+ (UIImage *)thumbnailWithImageWithoutScale:(UIImage *)image

                                       size:(CGSize)asize

{

    UIImage *newimage = nil;

    if (image)

    {

        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));

        [image drawInRect:rect];

        newimage = UIGraphicsGetImageFromCurrentImageContext();

        UIGraphicsEndImageContext();

    }

    return newimage;

}

等比压缩图片