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LibGfx: Make fill_path() less bad at filling paths

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This patchset fixes:
- Some parts of the path being skipped and not drawn (often horizontal)
- The filled shape moving around on an int grid depending on the winding
  number
- Winding number mess-up with four-way intersections
This commit is contained in:
AnotherTest 2020-12-12 21:04:39 +03:30 • committed by Andreas Kling
parent af80d71abc
commit 0918d8b1f8
1 changed files with 32 additions and 22 deletions
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54
Libraries/LibGfx/Painter.cpp
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@ -1449,6 +1449,28 @@ void Painter::stroke_path(const Path& path, Color color, int thickness)
//#define FILL_PATH_DEBUG
[[maybe_unused]] static void approximately_place_on_int_grid(FloatPoint ffrom, FloatPoint fto, IntPoint& from, IntPoint& to, Optional<IntPoint> previous_to)
{
auto diffs = fto - ffrom;
// Truncate all first (round down).
from = ffrom.to_type<int>();
to = fto.to_type<int>();
// There are 16 possible configurations, by deciding to round each
// coord up or down (and there are four coords, from.x from.y to.x to.y)
// we will simply choose one which most closely matches the correct slope
// with the following heuristic:
// - if the x diff is positive or zero (that is, a right-to-left slant), round 'from.x' up and 'to.x' down.
// - if the x diff is negative (that is, a left-to-right slant), round 'from.x' down and 'to.x' up.
// Note that we do not need to touch the 'y' attribute, as that is our scanline.
if (diffs.x() >= 0) {
from.set_x(from.x() + 1);
} else {
to.set_x(to.x() + 1);
}
if (previous_to.has_value() && from.x() != previous_to.value().x()) // The points have to line up, since we're using these lines to fill a shape.
from.set_x(previous_to.value().x());
}
void Painter::fill_path(Path& path, Color color, WindingRule winding_rule)
{
const auto& segments = path.split_lines();
@ -1460,9 +1482,9 @@ void Painter::fill_path(Path& path, Color color, WindingRule winding_rule)
active_list.ensure_capacity(segments.size());
// first, grab the segments for the very first scanline
auto first_y = segments.first().maximum_y;
auto last_y = segments.last().minimum_y;
auto scanline = first_y;
int first_y = path.bounding_box().bottom_right().y() + 1;
int last_y = path.bounding_box().top_left().y() - 1;
float scanline = first_y;
size_t last_active_segment { 0 };
@ -1501,6 +1523,7 @@ void Painter::fill_path(Path& path, Color color, WindingRule winding_rule)
};
while (scanline >= last_y) {
Optional<IntPoint> previous_to;
if (active_list.size()) {
// sort the active list by 'x' from right to left
quick_sort(active_list, [](const auto& line0, const auto& line1) {
@ -1518,21 +1541,10 @@ void Painter::fill_path(Path& path, Color color, WindingRule winding_rule)
auto& previous = active_list[i - 1];
auto& current = active_list[i];
int int_distance = fabs(current.x - previous.x);
IntPoint from(previous.x, scanline);
IntPoint to(current.x, scanline);
if (int_distance < 1) {
// the two lines intersect on an int grid
// so they should both be treated as a single line segment
goto skip_drawing;
}
if (int_distance == 1 && is_inside_shape(winding_number)) {
// The two lines form a singluar edge for the shape
// while they do not intersect, they connect together
goto skip_drawing;
}
IntPoint from, to;
IntPoint truncated_from { previous.x, scanline };
IntPoint truncated_to { current.x, scanline };
approximately_place_on_int_grid({ previous.x, scanline }, { current.x, scanline }, from, to, previous_to);
if (is_inside_shape(winding_number)) {
// The points between this segment and the previous are
@ -1543,8 +1555,6 @@ void Painter::fill_path(Path& path, Color color, WindingRule winding_rule)
draw_line(from, to, color, 1);
}
skip_drawing:;
auto is_passing_through_maxima = scanline == previous.maximum_y
|| scanline == previous.minimum_y
|| scanline == current.maximum_y
@ -1557,7 +1567,7 @@ void Painter::fill_path(Path& path, Color color, WindingRule winding_rule)
}
if (!is_passing_through_vertex || previous.inverse_slope * current.inverse_slope < 0)
increment_winding(winding_number, from, to);
increment_winding(winding_number, truncated_from, truncated_to);
// update the x coord
active_list[i - 1].x -= active_list[i - 1].inverse_slope;
@ -1595,7 +1605,7 @@ void Painter::fill_path(Path& path, Color color, WindingRule winding_rule)
#ifdef FILL_PATH_DEBUG
size_t i { 0 };
for (auto& segment : segments) {
draw_line(Point<int>(segment.from), Point<int>(segment.to), Color::from_hsv(++i / segments.size() * 360.0, 1.0, 1.0), 1);
draw_line(Point<int>(segment.from), Point<int>(segment.to), Color::from_hsv(i++ * 360.0 / segments.size(), 1.0, 1.0), 1);
}
#endif
}