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
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
- (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 = 1pixel；在retina显示屏中，1pt ＝ 4pixel。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
- (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:
@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
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
#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编码NSData(解决乱码问题)
NSString *imgStr = [[NSString alloc] initWithData:encode encoding:NSUTF8StringEncoding];
return imgStr;
}
@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
- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;