Convert BGR image into base64 string as jpeg

I have a color image represented in OpenCV convention where each pixel is represented as unsigned char in BGR order row after row:

const int BGR = 3;
const int rows= 256;
const int cols = 512;
unsigned char rawIm[BGR * rows *cols] = {'g', 't', 'y', // lots of chars.....}

I want to convert this stream into a base64 string which represents the corresponding jpeg image without actually writing the image on the disk, just "plain" bytes transformation. How can I do that in C++?

For the image to jpeg part, you can use toojpeg which is much easier to use than libjpeg. https://create.stephan-brumme.com/toojpeg/

But you'll have to reverse the BGR to RGB first, because toojpeg does not support BGR yet.

Here is an example:

#include <vector>
#include <string>
#include <iostream>
#include "toojpeg.h"

std::vector<unsigned char> jpeg_data;

void myOutput(unsigned char byte) {
    jpeg_data.push_back(byte);
}

int main() {
    const auto width = 800;
    const auto height = 600;
    const auto bytesPerPixel = 3;

    unsigned char bgr_data[width * height * bytesPerPixel];

    // put some sample data in bgr_data, just for the example
    for (unsigned i = 0; i < sizeof(bgr_data); i += 3) {
        bgr_data[i]     = i / width;
        bgr_data[i + 1] = i / width * 2;
        bgr_data[i + 2] = i / width * 3;
    }

    // convert BGR to RGB
    unsigned char rgb_data[sizeof(bgr_data)];
    for (unsigned i = 0; i < sizeof(bgr_data); i += 3) {
        rgb_data[i]     = bgr_data[i + 2];
        rgb_data[i + 1] = bgr_data[i + 1];
        rgb_data[i + 2] = bgr_data[i];
    }

    // convert the RGB data to jpeg
    bool isRGB = true;
    const auto quality = 90;
    const bool downsample = false;
    const char* comment = "example image";
    bool result_ok = TooJpeg::writeJpeg(myOutput, rgb_data, width, height, isRGB, quality, downsample, comment);
    if (result_ok) {
        // jpeg_data now contains jpeg-encoded image, which can be encoded as base 64
    }
    return 0;
}