiimagepickercontroller-result-image-orientation-after-upload
拉伸圖片
專案中使用的圖片素材如果能通過拉伸獲得就儘量這樣去做。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執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。
對於拉伸圖片，適配ios 5及之後可以使用
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- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets
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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，呼叫方法建立縮圖
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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;
}
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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:
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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
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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物件轉化為NSData再轉化為NSString的時候，NSString物件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物件
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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編碼NSData(解決亂碼問題)
NSString *imgStr = [[NSString alloc] initWithData:encode encoding:NSUTF8StringEncoding];
returnimgStr;
}
@end
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要將圖片儲存到本地磁碟中，需要先把圖片物件轉化為NSData物件，然後呼叫writeToFile:介面寫入
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- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;
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