ZXing识别本地图片二维码 识别率低

对图片做压缩处理 压缩到300左右识别率比较高（不知道原理）

- (UIImage *)commpressionImage:(UIImage *)theImage {
    UIImage* bigImage = theImage;
    float actualHeight = bigImage.size.height;
    float actualWidth = bigImage.size.width;
    float newWidth =0;
    float newHeight =0;
    if(actualWidth > actualHeight) {
        //宽图
        newHeight = 300;
        newWidth = actualWidth / actualHeight * newHeight;
    }
    else
    {
        //长图
        newWidth = 300;
        newHeight = actualHeight / actualWidth * newWidth;
    }
    CGRect rect =CGRectMake(0.0,0.0, newWidth, newHeight);
    UIGraphicsBeginImageContext(rect.size);
    [bigImage drawInRect:rect];// scales image to rect
    theImage =UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    //RETURN
    return theImage;
}