Implement segmenting of Line2D for better fidelity when using width curves

doc/classes/Line2D.xml

@@ -70,6 +70,9 @@
 			[b]Note:[/b] The shape of the closing segment is not guaranteed to be seamless if a [member width_curve] is provided.
 			[b]Note:[/b] The joint between the closing segment and the first segment is drawn first and it samples the [member gradient] and the [member width_curve] at the beginning. This is an implementation detail that might change in a future version.
 		</member>
+		<member name="curve_offset" type="float" setter="set_curve_offset" getter="get_curve_offset" default="0.0">
+			The horizontal offset value used when sampling from the width curve. Typically ranges from -1.0 to 1.0.
+		</member>
 		<member name="default_color" type="Color" setter="set_default_color" getter="get_default_color" default="Color(1, 1, 1, 1)">
 			The color of the polyline. Will not be used if a gradient is set.
 		</member>
@@ -82,6 +85,10 @@
 		<member name="joint_mode" type="int" setter="set_joint_mode" getter="get_joint_mode" enum="Line2D.LineJointMode" default="0">
 			The style of the connections between segments of the polyline. Use [enum LineJointMode] constants.
 		</member>
+		<member name="min_curve_line_segments" type="int" setter="set_min_curve_line_segments" getter="get_min_curve_line_segments" default="1">
+			Subdivide the line into the number of at least [member min_curve_line_segments] segments. The resulting segment count varies depending on the use of a fill gradient and the distances between points.
+			[b]Note:[/b] Using higher values results in more segments (and vertices/indices) that need to be generated which can degrade performance, particularly if you are animating properties on your line.
+		</member>
 		<member name="points" type="PackedVector2Array" setter="set_points" getter="get_points" default="PackedVector2Array()">
 			The points of the polyline, interpreted in local 2D coordinates. Segments are drawn between the adjacent points in this array.
 		</member>

scene/2d/line_2d.cpp

@@ -119,6 +119,16 @@ Ref<Curve> Line2D::get_curve() const {
 	return _curve;
 }
 
+void Line2D::set_min_curve_line_segments(int min_curve_line_segments) {
+	_min_curve_line_segments = min_curve_line_segments;
+	queue_redraw();
+}
+
+void Line2D::set_curve_offset(float curve_offset) {
+	_curve_offset = curve_offset;
+	queue_redraw();
+}
+
 Vector<Vector2> Line2D::get_points() const {
 	return _points;
 }
@@ -276,9 +286,115 @@ void Line2D::_draw() {
 		return;
 	}
 
-	// TODO Maybe have it as member rather than copying parameters and allocating memory?
+	// NOTE:
+	// Sublines refer to the lines between points before curve segmentation.
+
 	LineBuilder lb;
-	lb.points = _points;
+	LocalVector<Vector2> &generated_draw_points = lb.points;
+	LocalVector<Vector2> gradient_inclusive_points;
+	// We insert gradient points that correspond to a position on our normalized line (e.g. gradient point of 0.5 means we add a point in the at the half-way point of our total line).
+	if (_gradient.is_valid()) {
+		int gradient_point_count = _gradient->get_point_count();
+
+		int subline_count = len - 1;
+		LocalVector<float> subline_lengths;
+		LocalVector<Vector2> sublines;
+		subline_lengths.reserve(subline_count);
+		sublines.reserve(len + _gradient->get_point_count() - 1);
+
+		// Gather the total line length, and distances and directions for each subline
+		float total_line_length = 0.0f;
+		for (int i = 0; i < subline_count; ++i) {
+			Vector2 subline_dir = _points[i + 1] - _points[i];
+			float subline_length = subline_dir.length();
+			total_line_length += subline_length;
+			subline_lengths.push_back(subline_length);
+			sublines.push_back(subline_dir);
+		}
+
+		// Reserve the right amount of memory so we're not constantly reallocating as we're pushing items in the back.
+		gradient_inclusive_points.reserve(static_cast<unsigned int>(_points.size()) + static_cast<unsigned int>(gradient_point_count));
+
+		int gradient_idx = 0;
+
+		float cumulative_line_length = 0.0f;
+		for (int line_idx = 0; line_idx < subline_count; ++line_idx) {
+			// All current calculations are based on the current subline.
+			int wrapped_line_idx = line_idx % static_cast<int>(_points.size());
+			const Vector2 &curr_subline_start = _points[wrapped_line_idx];
+			const Vector2 &curr_subline = sublines[wrapped_line_idx];
+			const float &curr_subline_length = subline_lengths[wrapped_line_idx];
+
+			// This is where our current subline points lay on our normalized line scale.
+			float curr_subline_start_offset = cumulative_line_length / total_line_length;
+			float curr_subline_end_offset = (cumulative_line_length + curr_subline_length) / total_line_length;
+
+			gradient_inclusive_points.push_back(curr_subline_start);
+			// We check to see if there's a gradient point that has an offset between our current subline.
+			// For the curve offset, we only care about the X axis that ranges from 0 to 1, corresponding with our line normalization.
+			while (gradient_idx < _gradient->get_point_count()) {
+				float curr_gradient_point_offset = _gradient->get_offset(gradient_idx);
+				float relative_gradient_point_offset = (curr_gradient_point_offset - curr_subline_start_offset) / (curr_subline_end_offset - curr_subline_start_offset);
+				// Only add a point if the point offset is between our subline start and end.
+				if (relative_gradient_point_offset >= 1.0f) {
+					break;
+				}
+				if (relative_gradient_point_offset > 0.0f) {
+					gradient_inclusive_points.push_back(curr_subline_start + curr_subline * relative_gradient_point_offset);
+				}
+				++gradient_idx;
+			}
+			cumulative_line_length += curr_subline_length;
+		}
+		gradient_inclusive_points.push_back(_points[subline_count]);
+	} else {
+		gradient_inclusive_points = _points;
+	}
+
+	int subline_count = static_cast<int>(gradient_inclusive_points.size()) - 1;
+	int num_segments = _curve.is_valid() ? _min_curve_line_segments : -1;
+	num_segments = MAX(subline_count, num_segments);
+	generated_draw_points.reserve(num_segments);
+	generated_draw_points.clear();
+
+	// We have less segments than what's defined, so we just use the points.
+	if (num_segments <= subline_count) {
+		generated_draw_points = gradient_inclusive_points;
+	} else {
+		len = gradient_inclusive_points.size();
+
+		// TODO: Cache the memory somewhere so we don't keep allocating memory
+		int line_count = _closed ? len : len - 1;
+		LocalVector<float> subline_lengths;
+		subline_lengths.reserve(line_count);
+
+		// Gather the total line length, and distances and directions for each segment
+		float total_line_length = 0.0f;
+		for (int line_idx = 0; line_idx < line_count; ++line_idx) {
+			Vector2 line_dir = gradient_inclusive_points[(line_idx + 1) % len] - gradient_inclusive_points[line_idx];
+			float subline_length = line_dir.length();
+			total_line_length += subline_length;
+			subline_lengths.push_back(subline_length);
+		}
+
+		float segment_length = total_line_length / MAX(1, static_cast<float>(num_segments));
+		for (int line_idx = 0; line_idx < line_count; ++line_idx) {
+			float subline_length = subline_lengths[line_idx];
+			int subline_segments_count = static_cast<int>(Math::ceil(subline_length / segment_length));
+			Vector2 subline_start = gradient_inclusive_points[line_idx];
+			Vector2 subline_end = gradient_inclusive_points[(line_idx + 1) % len];
+			generated_draw_points.push_back(gradient_inclusive_points[line_idx]);
+			for (int l = 1; l < subline_segments_count; ++l) {
+				float current_segment_relative_start = segment_length * static_cast<float>(l);
+				generated_draw_points.push_back(subline_start + (subline_end - subline_start) * (current_segment_relative_start / subline_length));
+			}
+		}
+
+		// We don't add the last point so that the line builder can properly cap the ends
+		if (!_closed)
+			generated_draw_points.push_back(gradient_inclusive_points[line_count % len]);
+	}
+
 	lb.closed = _closed;
 	lb.default_color = _default_color;
 	lb.gradient = *_gradient;
@@ -290,6 +406,7 @@ void Line2D::_draw() {
 	lb.sharp_limit = _sharp_limit;
 	lb.width = _width;
 	lb.curve = *_curve;
+	lb.curve_offset = _curve_offset;
 
 	RID texture_rid;
 	if (_texture.is_valid()) {
@@ -358,6 +475,12 @@ void Line2D::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_curve", "curve"), &Line2D::set_curve);
 	ClassDB::bind_method(D_METHOD("get_curve"), &Line2D::get_curve);
 
+	ClassDB::bind_method(D_METHOD("set_min_curve_line_segments", "curve_offset"), &Line2D::set_min_curve_line_segments);
+	ClassDB::bind_method(D_METHOD("get_min_curve_line_segments"), &Line2D::get_min_curve_line_segments);
+
+	ClassDB::bind_method(D_METHOD("set_curve_offset", "curve_offset"), &Line2D::set_curve_offset);
+	ClassDB::bind_method(D_METHOD("get_curve_offset"), &Line2D::get_curve_offset);
+
 	ClassDB::bind_method(D_METHOD("set_default_color", "color"), &Line2D::set_default_color);
 	ClassDB::bind_method(D_METHOD("get_default_color"), &Line2D::get_default_color);
 
@@ -392,6 +515,8 @@ void Line2D::_bind_methods() {
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "closed"), "set_closed", "is_closed");
 	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "width", PROPERTY_HINT_NONE, "suffix:px"), "set_width", "get_width");
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "width_curve", PROPERTY_HINT_RESOURCE_TYPE, "Curve"), "set_curve", "get_curve");
+	ADD_PROPERTY(PropertyInfo(Variant::INT, "min_curve_line_segments", PROPERTY_HINT_RANGE, "1,64,1,or_greater"), "set_min_curve_line_segments", "get_min_curve_line_segments");
+	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "curve_offset", PROPERTY_HINT_RANGE, "-1.0,1.0,0.01"), "set_curve_offset", "get_curve_offset");
 	ADD_PROPERTY(PropertyInfo(Variant::COLOR, "default_color"), "set_default_color", "get_default_color");
 	ADD_GROUP("Fill", "");
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "gradient", PROPERTY_HINT_RESOURCE_TYPE, "Gradient"), "set_gradient", "get_gradient");

scene/2d/line_2d.h

@@ -85,6 +85,12 @@ class Line2D : public Node2D {
 	void set_curve(const Ref<Curve> &curve);
 	Ref<Curve> get_curve() const;
 
+	void set_min_curve_line_segments(int min_curve_line_segments);
+	int get_min_curve_line_segments() const { return _min_curve_line_segments; }
+
+	void set_curve_offset(float curve_offset);
+	float get_curve_offset() const { return _curve_offset; }
+
 	void set_default_color(Color color);
 	Color get_default_color() const;
 
@@ -133,6 +139,8 @@ class Line2D : public Node2D {
 	bool _closed = false;
 	float _width = 10.0;
 	Ref<Curve> _curve;
+	int _min_curve_line_segments = 1;
+	float _curve_offset = 0.0f;
 	Color _default_color = Color(1, 1, 1);
 	Ref<Gradient> _gradient;
 	Ref<Texture2D> _texture;

scene/2d/line_builder.cpp

@@ -57,7 +57,7 @@ void LineBuilder::build() {
 	const float hw = width / 2.f;
 	const float hw_sq = hw * hw;
 	const float sharp_limit_sq = sharp_limit * sharp_limit;
-	const int point_count = points.size();
+	const int point_count = static_cast<int>(points.size());
 	const bool wrap_around = closed && point_count > 2;
 
 	_interpolate_color = gradient != nullptr;
@@ -113,7 +113,7 @@ void LineBuilder::build() {
 	}
 
 	if (_interpolate_color) {
-		color0 = gradient->get_color(0);
+		color0 = gradient->get_color_at_offset(0.0f);
 	} else {
 		colors.push_back(default_color);
 	}
@@ -189,7 +189,8 @@ void LineBuilder::build() {
 			color1 = gradient->get_color_at_offset(current_distance1 / total_distance);
 		}
 		if (retrieve_curve) {
-			width_factor = curve->sample_baked(current_distance1 / total_distance);
+			float offset = CLAMP((current_distance1 / total_distance) - curve_offset, 0.0f, 1.0f);
+			width_factor = curve->sample_baked(offset);
 			modified_hw = hw * width_factor;
 		}
 
@@ -385,7 +386,7 @@ void LineBuilder::build() {
 			current_distance1 += pos0.distance_to(pos1);
 		}
 		if (_interpolate_color) {
-			color1 = gradient->get_color(gradient->get_point_count() - 1);
+			color1 = gradient->get_color_at_offset(1.0f);
 		}
 		if (retrieve_curve) {
 			width_factor = curve->sample_baked(1.f);
@@ -414,7 +415,7 @@ void LineBuilder::build() {
 		// Custom drawing for a round end cap.
 		if (end_cap_mode == Line2D::LINE_CAP_ROUND) {
 			// Note: color is not used in case we don't interpolate.
-			Color color = _interpolate_color ? gradient->get_color(gradient->get_point_count() - 1) : Color(0, 0, 0);
+			Color color = _interpolate_color ? gradient->get_color_at_offset(1.0f) : Color(0, 0, 0);
 			float dist = 0;
 			if (texture_mode == Line2D::LINE_TEXTURE_TILE) {
 				dist = width_factor / tile_aspect;

scene/2d/line_builder.h

@@ -37,13 +37,14 @@ class LineBuilder {
 public:
 	// TODO Move in a struct and reference it
 	// Input
-	Vector<Vector2> points;
+	LocalVector<Vector2> points;
 	Line2D::LineJointMode joint_mode = Line2D::LINE_JOINT_SHARP;
 	Line2D::LineCapMode begin_cap_mode = Line2D::LINE_CAP_NONE;
 	Line2D::LineCapMode end_cap_mode = Line2D::LINE_CAP_NONE;
 	bool closed = false;
 	float width = 10.0;
 	Curve *curve = nullptr;
+	float curve_offset = 0.0f;
 	Color default_color = Color(0.4, 0.5, 1);
 	Gradient *gradient = nullptr;
 	Line2D::LineTextureMode texture_mode = Line2D::LineTextureMode::LINE_TEXTURE_NONE;

scene/resources/curve.cpp

@@ -476,6 +476,7 @@ void Curve::set_bake_resolution(int p_resolution) {
 	ERR_FAIL_COND(p_resolution > 1000);
 	_bake_resolution = p_resolution;
 	_baked_cache_dirty = true;
+	emit_changed();
 }
 
 real_t Curve::sample_baked(real_t p_offset) const {