Add a Skew property to StyleBoxFlat

This makes it possible to create more aesthetically pleasing
styleboxes for GUI theming, especially in games that have
a futuristic appearance (where skewed buttons and progress bars
are common).

3 files changed, 47 insertions, 17 deletions

diff --git a/doc/classes/StyleBoxFlat.xml b/doc/classes/StyleBoxFlat.xml
index 20430f3205..c80f8dcbb1 100644
--- a/doc/classes/StyleBoxFlat.xml
+++ b/doc/classes/StyleBoxFlat.xml
@@ -119,7 +119,7 @@
 	</methods>
 	<members>
 		<member name="anti_aliasing" type="bool" setter="set_anti_aliased" getter="is_anti_aliased" default="true">
-			Antialiasing draws a small ring around the edges, which fades to transparency. As a result, edges look much smoother. This is only noticeable when using rounded corners.
+			Antialiasing draws a small ring around the edges, which fades to transparency. As a result, edges look much smoother. This is only noticeable when using rounded corners or [member skew].
 			[b]Note:[/b] When using beveled corners with 45-degree angles ([member corner_detail] = 1), it is recommended to set [member anti_aliasing] to [code]false[/code] to ensure crisp visuals and avoid possible visual glitches.
 		</member>
 		<member name="anti_aliasing_size" type="float" setter="set_aa_size" getter="get_aa_size" default="0.625">
@@ -168,15 +168,19 @@
 		</member>
 		<member name="expand_margin_bottom" type="float" setter="set_expand_margin" getter="get_expand_margin" default="0.0">
 			Expands the stylebox outside of the control rect on the bottom edge. Useful in combination with [member border_width_bottom] to draw a border outside the control rect.
+			[b]Note:[/b] Unlike [member StyleBox.content_margin_bottom], [member expand_margin_bottom] does [i]not[/i] affect the size of the clickable area for [Control]s. This can negatively impact usability if used wrong, as the user may try to click an area of the StyleBox that cannot actually receive clicks.
 		</member>
 		<member name="expand_margin_left" type="float" setter="set_expand_margin" getter="get_expand_margin" default="0.0">
 			Expands the stylebox outside of the control rect on the left edge. Useful in combination with [member border_width_left] to draw a border outside the control rect.
+			[b]Note:[/b] Unlike [member StyleBox.content_margin_left], [member expand_margin_left] does [i]not[/i] affect the size of the clickable area for [Control]s. This can negatively impact usability if used wrong, as the user may try to click an area of the StyleBox that cannot actually receive clicks.
 		</member>
 		<member name="expand_margin_right" type="float" setter="set_expand_margin" getter="get_expand_margin" default="0.0">
 			Expands the stylebox outside of the control rect on the right edge. Useful in combination with [member border_width_right] to draw a border outside the control rect.
+			[b]Note:[/b] Unlike [member StyleBox.content_margin_right], [member expand_margin_right] does [i]not[/i] affect the size of the clickable area for [Control]s. This can negatively impact usability if used wrong, as the user may try to click an area of the StyleBox that cannot actually receive clicks.
 		</member>
 		<member name="expand_margin_top" type="float" setter="set_expand_margin" getter="get_expand_margin" default="0.0">
 			Expands the stylebox outside of the control rect on the top edge. Useful in combination with [member border_width_top] to draw a border outside the control rect.
+			[b]Note:[/b] Unlike [member StyleBox.content_margin_top], [member expand_margin_top] does [i]not[/i] affect the size of the clickable area for [Control]s. This can negatively impact usability if used wrong, as the user may try to click an area of the StyleBox that cannot actually receive clicks.
 		</member>
 		<member name="shadow_color" type="Color" setter="set_shadow_color" getter="get_shadow_color" default="Color(0, 0, 0, 0.6)">
 			The color of the shadow. This has no effect if [member shadow_size] is lower than 1.
@@ -187,5 +191,9 @@
 		<member name="shadow_size" type="int" setter="set_shadow_size" getter="get_shadow_size" default="0">
 			The shadow size in pixels.
 		</member>
+		<member name="skew" type="Vector2" setter="set_skew" getter="get_skew" default="Vector2(0, 0)">
+			If set to a non-zero value on either axis, [member skew] distorts the StyleBox horizontally and/or vertically. This can be used for "futuristic"-style UIs. Positive values skew the StyleBox towards the right (X axis) and upwards (Y axis), while negative values skew the StyleBox towards the left (X axis) and downwards (Y axis).
+			[b]Note:[/b] To ensure text does not touch the StyleBox's edges, consider increasing the [StyleBox]'s content margin (see [member StyleBox.content_margin_bottom]). It is preferable to increase the content margin instead of the expand margin (see [member expand_margin_bottom]), as increasing the expand margin does not increase the size of the clickable area for [Control]s.
+		</member>
 	</members>
 </class>
diff --git a/scene/resources/style_box.cpp b/scene/resources/style_box.cpp
index 5afd424e03..ae8cfdb877 100644
--- a/scene/resources/style_box.cpp
+++ b/scene/resources/style_box.cpp
@@ -424,6 +424,15 @@ bool StyleBoxFlat::is_draw_center_enabled() const {
 	return draw_center;
 }
 
+void StyleBoxFlat::set_skew(Vector2 p_skew) {
+	skew = p_skew;
+	emit_changed();
+}
+
+Vector2 StyleBoxFlat::get_skew() const {
+	return skew;
+}
+
 void StyleBoxFlat::set_shadow_color(const Color &p_color) {
 	shadow_color = p_color;
 	emit_changed();
@@ -509,7 +518,7 @@ inline void set_inner_corner_radius(const Rect2 style_rect, const Rect2 inner_re
 }
 
 inline void draw_ring(Vector<Vector2> &verts, Vector<int> &indices, Vector<Color> &colors, const Rect2 &style_rect, const real_t corner_radius[4],
-		const Rect2 &ring_rect, const Rect2 &inner_rect, const Color &inner_color, const Color &outer_color, const int corner_detail, const bool fill_center = false) {
+		const Rect2 &ring_rect, const Rect2 &inner_rect, const Color &inner_color, const Color &outer_color, const int corner_detail, const Vector2 &skew, bool fill_center = false) {
 	int vert_offset = verts.size();
 	if (!vert_offset) {
 		vert_offset = 0;
@@ -553,9 +562,12 @@ inline void draw_ring(Vector<Vector2> &verts, Vector<int> &indices, Vector<Color
 					color = outer_color;
 					corner_point = outer_points[corner_index];
 				}
-				real_t x = radius * (real_t)cos((corner_index + detail / (double)adapted_corner_detail) * (Math_TAU / 4.0) + Math_PI) + corner_point.x;
-				real_t y = radius * (real_t)sin((corner_index + detail / (double)adapted_corner_detail) * (Math_TAU / 4.0) + Math_PI) + corner_point.y;
-				verts.push_back(Vector2(x, y));
+
+				const real_t x = radius * (real_t)cos((corner_index + detail / (double)adapted_corner_detail) * (Math_TAU / 4.0) + Math_PI) + corner_point.x;
+				const real_t y = radius * (real_t)sin((corner_index + detail / (double)adapted_corner_detail) * (Math_TAU / 4.0) + Math_PI) + corner_point.y;
+				const float x_skew = -skew.x * (y - ring_rect.get_center().y);
+				const float y_skew = -skew.y * (x - ring_rect.get_center().x);
+				verts.push_back(Vector2(x + x_skew, y + y_skew));
 				colors.push_back(color);
 			}
 		}
@@ -633,10 +645,12 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 		return;
 	}
 
-	bool rounded_corners = (corner_radius[0] > 0) || (corner_radius[1] > 0) || (corner_radius[2] > 0) || (corner_radius[3] > 0);
-	bool aa_on = rounded_corners && anti_aliased;
+	const bool rounded_corners = (corner_radius[0] > 0) || (corner_radius[1] > 0) || (corner_radius[2] > 0) || (corner_radius[3] > 0);
+	// Only enable antialiasing if it is actually needed. This improve performances
+	// and maximizes sharpness for non-skewed StyleBoxes with sharp corners.
+	const bool aa_on = (rounded_corners || !skew.is_equal_approx(Vector2())) && anti_aliased;
 
-	bool blend_on = blend_border && draw_border;
+	const bool blend_on = blend_border && draw_border;
 
 	Color border_color_alpha = Color(border_color.r, border_color.g, border_color.b, 0);
 	Color border_color_blend = (draw_center ? bg_color : border_color_alpha);
@@ -683,24 +697,24 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 		Color shadow_color_transparent = Color(shadow_color.r, shadow_color.g, shadow_color.b, 0);
 
 		draw_ring(verts, indices, colors, shadow_inner_rect, adapted_corner,
-				shadow_rect, shadow_inner_rect, shadow_color, shadow_color_transparent, corner_detail);
+				shadow_rect, shadow_inner_rect, shadow_color, shadow_color_transparent, corner_detail, skew);
 
 		if (draw_center) {
 			draw_ring(verts, indices, colors, shadow_inner_rect, adapted_corner,
-					shadow_inner_rect, shadow_inner_rect, shadow_color, shadow_color, corner_detail, true);
+					shadow_inner_rect, shadow_inner_rect, shadow_color, shadow_color, corner_detail, skew, true);
 		}
 	}
 
 	// Create border (no AA).
 	if (draw_border && !aa_on) {
 		draw_ring(verts, indices, colors, border_style_rect, adapted_corner,
-				border_style_rect, infill_rect, border_color_inner, border_color, corner_detail);
+				border_style_rect, infill_rect, border_color_inner, border_color, corner_detail, skew);
 	}
 
 	// Create infill (no AA).
 	if (draw_center && (!aa_on || blend_on || !draw_border)) {
 		draw_ring(verts, indices, colors, border_style_rect, adapted_corner,
-				infill_rect, infill_rect, bg_color, bg_color, corner_detail, true);
+				infill_rect, infill_rect, bg_color, bg_color, corner_detail, skew, true);
 	}
 
 	if (aa_on) {
@@ -732,7 +746,7 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 						aa_border_width[SIDE_RIGHT], aa_border_width[SIDE_BOTTOM]);
 				// Create infill within AA border.
 				draw_ring(verts, indices, colors, border_style_rect, adapted_corner,
-						infill_inner_rect_aa, infill_inner_rect_aa, bg_color, bg_color, corner_detail, true);
+						infill_inner_rect_aa, infill_inner_rect_aa, bg_color, bg_color, corner_detail, skew, true);
 			}
 
 			if (!blend_on || !draw_border) {
@@ -743,7 +757,7 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 
 				// Create infill fake AA gradient.
 				draw_ring(verts, indices, colors, style_rect, adapted_corner,
-						infill_rect_aa, infill_rect, bg_color, alpha_bg, corner_detail);
+						infill_rect_aa, infill_rect, bg_color, alpha_bg, corner_detail, skew);
 			}
 		}
 
@@ -757,17 +771,17 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 
 			// Create border.
 			draw_ring(verts, indices, colors, border_style_rect, adapted_corner,
-					border_style_rect_aa, ((blend_on) ? infill_rect : infill_rect_aa), border_color_inner, border_color, corner_detail);
+					border_style_rect_aa, ((blend_on) ? infill_rect : infill_rect_aa), border_color_inner, border_color, corner_detail, skew);
 
 			if (!blend_on) {
 				// Create inner border fake AA gradient.
 				draw_ring(verts, indices, colors, border_style_rect, adapted_corner,
-						infill_rect_aa, infill_rect, border_color_blend, border_color, corner_detail);
+						infill_rect_aa, infill_rect, border_color_blend, border_color, corner_detail, skew);
 			}
 
 			// Create outer border fake AA gradient.
 			draw_ring(verts, indices, colors, border_style_rect, adapted_corner,
-					style_rect_aa, border_style_rect_aa, border_color, border_color_alpha, corner_detail);
+					style_rect_aa, border_style_rect_aa, border_color, border_color_alpha, corner_detail, skew);
 		}
 	}
 
@@ -825,6 +839,9 @@ void StyleBoxFlat::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_draw_center", "draw_center"), &StyleBoxFlat::set_draw_center);
 	ClassDB::bind_method(D_METHOD("is_draw_center_enabled"), &StyleBoxFlat::is_draw_center_enabled);
 
+	ClassDB::bind_method(D_METHOD("set_skew", "skew"), &StyleBoxFlat::set_skew);
+	ClassDB::bind_method(D_METHOD("get_skew"), &StyleBoxFlat::get_skew);
+
 	ClassDB::bind_method(D_METHOD("set_shadow_color", "color"), &StyleBoxFlat::set_shadow_color);
 	ClassDB::bind_method(D_METHOD("get_shadow_color"), &StyleBoxFlat::get_shadow_color);
 
@@ -846,6 +863,7 @@ void StyleBoxFlat::_bind_methods() {
 	ADD_PROPERTY(PropertyInfo(Variant::COLOR, "bg_color"), "set_bg_color", "get_bg_color");
 
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "draw_center"), "set_draw_center", "is_draw_center_enabled");
+	ADD_PROPERTY(PropertyInfo(Variant::VECTOR2, "skew"), "set_skew", "get_skew");
 
 	ADD_GROUP("Border Width", "border_width_");
 	ADD_PROPERTYI(PropertyInfo(Variant::INT, "border_width_left", PROPERTY_HINT_RANGE, "0,1024,1"), "set_border_width", "get_border_width", SIDE_LEFT);
diff --git a/scene/resources/style_box.h b/scene/resources/style_box.h
index 4c41f42293..358f7ca596 100644
--- a/scene/resources/style_box.h
+++ b/scene/resources/style_box.h
@@ -149,6 +149,7 @@ class StyleBoxFlat : public StyleBox {
 
 	bool draw_center = true;
 	bool blend_border = false;
+	Vector2 skew;
 	bool anti_aliased = true;
 
 	int corner_detail = 8;
@@ -194,6 +195,9 @@ public:
 	void set_draw_center(bool p_enabled);
 	bool is_draw_center_enabled() const;
 
+	void set_skew(Vector2 p_skew);
+	Vector2 get_skew() const;
+
 	void set_shadow_color(const Color &p_color);
 	Color get_shadow_color() const;