Function writeImageSixel

src/Formats/PNGTerminalOutput.cpp:186–312

}

void writeImageSixel(WriteBuffer & out, const UInt8 * pixels, size_t width, size_t height, size_t channels)
{
    auto to_level = [](UInt8 c) -> int { return (c * (SIXEL_LEVELS - 1) + 127) / 255; };

    auto pixel_color = [&](size_t x, size_t y) -> int
    {
        const UInt8 * p = pixels + (y * width + x) * channels;
        UInt8 r = 0;
        UInt8 g = 0;
        UInt8 b = 0;
        if (channels == 1)
        {
            r = g = b = p[0];
        }
        else
        {
            r = p[0];
            g = p[1];
            b = p[2];
            if (channels == 4)
            {
                /// Composite over a black background using the alpha channel.
                const UInt32 a = p[3];
                r = static_cast<UInt8>(r * a / 255);
                g = static_cast<UInt8>(g * a / 255);
                b = static_cast<UInt8>(b * a / 255);
            }
        }
        return (to_level(r) * SIXEL_LEVELS + to_level(g)) * SIXEL_LEVELS + to_level(b);
    };

    /// Start of the Sixel data: DCS, then raster attributes (1:1 pixel aspect ratio and the image size).
    writeCString("\033Pq", out);
    writeCString("\"1;1;", out);
    writeIntText(width, out);
    writeChar(';', out);
    writeIntText(height, out);

    /// Define the full fixed palette up front (color number; RGB color space [2]; R, G, B in percent [0, 100]).
    for (int c = 0; c < SIXEL_LEVELS * SIXEL_LEVELS * SIXEL_LEVELS; ++c)
    {
        const int r = c / (SIXEL_LEVELS * SIXEL_LEVELS);
        const int g = (c / SIXEL_LEVELS) % SIXEL_LEVELS;
        const int b = c % SIXEL_LEVELS;
        writeChar('#', out);
        writeIntText(c, out);
        writeCString(";2;", out);
        writeIntText(r * 100 / (SIXEL_LEVELS - 1), out);
        writeChar(';', out);
        writeIntText(g * 100 / (SIXEL_LEVELS - 1), out);
        writeChar(';', out);
        writeIntText(b * 100 / (SIXEL_LEVELS - 1), out);
    }

    /// Sixels encode 6 vertical pixels at a time, so the image is processed in horizontal bands of 6 rows.
    VectorWithMemoryTracking<int> band_colors(width * 6);
    std::array<bool, SIXEL_LEVELS * SIXEL_LEVELS * SIXEL_LEVELS> present{};
    StringWithMemoryTracking line;