Question

I’m using a zxing library in my iOS project. It’s a library for reading and creating QR Codes.

From my researching and browsing around the web the process of decoding the image is made of this steps:

• takes an image from source,
• converts all the pixels to 255 grayscale
• decodes the data

One thing which is not supported by this specific library is reading/decoding (and I’m pretty sure that this is missing in creating also) of inverted QRCodes.

Inverted QR Codes are basicly the same as normal codes -> but with inverted colors (white is black and black is white). But because the QRCodes standard doesn’t describe the inverted QRCodes implementation and on zxing project website there is a few requests and issues, I must implement this by myself.

The method below is a good place to insert some logic to invert the pixels (unsigned char*), but because of my non-experience with C++ the result is this writing.

The grayData_ is a unsigned char* data type. And inside this variable there are grayScaled pixels from source.

What I want to do is invert these pixels.

If I’m correct this is done by unsigned char cz = (255 - val);?

unsigned char* GreyscaleLuminanceSource::getMatrix() {
    int size = width_ * height_;
    unsigned char* result = new unsigned char[size];
    if (left_ == 0 && top_ == 0 && dataWidth_ == width_ && dataHeight_ == height_) {
        memcpy(result, greyData_, size);
    } else {
        for (int row = 0; row < height_; row++) {
            memcpy(result + row * width_, greyData_ + (top_ + row) * dataWidth_ + left_, width_);
        }
    }

    //return result;
    //from this point down is my implementation

    printf(" %c", result[200]);
    printf(" %c", result[600]);
    printf(" %c", result[6000]);
    printf(" %c", result[7000]);

    for (int i = 0; i < size; i++)
    {
        int val = static_cast<int>(result[i]); 
        unsigned char cz = (255 -  val);
        result[i] = cz;
    }
    printf("******\n");
    printf(" %c", result[200]); //prints a " " char to console/terminal
    printf(" %c", result[600]); //prints a " " char to console/terminal
    printf(" %c", result[6000]); //prints a " " char to console/terminal
    printf(" %c", result[7000]); //prints a " " char to console/terminal

    return result;
}

Is this the right way to invert the pixels? And I’m not so happy with changing the data in the result variable.

Answer 1

You don’t want to print the values as characters because they aren’t ASCII. Print them as unsigneds:

printf(" %u", ...);

You can simplify the loop to simply

result[i] = static_cast<unsigned char>(255 - result[i]);

The other conversions are normal integral promotion.

And you should note that ZXing uses some asymmetric heuristics when identifying codes. If you don’t have a guard region surrounding the code that is, in this case, black (since the guard region is supposed to be white), it may fail to identify the code.