unsigned short calculate_stride(unsigned int w, unsigned char bypp)
{
    unsigned short pitch = 0;

    pitch = w * bypp;
    switch (bypp<<3)
    {
    case 1: pitch = (pitch+7)/8; break;
    case 4: pitch = (pitch+1)/2; break;
    }

    // 4-byte aligning
    pitch = (pitch + 3) & ~3;
    return pitch;
}


struct cm_RGB_2_RGB {
    enum {
        r = 0, g = 1, b = 2
    };
};

struct cm_BRG_2_RGB {
    enum {
        r = 1, g = 2, b = 0
    };
};

struct cm_BGR_2_RGB {
    enum {
        r = 2, g = 1, b = 0
    };
};

template< typename color_model >
bool    graphics_algs::export_2ppm( unsigned int w, unsigned int h, int stride, unsigned char bypp, const void* data, std::ostream& stream )
{
    assert( stream.good() && stream.flags() & std::ios::binary|std::ios::out);
    if( !stream.good() ) return false;

    stream << (1 == bypp ? "P5" : "P6") << "\n";
    stream << w << "\n" << h << "\n";
    stream << 255 << "\n";

    if(4 == bypp) { //convert to 24 bit PPM

        const unsigned short new_stride = calculate_stride(w, 3);

        for(unsigned int r = 0;  r < h; ++r)
        {
            const unsigned char *_data = (const unsigned char*)data + (r*stride);
            for(unsigned int c = 0; c < w; ++c)
            {
                stream.put(*(_data+c*4+color_model::r));
                stream.put(*(_data+c*4+color_model::g));
                stream.put(*(_data+c*4+color_model::b));
            }

            for(int s = 0; s < new_stride - (w*3); ++s) {
                stream.put(0);
            }
        }

    } else {
        for(unsigned int y = 0; y < h; y++) {
            stream.write( (const char*)data + y * stride, w * bypp);
        }
    }

    return true;
}