libqrencode doesn't make QR Codes

Question

This may seem naive, but I've been banging my head against the wall for two days trying to figure out how to make a QR code with libqrencode. Everything I find online that asks "how do I make an image with libqrencode" has a ho-hum 'maybe you could do this, but it's not really going to make a useable image'

I can:

QRcode *qrcode;
qrcode = QRcode_encodeString("This is my text", 4, QR_ECLEVEL_H, QR_MODE_8, 0);

all day, and encode the data into a struct where it sits effectively useless because my QR code scanner doesn't scan structs, it scans QR codes.

The only way I've been able to achieve it is this:

string name= "\'this is my text\'";
string s2 = "qrencode -l L -v 1 -o output.png " + name;
system(s2.c_str());

which makes me shudder because I hate making system calls (and I want this little program to be portable).

I know that there's something obvious I'm missing, because libqrencode is useless without actually making QR codes, but what is it that I'm missing? How do I make an actual QR code with libqrencode?

Answer 1

Still frustrated, I did some deeper searching and found the code for qrenc.c here. I pilfered just the part that makes a png. A little finagling got it to work. I figured I'd share this snippet since I couldn't find a clear answer on how to do it easily myself. I have all the variables declared here and this code should run out-of-the box (copy-paste) as long as you have libqrenc and libpng installed.

Bob's your uncle. :)

#include <stdio.h>
#include "qrencode.h"
#include <errno.h>
#include <stdlib.h>
#include <fstream>
#include <string>
#include<iostream>
#include<numeric>
#include <png.h>

using namespace std;
#define INCHES_PER_METER (100.0/2.54)

static int casesensitive = 0;
static int eightbit = 0;
static int version = 0;
static int size = 3;
static int margin = 3;
static int dpi = 72;
static int structured = 0;
static int rle = 0;
static int micro = 0;
static QRecLevel level = QR_ECLEVEL_L;
static QRencodeMode hint = QR_MODE_8;
static unsigned int fg_color[4] = {0, 0, 0, 255};
static unsigned int bg_color[4] = {255, 255, 255, 255};

static int writePNG(QRcode *qrcode, const char *outfile);

int main()
{
const char * line = "The stuff you want to encode";
        QRcode *myqrcode;
        myqrcode = QRcode_encodeString(line, 4, QR_ECLEVEL_H, QR_MODE_8,1);
        writePNG(myqrcode,"filename.png");
        QRcode_free(myqrcode);
return 0;
}



static int writePNG(QRcode *qrcode, const char *outfile)
{
    static FILE *fp; // avoid clobbering by setjmp.
    png_structp png_ptr;
    png_infop info_ptr;
    png_colorp palette;
    png_byte alpha_values[2];
    unsigned char *row, *p, *q;
    int x, y, xx, yy, bit;
    int realwidth;

    realwidth = (qrcode->width + margin * 2) * size;
    row = (unsigned char *)malloc((realwidth + 7) / 8);
    if(row == NULL) {
        fprintf(stderr, "Failed to allocate memory.\n");
        exit(EXIT_FAILURE);
    }

    if(outfile[0] == '-' && outfile[1] == '\0') {
        fp = stdout;
    } else {
        fp = fopen(outfile, "wb");
        if(fp == NULL) {
            fprintf(stderr, "Failed to create file: %s\n", outfile);
            perror(NULL);
            exit(EXIT_FAILURE);
        }
    }

    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if(png_ptr == NULL) {
        fprintf(stderr, "Failed to initialize PNG writer.\n");
        exit(EXIT_FAILURE);
    }

    info_ptr = png_create_info_struct(png_ptr);
    if(info_ptr == NULL) {
        fprintf(stderr, "Failed to initialize PNG write.\n");
        exit(EXIT_FAILURE);
    }

    if(setjmp(png_jmpbuf(png_ptr))) {
        png_destroy_write_struct(&png_ptr, &info_ptr);
        fprintf(stderr, "Failed to write PNG image.\n");
        exit(EXIT_FAILURE);
    }

    palette = (png_colorp) malloc(sizeof(png_color) * 2);
    if(palette == NULL) {
        fprintf(stderr, "Failed to allocate memory.\n");
        exit(EXIT_FAILURE);
    }
    palette[0].red   = fg_color[0];
    palette[0].green = fg_color[1];
    palette[0].blue  = fg_color[2];
    palette[1].red   = bg_color[0];
    palette[1].green = bg_color[1];
    palette[1].blue  = bg_color[2];
    alpha_values[0] = fg_color[3];
    alpha_values[1] = bg_color[3];
    png_set_PLTE(png_ptr, info_ptr, palette, 2);
    png_set_tRNS(png_ptr, info_ptr, alpha_values, 2, NULL);

    png_init_io(png_ptr, fp);
    png_set_IHDR(png_ptr, info_ptr,
        realwidth, realwidth,
        1,
        PNG_COLOR_TYPE_PALETTE,
        PNG_INTERLACE_NONE,
        PNG_COMPRESSION_TYPE_DEFAULT,
        PNG_FILTER_TYPE_DEFAULT);
png_set_pHYs(png_ptr, info_ptr,
        dpi * INCHES_PER_METER,
        dpi * INCHES_PER_METER,
        PNG_RESOLUTION_METER);
png_write_info(png_ptr, info_ptr);

/* top margin */
memset(row, 0xff, (realwidth + 7) / 8);
for(y=0; y<margin * size; y++) {
    png_write_row(png_ptr, row);
}

/* data */
p = qrcode->data;
for(y=0; y<qrcode->width; y++) {
    bit = 7;
    memset(row, 0xff, (realwidth + 7) / 8);
    q = row;
    q += margin * size / 8;
    bit = 7 - (margin * size % 8);
    for(x=0; x<qrcode->width; x++) {
        for(xx=0; xx<size; xx++) {
            *q ^= (*p & 1) << bit;
            bit--;
            if(bit < 0) {
                q++;
                bit = 7;
            }
        }
        p++;
    }
    for(yy=0; yy<size; yy++) {
        png_write_row(png_ptr, row);
    }
}
/* bottom margin */
memset(row, 0xff, (realwidth + 7) / 8);
for(y=0; y<margin * size; y++) {
    png_write_row(png_ptr, row);
}

png_write_end(png_ptr, info_ptr);
png_destroy_write_struct(&png_ptr, &info_ptr);

fclose(fp);
free(row);
free(palette);

return 0;

}

Answer 2

According to the documentation found here libqrencode just returns you an array with the information needed to generate the image. To get an actual image you need to do something like this:

for(int i = 0; i < qrcode->width; ++i)
{
    for(int j = 0; j < qrcode->width; ++j)
    {
        if(qrcode->data[i * qrcode->width + j] & 1)
        {
            // draw black dot at (i,j)
        }
        else
        {
            // draw white dot at (i,j)
        }
    }
}