压缩图片的算法

#pragma mark image scale utility 《完整！！！！》
- (UIImage *)imageByScalingToMaxSize:(UIImage *)sourceImage {
if (sourceImage.size.width < ORIGINAL_MAX_WIDTH) return sourceImage;
CGFloat btWidth = 0.0f;
CGFloat btHeight = 0.0f;
if (sourceImage.size.width > sourceImage.size.height) {
btHeight = ORIGINAL_MAX_WIDTH;
btWidth = sourceImage.size.width * (ORIGINAL_MAX_WIDTH / sourceImage.size.height);
} else {
btWidth = ORIGINAL_MAX_WIDTH;
btHeight = sourceImage.size.height * (ORIGINAL_MAX_WIDTH / sourceImage.size.width);
}
CGSize targetSize = CGSizeMake(btWidth, btHeight);
return [self imageByScalingAndCroppingForSourceImage:sourceImage targetSize:targetSize];
}

- (UIImage *)imageByScalingAndCroppingForSourceImage:(UIImage *)sourceImage targetSize:(CGSize)targetSize {
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;
}