Modifying pixels in RenderTarget to draw bounding box

[anonymous_user_c3e69a5a]

Hi,

I have a SceneCapture2D pointing to a staticmesh (the small metallic thing on the table)

I also have an algorithm to calculate a 2D Bounding Box around the mesh in the texture. So now I want to see whether the algorithm works, and therefore draw this bounding box onto the Rendertarget / texture.

This is my code for doing that, it is called every tick:

static void makeRed(TArray<FLinearColor> & ImageData, int i) {
	ImageData[i].R = 1;
	ImageData[i].G = 0;
	ImageData[i].B = 0;
};

void UTryPixelAccess::drawBoundingBox(UTextureRenderTarget2D * RenderTexture, FBox2D Bounds)
{
	TArray<FLinearColor> ImageData;
	FRenderTarget *RenderTarget = RenderTexture->GameThread_GetRenderTargetResource();
	RenderTarget->ReadLinearColorPixels(ImageData);

	//todo: lock data?

	int x1 = Bounds.Min.X;
	int y1 = Bounds.Min.Y;
	int x2 = Bounds.Max.X;
	int y2 = Bounds.Max.Y;

	// lambda for array access
	auto toIndex = [RenderTexture](int x, int y) {
		return x + y * RenderTexture->SizeX;
	};
	
	// horizontal lines
	for (x1 = Bounds.Min.X; x1 <= Bounds.Max.X; x1++) {
		makeRed(ImageData, toIndex(x1, y1));
		makeRed(ImageData, toIndex(x1, y2));
	}

	// vertical lines
	x1 = Bounds.Min.X;
	for (y1 = Bounds.Min.Y; y1 <= Bounds.Max.X; y1++) {
		makeRed(ImageData, toIndex(x1, y1));
		makeRed(ImageData, toIndex(x2, y1));
	}
}

The problem is: it does not work! With debugging the code I can see that the pixel values are modified as wished. But the plane showing that RenderTarget Texture still does not have a red bounding box drawn onto it…

So for me there are possibilities what could be the problem:

• I´m not accessing the real data, but only a copy of it (which is wondering, because in [this post][2] they make a copy themself after reading the pixels - so it should be the real data?)

• The pixels are overritten by the engine before they are rendered, so I need some kind of write protection

• maybe I completly misunderstood the concept of a RenderTarget (I´m very new to Unreal)

Or is there another, simpler way of drawing the box onto the material/texture/renderTarget itself? Maybe with blueprint scripting?

I´m thankful for any advice!

[anonymous_user_c3e69a5a]

Found out that you can use Blueprint functions for drawing on render targets. But the do not work if the SceneCaputer is set to capture every frame.

[anonymous_user_c3e69a5a]

For the sake of completeness: I found a way to do this even when “capture every frame” is active. I derived an own class from USceneCaptureComponent2D and extended its UpdateSceneReferred():

void UMySceneCaptureComponent2D::UpdateSceneCaptureContents(FSceneInterface * Scene)
{
	Super::UpdateSceneCaptureContents(Scene);
	if (TargetActors.Num() > 0) {
		DrawTargetActorBounds();
	}
}

void UMySceneCaptureComponent2D::DrawTargetActorBounds()
{
	UObject* WorldContext = this;
	UCanvas* Canvas;
	FVector2D Size;
	FDrawToRenderTargetContext Context;
	UKismetRenderingLibrary::BeginDrawCanvasToRenderTarget(WorldContext, this->TextureTarget, Canvas, Size, Context);
	if (Canvas) {

		for (size_t i = 0; i < TargetActors.Num(); i++)
		{
			FLinearColor color = (TargetColors.Num() > i ? TargetColors[i] : DefaultColor);
			FVector Origin, Extend;
			TArray<FVector> Points;
			TArray<FVector2D> Pixels;
			FBox2D Bounds;
			(TargetActors[i])->GetActorBounds(false, Origin, Extend);
			if (UMyPixelUtility::calcBoundingFromViewInfo(this, Origin, Extend, Bounds, Points, Pixels)) {
				// not possible in UE 4.19:
				//FVector2D BoundExtend = (Bounds.Max - Bounds.Min) / 2.0;
				//Canvas->K2_DrawBox((Bounds.Min + BoundExtend), BoundExtend, LineThickness, color);
				FVector2D TopRight(Bounds.Max.X, Bounds.Min.Y);
				FVector2D BottomLeft(Bounds.Min.X, Bounds.Max.Y);
				Canvas->K2_DrawLine(Bounds.Min, TopRight, LineThickness, color);
				Canvas->K2_DrawLine(Bounds.Min, BottomLeft, LineThickness, color);
				Canvas->K2_DrawLine(Bounds.Max, TopRight, LineThickness, color);
				Canvas->K2_DrawLine(Bounds.Max, BottomLeft, LineThickness, color);
			}
		}
	}
	else {
		UE_LOG(LogTemp, Warning, TEXT("No Canvas could be created from TextureTarget"));
	}
	UKismetRenderingLibrary::EndDrawCanvasToRenderTarget(WorldContext, Context);
}

If you want to know whats in the calcBoundsFromViewInfo method, you can take a look in this post