Interpolating orthograpfic and perspective projection matrices

Development Cinematics & Media
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1

I want to have a smooth transition between two different projection modes, orthographic and perspective.

After some researching I discovered that it is possible to feed different projection matrices overriding ULocalPlayer::GetProjectionData, so I made something like this:

bool UOrthoLocalPlayer::GetProjectionData(FViewport* Viewport, EStereoscopicPass StereoPass, FSceneViewProjectionData& ProjectionData) const
{
	bool ret = Super::GetProjectionData(Viewport, StereoPass, ProjectionData);

	FMinimalViewInfo ViewInfo;
	GetViewPoint(/*out*/ ViewInfo, StereoPass);

	FMatrix originalMatrix = ProjectionData.ProjectionMatrix;

	ViewInfo.ProjectionMode = ECameraProjectionMode::Orthographic;
	FMinimalViewInfo::CalculateProjectionMatrixGivenView(ViewInfo, AspectRatioAxisConstraint, ViewportClient->Viewport, /*inout*/ ProjectionData);

	for (int i = 0; i < 4; i++)
		for (int j = 0; j < 4; j++)
			ProjectionData.ProjectionMatrix.M[i][j] = ProjectionData.ProjectionMatrix.M[i][j] * OrthoQuantity + originalMatrix.M[i][j] * (1 - OrthoQuantity);

	return ret;
}

This should interpolate between the two projection matrices changing the OrthoQuantity parameter from 0 to 1. And for these very values it works perfectly, but when I change the value in something between them I only obtain a very strange result: I simply have the scene like if it was with the perspective matrix but moving, like the camera was changing the direction it seens, more and more as I approach the value 1. I suppose something happens behind the curtrain, but I can’t understand what.