Clipping a HUD/Canvas DrawText

anon93249095 · November 30, 2014, 4:20pm

Here is one way to achieve clipping of the canvas / HUD using RHI scissor rects.

gist.github.com

https://gist.github.com/pixeltris/40fde2a6b4d8b68caeee

ClipCanvas.cpp

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

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

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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);

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HUDEx.h

#pragma once

#include "HUDEx.generated.h"

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

	UFUNCTION(BlueprintCallable, Category = HUD, meta = (FriendlyName = "Clip"))

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I added blueprint functionality as well but you will need to re-parent your HUD to HUDEx as it requires access to the protected member Canvas.

I believe Slate can also clip non axis aligned rectangles. To do this with Canvas you would probably need to either use masks with materials or cut up the triangles and update the UV coords. This would require replacing all existing canvas items to work correctly so materials may be easier with better masking options if anyone is interested in going beyond singular axis aligned clipping rects.

This should answer these related questions