压缩图片

// 生成图片缩略图
+ (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));
        [image drawInRect:rect];
        newimage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }
    return newimage;

}

thImage = [WeChatShare thumbnailWithImageWithoutScale:thImage size:CGSizeMake(thImage.size.width*0.2, thImage.size.height*0.2)];  0.2倍缩放