Add bump map from normal map effect

[yuripourre]

Snippet:

const int w = normalMap->width();
    const int h = normalMap->height();

    // Compute the laplacian once; it never changes
    Image1::Ref laplacian = Image1::createEmpty(w, h, WrapMode::ERROR);

    for (int x = 0; x < w; ++x) {
        for (int y = 0; y < h; ++y) {
            float ddx = normalMap->get(x + 1, y).r - normalMap->get(x - 1, y).r;
            float ddy = normalMap->get(x, y + 1).g - normalMap->get(x, y - 1).g;
            laplacian->set(x, y, Color1(ddx + signConvention * ddy) / 2.0f);
        }
    }

    // Ping-pong buffers
    Image1::Ref src = Image1::createEmpty(w, h, WrapMode::TILE);
    Image1::Ref dst = Image1::createEmpty(w, h, WrapMode::TILE);

    for (int x = 0; x < w; ++x) {
        for (int y = 0; y < h; ++y) {
            dst->set(x, y, Color1(0.5));
        }
    }

    // Number of Poisson iterations
    const int N = 100;
    for (int i = 0; i < N; ++i) {
        // Swap buffers
        Image1::Ref tmp = src;
        src = dst;
        dst = tmp;

        for (int x = 0; x < w; ++x) {
            for (int y = 0; y < h; ++y) {
                dst->set(x, y,
                         Color1((src->get(x - 1, y).value + src->get(x, y - 1).value +
                                 src->get(x + 1, y).value + src->get(x, y + 1).value + 
                                 laplacian->get(x, y).value) * 0.25f));
            }
        }
    }

    float lo = inf(), hi = -inf();
    for (int x = 0; x < w; ++x) {
        for (int y = 0; y < h; ++y) {
            float v = dst->get(x, y).value;
            lo = min(lo, v);
            hi = max(hi, v);
        }
    }
    for (int x = 0; x < w; ++x) {
        for (int y = 0; y < h; ++y) {
            dst->set(x, y, Color1((dst->get(x, y).value - lo) / (hi - lo)));
        }
    }

Source: https://github.com/morgan3d/misc/blob/master/normal2bump/normal2bump.cpp