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  • Writer's pictureEngineer Luna

Creating Splines for UMG, leveraging the internal drawing API

Note: This is a fascinating post from a few years ago, I feel I would massively improve this system and the code standards now due to much more experience. I would also like to bonk past me for all of these autos.

I created UMGSplines leveraging the already existing slate function in FSlateDrawElement, giving the option of Cubic or Hermite 2 point splines.


I created a Base Slate Spline class and a base UMG Spline class. These base classes will hold core functionality, while I am able to derive from there and build layers of functionality on top or override existing functionality to easily create Splines with different behaviour.

// slate
class SSpline;
// uwidget
class USpline;

Here is a snippet of the base Slate Spline class and the mandatory SplineData struct. This struct is required for all spline classes, and is passed through to construct while using SNew or SAssignNew SNew(USpline, SplineData) and cannot be set using the postfix .Member(Value) notation (named parameter pattern) - as it is not an optional parameter.

struct FSplineData
	FSplineData(const TArray<FVector2D>& InPoints,
		const FSlateColor& InColor,
		const float& InThickness)
		: Points(InPoints)
		, Color(InColor)

	TArray<FVector2D> Points;
	FSlateColor Color;
	float Thickness;

class UMGSPLINESRUNTIME_API SSpline : public SLeafWidget
	// No Slate constructor since this class should be treated as abstract
	virtual FVector2D ComputeDesiredSize(float) const override;
	// Default validation for spline functionality
	virtual const bool IsSplineDataValid() const = 0;

	virtual void CreateSpline(const FPaintArgs& Args, const FGeometry& AllottedGeometry, const FSlateRect& MyCullingRect, FSlateWindowElementList& OutDrawElements, int32 LayerId, const FWidgetStyle& InWidgetStyle, bool bParentEnabled) const;

	virtual int32 OnPaint(const FPaintArgs& Args, const FGeometry& AllottedGeometry, const FSlateRect& MyCullingRect, FSlateWindowElementList& OutDrawElements, int32 LayerId, const FWidgetStyle& InWidgetStyle, bool bParentEnabled) const override;
	// Spline Data
	FSplineData Data;

Whereas Cubic Bezier Splines takes normally 4 points, start, end and handles, a lot of users might want to control their spline with just 1 control point to simplify things. There is where creating a second point that is evenly distributed would help, building on top of evenly placing the control points I am able to allow for evenly distributed bends.

Spline auto bending and adjusting the positions for an evenly distributed curve.
Debug Point 1 and 2 show with trigonometry Lines

some code snippets

few pieces of basic code that help to drive the behaviour of the TriBezierSpline, taking point0 and breaking it down in to a triangle using the start and end points.

void UTriBezierSpline::ConvertPointToTri(const FVector2D& TriPoint, FVector2D& _Point0, FVector2D& _Point1)
	FVector2D PointTri(TriPoint);
	PointTri = TriPoint + GetTriOffset();

	_Point0 = FVector2D((Start.X + PointTri.X) * 0.5f, (Start.Y + PointTri.Y) * 0.5f);
	_Point1 = FVector2D((End.X + PointTri.X) * 0.5f, (End.Y + PointTri.Y) * 0.5f);
// Generating a even curve to point north
void UTriBezierSpline::BendNorth()
	FVector2D NewPoint0 = FMath::Abs(End - Start);

	NewPoint0 = NewPoint0 * 0.5f;
	NewPoint0 = NewPoint0 * -1.f;

	NewPoint0 = NewPoint0 * CurveScale;
	Point0 = NewPoint0;

This system was expanded on by creating custom containers that then divide themselves into sections and drive the auto-bend configuration of any splines within it.

Spline auto bending from container driving
debug lines show you how the space is split up in the theatre sections

some code snippets that drive this

ESplineDirection USplineMathLibrary::GetSplineDirectionState(const FVector2D Direction)
	float SplineDir = GetSplineDirectionAngle(Direction);

	static const TArray<ESplineDirection> SectorDirectionMappings = {
	ESplineDirection::East, ESplineDirection::SouthEast, ESplineDirection::South,
	ESplineDirection::SouthWest, ESplineDirection::West, ESplineDirection::NorthWest,
	ESplineDirection::North, ESplineDirection::NorthEast };

	return SectorDirectionMappings[SplineDir];

float USplineMathLibrary::GetSplineDirectionAngle(const FVector2D Direction)
	FVector2D _Vec = (Direction - FVector2D(250.f, 250.f)).GetSafeNormal();
	float Angle = FMath::Acos(FVector2D::DotProduct(_Vec, FVector2D(1.f, 0.f)));
	Angle = FMath::RadiansToDegrees(Angle);

	if (_Vec.Y < 0.f)
		Angle = 360.f - Angle;

	const int32 SectorCount = 8;
	const float SectorTheta = 360.0f / SectorCount;
	const float HalfSectorTheta = SectorTheta / 2.0f;

	for (int32 i = 0; i < SectorCount; ++i)
		const float CurrentSectorMin = (SectorTheta * i) - HalfSectorTheta;
		const float CurrentSectorMax = (SectorTheta * i) + HalfSectorTheta;

		if (Angle >= CurrentSectorMin && Angle <= CurrentSectorMax)
			return i;
	return 0;

Working around the restrictions of UMGEditor for handles

One of the main challenges I had to face was the restriction of not being able to easily add a handle based control system in the UMGEditor, there is a system for extending this, but it is not yet finished.

You can find my PR here that finishes exposing the system.

After much testing and discovery of limitations, the stock-ue4 solution ended up being a function that can be used during designer time to pass an array of handle targets to the slate widgets. These don’t need to exist during runtime. This should work until the FDesignerExtension system has been exposed for non-engine modules, and should be easy to rework later on.

Some important things to consider when dealing with robust maintainable code

One thing I had to consider for the future was when I would have access to the FDesignerExtension in the engine, so I decided to make the Handle system for the underline slate widgets be an interface. Having the ability to easily pass the Handle functionality to different spline types and keeping the code much easier to read.

Code Snippet, Slim version of the Handle interface

class ISHandle
	TArray <TSharedPtr<SWidget>> Handles;
	TSharedPtr<SWidget> MyCanvas;

	virtual void AnchorHandles(FSplineData& Data)
		if (!MyCanvas.IsValid())
			UE_LOG(SplineLog, Warning, TEXT("Warning when attempting to anchor Handles Canvas is invalid"));

		const auto AllocatedCanvasGeo = MyCanvas->GetCachedGeometry();
		int index = 0;
		for (auto& Handle : Handles)
			if (Handle.IsValid())
				const auto AnchorGeo = Handle->GetCachedGeometry();
				Data.Points[index] = AllocatedCanvasGeo.AbsoluteToLocal(
					AnchorGeo.GetAbsolutePositionAtCoordinates({ 0.f, 0.f }));

Final thoughts & tackling a robust Spline inheritance system in slate

One thing I really had to consider was the base foundation for SSpline class, we have our slate macro constructor and are non-macro slate constructor which is mandatory to create the widget. These are not to be confused with the class constructor. For the sake of maintainable code, I decided to wrap the core parameters for a Spline in a struct that would be paired with the non-macro constructor, this being the same for every derived slate class. Extra parameter flexibility will come from slate macro constructor, allowing for any extra information to be added on to any classes.

Encapsulation, ain't nobody got time for that

1, 2, 3 and easy!

Even the most simple sections of code can lead to system rot without the correct supporting framework.


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