void draw_color_wheel(SkCanvas* canvas, float scale) {

    const float stroke = scale * 3 / 64;

    const float size = scale * 9 / 32;

    SkAutoCanvasRestore autoCanvasRestore(canvas, true);

    canvas->translate(0.5f * scale, 0.5f * scale);

    canvas->clear(0);

    canvas->drawCircle({0, 0}, (scale - stroke) * 0.5f, SkPaint(SkColors::kWhite));

    {

        SkPaint sweep;

        const SkMatrix rotate = SkMatrix::MakeAll(0, -1, 0, 1, 0, 0, 0, 0, 1);

        constexpr unsigned kColorCount = 7;

        static const SkColor4f kColors[kColorCount] = {

            SkColors::kRed,  SkColors::kYellow,  SkColors::kGreen, SkColors::kCyan,

            SkColors::kBlue, SkColors::kMagenta, SkColors::kRed};

        sweep.setShader(SkGradientShader::MakeSweep(0, 0, kColors, nullptr, nullptr,

                                                    kColorCount, 0, &rotate));

        sweep.setStyle(SkPaint::kStroke_Style);

        sweep.setStrokeWidth(stroke);

        sweep.setAntiAlias(true);

        canvas->drawCircle({0, 0}, (scale - stroke) * 0.5f, sweep);

    }

​

    SkFont font(fontMgr->legacyMakeTypeface(nullptr, SkFontStyle::Bold()), size);

    const struct { const char* str; float radius; float angle; SkColor4f color;} kLetters[] = {

        {"K", 0,   0, SkColors::kBlack},

        {"R", 1,  90, SkColors::kRed},

        {"Y", 1, 150, SkColors::kYellow},

        {"G", 1, 210, SkColors::kGreen},

        {"C", 1, 270, SkColors::kCyan},

        {"B", 1, 330, SkColors::kBlue},

        {"M", 1,  30, SkColors::kMagenta},

    };

    for (const auto& v : kLetters) {

        SkRect bnds;

        font.measureText(v.str, strlen(v.str), SkTextEncoding::kUTF8, &bnds);

        constexpr double pi_over_180 = 3.141592653589793 / 180;

        float x = (float)(0.3 * scale * v.radius * cos(pi_over_180 * v.angle) - bnds.centerX());

        float y = (float)(0.3 * scale * v.radius * sin(pi_over_180 * v.angle) - bnds.centerY());

        canvas->drawString(v.str, x, y, font, SkPaint(v.color));

    }

}

void draw(SkCanvas* canvas) { draw_color_wheel(canvas, 256); }