iOS 压缩图片

    sendImageData = UIImageJPEGRepresentation(self.shareImage, 1.0);
    NSUInteger sizeOrigin = [sendImageData length];
    NSUInteger sizeOriginKB = sizeOrigin / 1024;

if (sizeOriginKB > 500) {
        float a = 500.0000;
        float b = (float)sizeOriginKB;
        float q = sqrtf(a / b);
        
        CGSize sizeImage = [self.shareImage size];
        CGFloat widthSmall = sizeImage.width * q;
        CGFloat heighSmall = sizeImage.height * q;
        CGSize sizeImageSmall = CGSizeMake(widthSmall, heighSmall);
        
        UIGraphicsBeginImageContext(sizeImageSmall);
        CGRect smallImageRect = CGRectMake(0, 0, sizeImageSmall.width, sizeImageSmall.height);
        [self.shareImage drawInRect:smallImageRect];
        UIImage *smallImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        
        NSData *dataImage = UIImageJPEGRepresentation(smallImage, 1.0);
        sendImageData = dataImage;
        
        sendImage = smallImage;
        [sendImage retain];
    }