PNG encode

Takes a btprnt buffer, and creates a PNG file encoded as a base64 string.

#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include "btprnt/btprnt.h"
#include "lodepng/lodepng.h"

char *base64_encode(const unsigned char *data,
                    size_t input_length,
                    size_t *output_length);
char * btprnt_buf_pngencode(btprnt_buf *buf, size_t *sz);
<<funcs>>

This allocates a string with malloc. One must ensure that is is freed after it is used.

char * btprnt_buf_pngencode(btprnt_buf *buf, size_t *b64_sz)
{
    int w, h;
    unsigned char *img;
    unsigned char *out;
    size_t sz;
    int x, y;
    char *b64;

    w = btprnt_buf_width(buf);
    h = btprnt_buf_height(buf);

    img = malloc(w*h*4);

    for (y = 0; y < h; y++) {
        for (x = 0; x < w; x++) {
            unsigned char s;
            unsigned char *p;
            s = btprnt_buf_read(buf, x, y);
            p = &img[4 * ((y * w) + x)];

            p[3] = 255;

            if (s) {
                p[0] = 0;
                p[1] = 0;
                p[2] = 0;
            } else {
                p[0] = 255;
                p[1] = 255;
                p[2] = 255;
            }
        }
    }

    lodepng_encode32(&out, &sz, img, w, h);
    b64 = NULL;
    b64 = base64_encode(out, sz, b64_sz);

    free(img);
    free(out);
    return b64;
}

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