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LibGfx: Use the Midpoint Ellipse Algorithm

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It is only used to draw non-antialiased and non-filled ellipses.
This commit is contained in:
Lucas CHOLLET 2022-11-08 17:07:14 +01:00 • committed by Andreas Kling
parent 09841f56ed
commit 4219d50a21
2 changed files with 51 additions and 17 deletions
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57
Userland/Libraries/LibGfx/Painter.cpp
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@ -496,17 +496,58 @@ void Painter::draw_ellipse_intersecting(IntRect const& rect, Color color, int th
if (thickness <= 0)
return;
constexpr int number_samples = 100; // FIXME: dynamically work out the number of samples based upon the rect size
float increment = AK::Pi<float> / number_samples;
auto const center = rect.center();
auto ellipse_xy = [&rect](float theta) -> IntPoint {
float s, c;
AK::sincos(theta, s, c);
return IntPoint { (c * rect.width() * AK::Sqrt1_2<float>), (s * rect.height() * AK::Sqrt1_2<float>)} + rect.center();
auto const draw_real_world_x4 = [this, &color, thickness, center](int x, int y) {
IntPoint const directions[4] = { { x, y }, { x, -y }, { -x, y }, { -x, -y } };
for (auto const& delta : directions) {
auto const point = center + delta;
draw_line(point, point, color, thickness);
}
};
for (auto theta = 0.f; theta < 2 * AK::Pi<float>; theta += increment) {
draw_line(ellipse_xy(theta), ellipse_xy(theta + increment), color, thickness);
// Note: This is an implementation of the Midpoint Ellipse Algorithm:
double const a = rect.width() / 2;
double const a_square = a * a;
double const b = rect.height() / 2;
double const b_square = b * b;
int x = 0;
auto y = static_cast<int>(b);
double dx = 2 * b_square * x;
double dy = 2 * a_square * y;
// For region 1:
auto decision_parameter = b_square - a_square * b + .25 * a_square;
while (dx < dy) {
draw_real_world_x4(x, y);
if (decision_parameter >= 0) {
y--;
dy -= 2 * a_square;
decision_parameter -= dy;
}
x++;
dx += 2 * b_square;
decision_parameter += dx + b_square;
}
// For region 2:
decision_parameter = b_square * ((x + 0.5) * (x + 0.5)) + a_square * ((y - 1) * (y - 1)) - a_square * b_square;
while (y >= 0) {
draw_real_world_x4(x, y);
if (decision_parameter <= 0) {
x++;
dx += 2 * b_square;
decision_parameter += dx;
}
y--;
dy -= 2 * a_square;
decision_parameter += a_square - dy;
}
}