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LibGfx: Use the Midpoint Ellipse Algorithm
It is only used to draw non-antialiased and non-filled ellipses.
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2 changed files with 51 additions and 17 deletions
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@ -496,17 +496,58 @@ void Painter::draw_ellipse_intersecting(IntRect const& rect, Color color, int th
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if (thickness <= 0)
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return;
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constexpr int number_samples = 100; // FIXME: dynamically work out the number of samples based upon the rect size
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float increment = AK::Pi<float> / number_samples;
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auto const center = rect.center();
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auto ellipse_xy = [&rect](float theta) -> IntPoint {
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float s, c;
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AK::sincos(theta, s, c);
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return IntPoint { (c * rect.width() * AK::Sqrt1_2<float>), (s * rect.height() * AK::Sqrt1_2<float>)} + rect.center();
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auto const draw_real_world_x4 = [this, &color, thickness, center](int x, int y) {
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IntPoint const directions[4] = { { x, y }, { x, -y }, { -x, y }, { -x, -y } };
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for (auto const& delta : directions) {
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auto const point = center + delta;
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draw_line(point, point, color, thickness);
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}
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};
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for (auto theta = 0.f; theta < 2 * AK::Pi<float>; theta += increment) {
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draw_line(ellipse_xy(theta), ellipse_xy(theta + increment), color, thickness);
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// Note: This is an implementation of the Midpoint Ellipse Algorithm:
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double const a = rect.width() / 2;
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double const a_square = a * a;
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double const b = rect.height() / 2;
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double const b_square = b * b;
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int x = 0;
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auto y = static_cast<int>(b);
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double dx = 2 * b_square * x;
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double dy = 2 * a_square * y;
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// For region 1:
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auto decision_parameter = b_square - a_square * b + .25 * a_square;
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while (dx < dy) {
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draw_real_world_x4(x, y);
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if (decision_parameter >= 0) {
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y--;
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dy -= 2 * a_square;
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decision_parameter -= dy;
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}
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x++;
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dx += 2 * b_square;
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decision_parameter += dx + b_square;
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}
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// For region 2:
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decision_parameter = b_square * ((x + 0.5) * (x + 0.5)) + a_square * ((y - 1) * (y - 1)) - a_square * b_square;
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while (y >= 0) {
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draw_real_world_x4(x, y);
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if (decision_parameter <= 0) {
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x++;
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dx += 2 * b_square;
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decision_parameter += dx;
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}
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y--;
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dy -= 2 * a_square;
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decision_parameter += a_square - dy;
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}
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}
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