这两天研究OpenFlow，认为几个UIImage相关的函数很有意义，以后可能会用到，故拿出来分享，可以用来建立自己的“Category”库
1 将图片scale为新的size
- (UIImage *)rescaleImageToSize:(CGSize)size { CGRect rect = CGRectMake(0.0, 0.0, size.width, size.height); UIGraphicsBeginImageContext(rect.size); [self drawInRect:rect]; // scales image to rect UIImage *resImage = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); return resImage;}2 截取图像中的指定区域，返回结果图像
- (UIImage *)cropImageToRect:(CGRect)cropRect { // Begin the drawing (again) UIGraphicsBeginImageContext(cropRect.size); CGContextRef ctx = UIGraphicsGetCurrentContext(); // Tanslate and scale upside-down to compensate for //Quartz's inverted coordinate system CGContextTranslateCTM(ctx, 0.0, cropRect.size.height); CGContextScaleCTM(ctx, 1.0, -1.0); // Draw view into context CGRect drawRect = CGRectMake( -cropRect.origin.x, cropRect.origin.y-(self.size.height-cropRect.size.height), self.size.width, self.size.height); CGContextDrawImage(ctx, drawRect, self.CGImage); // Create the new UIImage from the context UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext(); // End the drawing UIGraphicsEndImageContext(); return newImage;}3 通过一个选定区域大小，计算和图像“等比”的相应区域大小
- (CGSize)calculateNewSizeForCroppingBox:(CGSize)croppingBox { // Make the shortest side be equivalent to the cropping box. CGFloat newHeight, newWidth; if (self.size.width < self.size.height) { newWidth = croppingBox.width; newHeight = (self.size.height / self.size.width) * croppingBox.width; } else { newHeight = croppingBox.height; newWidth = (self.size.width / self.size.height) * croppingBox.height; } return CGSizeMake(newWidth, newHeight);}4 通过一个选定区域，得到和这个选定区域相应等比大小的图像（与原图像的中心对齐）
- (UIImage *)cropCenterAndScaleImageToSize:(CGSize)cropSize { UIImage *scaledImage = [self rescaleImageToSize: [self calculateNewSizeForCroppingBox:cropSize]]; return [scaledImage cropImageToRect:CGRectMake( (scaledImage.size.width-cropSize.width)/2, (scaledImage.size.height-cropSize.height)/2, cropSize.width, cropSize.height)];}
