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LibGUI: Reimplement Painter::draw_triangle to be more symmetrical

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This is a reimplementation of draw_triangle that manages without
floating point arithmetic.
It's most important property, compared to the previous implementation is
that rotating the same triangle 90 degrees won't drastically change the
appearance of that triangle. (it did before)
This commit is contained in:
FrHun 2022-06-15 03:55:34 +02:00 committed by Sam Atkins
parent 586a94818d
commit 565f68f8bb
Notes: sideshowbarker 2024-07-17 10:04:11 +09:00 
Author: https://github.com/frhun
Commit: https://github.com/SerenityOS/serenity/commit/565f68f8bb
Pull-request: https://github.com/SerenityOS/serenity/pull/14282
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/MacDue
1 changed files with 65 additions and 39 deletions
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104
Userland/Libraries/LibGfx/Painter.cpp
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@ -695,6 +695,10 @@ void Painter::draw_triangle(IntPoint const& a, IntPoint const& b, IntPoint const
if (p0.y() == p2.y())
return;
// return if all points are on the same line vertically
if (p0.x() == p1.x() && p1.x() == p2.x())
return;
// return if top is below clip rect or bottom is above clip rect
auto clip = clip_rect();
if (p0.y() >= clip.bottom())
@ -702,56 +706,78 @@ void Painter::draw_triangle(IntPoint const& a, IntPoint const& b, IntPoint const
if (p2.y() < clip.top())
return;
class BoundaryLine {
private:
IntPoint m_base {};
IntPoint m_path {};
public:
BoundaryLine(IntPoint a, IntPoint b)
{
VERIFY(a.y() <= b.y());
m_base = a;
m_path = b - a;
}
int top_y() const { return m_base.y(); }
int bottom_y() const { return m_base.y() + m_path.y(); }
bool is_vertical() const { return m_path.x() == 0; }
bool is_horizontal() const { return m_path.y() == 0; }
bool in_y_range(int y) const { return y >= top_y() && y <= bottom_y(); }
Optional<int> intersection_on_x(int y) const
{
if (!in_y_range(y))
return {};
if (is_horizontal())
return {};
if (is_vertical())
return m_base.x();
int y_diff = y - top_y();
int x_d = m_path.x() * y_diff, y_d = m_path.y();
return (x_d / y_d) + m_base.x();
}
};
BoundaryLine l0(p0, p1), l1(p0, p2), l2(p1, p2);
int rgba = color.value();
float dx02 = (float)(p2.x() - p0.x()) / (p2.y() - p0.y());
float x01 = p0.x();
float x02 = p0.x();
for (int y = max(p0.y(), clip.top()); y <= min(p2.y(), clip.bottom()); y++) {
Optional<int>
x0 = l0.intersection_on_x(y),
x1 = l1.intersection_on_x(y),
x2 = l2.intersection_on_x(y);
if (p0.y() != p1.y()) { // p0 and p1 are on different lines
float dx01 = (float)(p1.x() - p0.x()) / (p1.y() - p0.y());
int result_a = 0, result_b = 0;
int top = p0.y();
if (top < clip.top()) {
x01 += dx01 * (clip.top() - top);
x02 += dx02 * (clip.top() - top);
top = clip.top();
}
for (int y = top; y < p1.y() && y < clip.bottom(); ++y) { // XXX <=?
int start = x01 > x02 ? max((int)x02, clip.left()) : max((int)x01, clip.left());
int end = x01 > x02 ? min((int)x01, clip.right()) : min((int)x02, clip.right());
auto* scanline = m_target->scanline(y);
for (int x = start; x < end; x++) {
scanline[x] = rgba;
if (x0.has_value()) {
result_a = x0.value();
if (x1.has_value() && ((!x2.has_value()) || (result_a != x1.value()))) {
result_b = x1.value();
} else {
result_b = x2.value();
}
x01 += dx01;
x02 += dx02;
} else if (x1.has_value()) {
result_a = x1.value();
result_b = x2.value();
}
}
// return if middle point and bottom point are on same line
if (p1.y() == p2.y())
return;
if (result_a > result_b)
swap(result_a, result_b);
float x12 = p1.x();
float dx12 = (float)(p2.x() - p1.x()) / (p2.y() - p1.y());
int top = p1.y();
if (top < clip.top()) {
x02 += dx02 * (clip.top() - top);
x12 += dx12 * (clip.top() - top);
top = clip.top();
}
int left_bound = result_a, right_bound = result_b;
for (int y = top; y < p2.y() && y < clip.bottom(); ++y) { // XXX <=?
int start = x12 > x02 ? max((int)x02, clip.left()) : max((int)x12, clip.left());
int end = x12 > x02 ? min((int)x12, clip.right()) : min((int)x02, clip.right());
auto* scanline = m_target->scanline(y);
for (int x = start; x < end; x++) {
ARGB32* scanline = m_target->scanline(y);
for (int x = max(left_bound, clip.left()); x <= min(right_bound, clip.right()); x++) {
scanline[x] = rgba;
}
x02 += dx02;
x12 += dx12;
}
}