Canvas / HUD clipping for Unreal Engine using scissor rects.

ClipCanvas.cpp

#include "ClipCanvas.h"
#include "HUDEx.h"

class FCanvasScissorRectItem : public FCanvasItem
{
public:
	bool bEnable;
	int32 MinX;
	int32 MaxX;
	int32 MinY;
	int32 MaxY;

	FCanvasScissorRectItem(bool enable, int32 minX, int32 minY, int32 maxX, int32 maxY) :
		FCanvasItem(FVector2D::ZeroVector),
		bEnable(enable),
		MinX(minX),
		MaxX(maxX),
		MinY(minY),
		MaxY(maxY)
	{
	}

	FCanvasScissorRectItem() :
		FCanvasScissorRectItem(false, 0, 0, 0, 0)
	{
	}

public:
	void Draw(FCanvas* InCanvas);
};

class FCanvasScissorRectRenderItem : public FCanvasBaseRenderItem
{
public:
	FCanvasScissorRectItem ScissorInfo;

	FCanvasScissorRectRenderItem(FCanvasScissorRectItem& scissorInfo)
	{
		ScissorInfo = scissorInfo;
	}

	virtual bool Render_RenderThread(FRHICommandListImmediate& RHICmdList, const FCanvas* Canvas) override
	{
		RHICmdList.SetScissorRect(ScissorInfo.bEnable, ScissorInfo.MinX, ScissorInfo.MinY, ScissorInfo.MaxX, ScissorInfo.MaxY);
		return false;
	}

	virtual bool Render_GameThread(const FCanvas* Canvas) override
	{
		struct FCanvasScissorRectParameters
		{
			bool bEnable;
			int32 MinX;
			int32 MaxX;
			int32 MinY;
			int32 MaxY;
		};
		FCanvasScissorRectParameters ScissorParameters =
		{
			ScissorInfo.bEnable,
			ScissorInfo.MinX,
			ScissorInfo.MaxX,
			ScissorInfo.MinY,
			ScissorInfo.MaxY
		};

		ENQUEUE_UNIQUE_RENDER_COMMAND_ONEPARAMETER(
			CanvasScissorRectCommand,
			FCanvasScissorRectParameters, Parameters, ScissorParameters,
			{
				RHICmdList.SetScissorRect(Parameters.bEnable, Parameters.MinX, Parameters.MinY, Parameters.MaxX, Parameters.MaxY);
			});

		return false;
	}
};

void FCanvasScissorRectItem::Draw(FCanvas* InCanvas)
{
	Canvas_GetSortElement(*InCanvas, InCanvas->TopDepthSortKey()).RenderBatchArray.Add(new FCanvasScissorRectRenderItem(*this));
}

FCanvas::FCanvasSortElement& Canvas_GetSortElement(FCanvas& canvas, int32 DepthSortKey)
{
	// Optimization to resuse last index so that the more expensive Find() does not
	// need to be called as much.
	if (canvas.SortedElements.IsValidIndex(canvas.LastElementIndex))
	{
		FCanvas::FCanvasSortElement & LastElement = canvas.SortedElements[canvas.LastElementIndex];
		if (LastElement.DepthSortKey == DepthSortKey)
		{
			return LastElement;
		}
	}

	// find the FCanvasSortElement array entry based on the sortkey
	int32 ElementIdx = INDEX_NONE;
	int32* ElementIdxFromMap = canvas.SortedElementLookupMap.Find(DepthSortKey);
	if (ElementIdxFromMap)
	{
		ElementIdx = *ElementIdxFromMap;
		checkSlow(canvas.SortedElements.IsValidIndex(ElementIdx));
	}
	// if it doesn't exist then add a new entry (no duplicates allowed)
	else
	{
		new(canvas.SortedElements)FCanvas::FCanvasSortElement(DepthSortKey);
		ElementIdx = canvas.SortedElements.Num() - 1;
		// keep track of newly added array index for later lookup
		canvas.SortedElementLookupMap.Add(DepthSortKey, ElementIdx);
	}
	canvas.LastElementIndex = ElementIdx;
	return canvas.SortedElements[ElementIdx];
}

void ClipCanvas(UCanvas* canvas, bool enable, int32 minX, int32 minY, int32 maxX, int32 maxY)
{
	if (canvas)
	{
		ClipCanvas(canvas->Canvas, enable, minX, minY, maxX, maxY);
	}
}

void ClipCanvas(FCanvas* canvas, bool enable, int32 minX, int32 minY, int32 maxX, int32 maxY)
{
	if (canvas)
	{
		FCanvasScissorRectItem item(enable, minX, minY, maxX, maxY);
		canvas->DrawItem(item);
	}
}

void ClipCanvas(UCanvas* canvas, int32 minX, int32 minY, int32 maxX, int32 maxY)
{
	ClipCanvas(canvas, true, minX, minY, maxX, maxY);
}

void ClipCanvas(FCanvas* canvas, int32 minX, int32 minY, int32 maxX, int32 maxY)
{
	ClipCanvas(canvas, true, minX, minY, maxX, maxY);
}

void ClearCanvasClip(UCanvas* canvas)
{
	ClipCanvas(canvas, false, 0, 0, 0, 0);
}

void ClearCanvasClip(FCanvas* canvas)
{
	ClipCanvas(canvas, false, 0, 0, 0, 0);
}

// Blueprint related code below, remove if not required

AHUDEx::AHUDEx(const class FPostConstructInitializeProperties& PCIP) : Super(PCIP)
{
}

void AHUDEx::ClipHUD(int32 minX, int32 minY, int32 maxX, int32 maxY)
{
	ClipCanvas(Canvas, true, minX, minY, maxX, maxY);
}

void AHUDEx::ClearHUDClip()
{
	ClearCanvasClip(Canvas);
}

ClipCanvas.h

#pragma once

// Could call GetSortElement directly but it seems to cause a linker errors
FCanvas::FCanvasSortElement& Canvas_GetSortElement(FCanvas& canvas, int32 DepthSortKey);

void ClipCanvas(UCanvas* canvas, bool enable, int32 minX, int32 minY, int32 maxX, int32 maxY);
void ClipCanvas(FCanvas* canvas, bool enable, int32 minX, int32 minY, int32 maxX, int32 maxY);
void ClipCanvas(UCanvas* canvas, int32 minX, int32 minY, int32 maxX, int32 maxY);
void ClipCanvas(FCanvas* canvas, int32 minX, int32 minY, int32 maxX, int32 maxY);
void ClearCanvasClip(UCanvas* canvas);
void ClearCanvasClip(FCanvas* canvas);

HUDEx.h

#pragma once

#include "HUDEx.generated.h"

UCLASS()
class AHUDEx : public AHUD
{
	GENERATED_UCLASS_BODY()

	UFUNCTION(BlueprintCallable, Category = HUD, meta = (FriendlyName = "Clip"))
	void ClipHUD(int32 minX, int32 minY, int32 maxX, int32 maxY);

	UFUNCTION(BlueprintCallable, Category = HUD, meta = (FriendlyName = "Clear Clip"))
	void ClearHUDClip();
};

With Unreal 4.24 ENQUEUE_UNIQUE_RENDER_COMMAND_ONEPARAMETER will look like this:

	RenderScope.EnqueueRenderCommand([ScissorParameters](FRHICommandListImmediate& RHICmdList)
	{
		RHICmdList.SetScissorRect(ScissorParameters.bEnable, ScissorParameters.MinX, ScissorParameters.MinY, ScissorParameters.MaxX, ScissorParameters.MaxY);
	});