改变图片尺寸

// 改变图片尺寸
-(UIImage *)thumbnailWithImageWithoutScale:(UIImage *)image size:(CGSize)asize{

    UIImage *newimage;
    if (nil == image) {
        newimage = nil;
    }else{
        CGSize oldsize = image.size;
        CGRect rect;
        if (asize.width/asize.height > oldsize.width/oldsize.height) {
            
            rect.size.width = asize.height*oldsize.width/oldsize.height;
            rect.size.height = asize.height;
            rect.origin.x = (asize.width - rect.size.width)/2;
            rect.origin.y =0;
        }else{
            rect.size.width = asize.width;
            rect.size.height = asize.width*oldsize.height/oldsize.width;
            rect.origin.x =0;
            rect.origin.y = (asize.height - rect.size.height)/2;
        }
        
        UIGraphicsBeginImageContext(asize);
        CGContextRef context =UIGraphicsGetCurrentContext();
        CGContextSetFillColorWithColor(context, [[UIColor clearColor] CGColor]);
        UIRectFill(CGRectMake(0,0, asize.width, asize.height));//clear background
        [image drawInRect:rect];
        newimage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
   }
   return newimage;
   
}