How to scale a bitmap (uint8_t array) in Arduino?

Question

So I have a project where I send text to an Arduino, which is then printed out as a QR code on a thermal printer that can take a bitmap image with a given width and height.

Is there a way I can "scale up" the "image," making it so one pixel is scaled up to 4 or 8 pixels?

Additionally, would it be possible to add a "quiet zone" of sorts on the left side of the image, as I am not able to offset/justify it to the middle.

QR Code lib: https://github.com/ricmoo/QRCode

Thermal printer lib: https://github.com/adafruit/Adafruit-Thermal-Printer-Library

Answer 1

You didn't tell use the size of your QRCode, nor the type of Arduino you are using. So I am just going to guess that your problem is that you may have enough RAM to store the raw QRCode, but not enough to store a scaled bitmap version.

The Adafruit tutorial for their thermal printer library suggests printing bitmaps with the method:

printBitmap(width, height, tablename)

where tablename is an array holding the bitmap data. This will obviously not work if you don't have enough RAM to store the array. Fortunately, there is an overloaded method:

void Adafruit_Thermal::printBitmap(int w, int h, Stream *fromStream)

that allows you to stream the image data instead of preparing it in RAM. You can create a stream that delivers arbitrary data by writing a class that derives from Stream. Here is a tentative, untested implementation, that delivers a scaled-up QRCode one pixel at a time:

// Stream a scaled QRCode one pixel at a time.
class QRCodeStream : public Stream
{
public:
    QRCodeStream(const QRCode &code, uint8_t scale = 1)
    : qrcode(code), scale(scale) {}

    // Output: writing to this stream is a no-op.
    size_t write(uint8_t) {}
    void flush() {}

    // Input delivers the QRCode pixels.
    int available();
    int peek();
    int read();
private:
    QRCode &qrcode;
    uint8_t scale;
    bool done = false;
    uint8_t code_x = 0;  // coordinates of the QRCode module
    uint8_t code_y = 0;
    uint8_t pix_x = 0;  // pixel coordinates within the module
    uint8_t pix_y = 0;
};

int QRCodeStream::available()
{
    // For simplicity, pretend the bytes come one at a time.
    if (done) return 0;
    else return 1;
}

int QRCodeStream::peek()
{
    if (done) return -1;
    if (qrcode_getModule(&qrcode, code_x, code_y)) {
        return 0;  // black
    } else {
        return 255;  // white
    }
}

int QRCodeStream::read()
{
    if (done) return -1;
    int data = peek();

    // Move to the next pixel.
    if (++pix_x >= scale) {
        pix_x = 0;
        if (++code_x >= qrcode.size) {
            code_x = 0;
            if (++pix_y >= scale) {
                pix_y = 0;
                if (++code_y >= qrcode.size) {
                    done = true;
                }
            }
        }
    }

    return data;
}

This would be used as

printer.printBitmap(width, height, QRCodeStream(qrcode, scale));

Note that QRCodeStream delivers the image data as 8-bit gray scale, where each pixel is returned as one byte, either 0 (for black) or 255 (for white). This is likely not the format expected by the thermal printer library, which seems to expect 1-bit black and white data, with 8 pixels packed in each byte. You will have to adapt the code for the right format, which will likely be tedious, but not overly complex. I leave that “as an exercise to the reader”.