// Copyright Epic Games, Inc. All Rights Reserved.

#include "BaseGizmos/GizmoRenderingUtil.h"
#include "BaseGizmos/GizmoViewContext.h"
#include "RHI.h"
#include "Math/UnrealMathUtility.h"
#include "Math/NumericLimits.h"


// yuck global value set by Editor
static const FSceneView* GlobalCurrentSceneView = nullptr;

static FCriticalSection GlobalCurrentSceneViewLock;

#if WITH_EDITOR
static bool bGlobalUseCurrentSceneViewTracking = true;
#else
static bool bGlobalUseCurrentSceneViewTracking = false;
#endif

namespace GizmoRenderingUtilLocals
{
	static double VectorDifferenceSqr(const FVector2D& A, const FVector2D& B)
	{
		double ax = A.X, ay = A.Y;
		double bx = B.X, by = B.Y;
		ax -= bx;
		ay -= by;
		return ax * ax + ay * ay;
	}

	static double VectorDifferenceSqr(const FVector& A, const FVector& B)
	{
		double ax = A.X, ay = A.Y, az = A.Z;
		double bx = B.X, by = B.Y, bz = B.Z;
		ax -= bx;
		ay -= by;
		az -= bz;
		return ax * ax + ay * ay + az * az;
	}

	// duplicates FSceneView::WorldToPixel but in double where possible (unfortunately WorldToScreen still in float)
	template <typename SceneViewOrGizmoViewContext>
	static FVector2D WorldToPixelDouble(const SceneViewOrGizmoViewContext* View, const FVector& Location)
	{
		FVector4 ScreenPoint = View->WorldToScreen(Location);

		double InvW = (ScreenPoint.W > 0.0 ? 1.0 : -1.0) / (double)ScreenPoint.W;
		double Y = (GProjectionSignY > 0.0) ? (double)ScreenPoint.Y : 1.0 - (double)ScreenPoint.Y;

		const FIntRect& UnscaledViewRect = View->UnscaledViewRect;
		double PosX = (double)UnscaledViewRect.Min.X + (0.5 + (double)ScreenPoint.X * 0.5 * InvW) * (double)UnscaledViewRect.Width();
		double PosY = (double)UnscaledViewRect.Min.Y + (0.5 - Y * 0.5 * InvW) * (double)UnscaledViewRect.Height();

		return FVector2D((float)PosX, (float)PosY);
	}

	// Shared code between the GizmoViewContext and FSceneView versions of CalculateLocalPixelToWorldScale. We don't expose
	// this template itself to the user since it's not entirely intuitive that this works on two very different types.
	template <typename SceneViewOrGizmoViewContext>
	float CalculateLocalPixelToWorldScale(
		const SceneViewOrGizmoViewContext* View,
		const FVector& Location)
	{
		// To calculate this scale at Location, we project Location to screen and also project a second
		// point at a small distance from Location in a camera-perpendicular plane, then measure 2D/3D distance ratio. 
		// However, because some of the computations are done in float, there will be enormous numerical error 
		// when the camera is very far from the location if the distance is relatively small. The "W" value
		// below gives us a sense of this distance, so we make the offset relative to that
		// (this does do one redundant WorldToScreen)
		FVector4 LocationScreenPoint = View->WorldToScreen(Location);
		float OffsetDelta = LocationScreenPoint.W * 0.01f;

		FVector2D PixelA = WorldToPixelDouble(View, Location);
		FVector OffsetPointWorld = Location + OffsetDelta * View->GetViewRight() + OffsetDelta * View->GetViewUp();
		FVector2D PixelB = WorldToPixelDouble(View, OffsetPointWorld);

		double PixelDeltaSqr = VectorDifferenceSqr(PixelA, PixelB);
		double WorldDeltaSqr = VectorDifferenceSqr(Location, OffsetPointWorld);
		return (float)(sqrt(WorldDeltaSqr / PixelDeltaSqr));
	}
}

float GizmoRenderingUtil::CalculateLocalPixelToWorldScale(
	const FSceneView* View,
	const FVector& Location)
{
	return GizmoRenderingUtilLocals::CalculateLocalPixelToWorldScale(View, Location);
}

float GizmoRenderingUtil::CalculateLocalPixelToWorldScale(
	const UGizmoViewContext* ViewContext,
	const FVector& Location)
{
	return GizmoRenderingUtilLocals::CalculateLocalPixelToWorldScale(ViewContext, Location);
}

float GizmoRenderingUtil::CalculateViewDependentScaleAndFlatten(
	const FSceneView* View,
	const FVector& Location,
	const float InScale,
	FVector& OutFlattenScale)
{
	const FMatrix& ViewMatrix = View->ViewMatrices.GetViewMatrix();
	bool bIsPerspective = View->ViewMatrices.GetProjectionMatrix().M[3][3] < 1.0f;
	bool bIsOrthoXY = !bIsPerspective && FMath::Abs(ViewMatrix.M[2][2]) > 0.0f;
	bool bIsOrthoXZ = !bIsPerspective && FMath::Abs(ViewMatrix.M[1][2]) > 0.0f;
	bool bIsOrthoYZ = !bIsPerspective && FMath::Abs(ViewMatrix.M[0][2]) > 0.0f;
	float UniformScale = InScale * View->WorldToScreen(Location).W * (4.0f / View->UnscaledViewRect.Width() / View->ViewMatrices.GetProjectionMatrix().M[0][0]);

	// Clamp to tolerance to prevent division by zero.
	// @todo change to use MathUtil<RealType>::ZeroTolerance and TMathUtil<RealType>::SignNonZero(Value)
	float MinimumScale = TNumericLimits<float>::Lowest();
	if (FMath::Abs(UniformScale) < MinimumScale)
	{
		UniformScale = MinimumScale * (UniformScale < 0.0f ? -1.0f : 1.0f);
	}

	if (bIsPerspective)
	{
		OutFlattenScale = FVector(1.0f, 1.0f, 1.0f);
	}
	else
	{
		// Flatten scale prevents scaling in the direction of the camera and thus intersecting the near plane.
		// Based on legacy FWidget render code but is flatten actually necessary?? that axis wasn't scaled anyways!
		if (bIsOrthoXY)
		{
			OutFlattenScale = FVector(1.0f, 1.0f, 1.0f / UniformScale);
		}
		else if (bIsOrthoXZ)
		{
			OutFlattenScale = FVector(1.0f, 1.0f / UniformScale, 1.0f);
		}
		else if (bIsOrthoYZ)
		{
			OutFlattenScale = FVector(1.0f / UniformScale, 1.0f, 1.0f);
		}
	}
	return UniformScale;
}