Bug in BDPT when using a point light in a Cornell Box

[honzukka]

I have discovered a strange behavior of BDPT when rendering a Cornell Box with a point light.

There is a single firefly on the back wall whose position depends of the light position. Its brightness is over 80000, even though the point light intensity is just 1. The firefly disappears when using a unidirectional path tracer.

Please, find below the scene file which reproduces the issue. I have downloaded the latest version of Mitsuba from this repository and compiled on Windows. I have tried both single precision and double precision flags but the behavior was unaffected by this.

<?xml version="1.0" encoding="utf-8"?>

<scene version="0.6.0">
	<integrator type="bdpt">
		<integer name="maxDepth" value="65"/>
		<boolean name="strictNormals" value="true"/>
		<boolean name="sampleDirect" value="true"/>
		<boolean name="lightImage" value="true"/>
	</integrator>

	<sensor type="perspective">
		<float name="fov" value="19.5"/>
		<transform name="toWorld">
			<matrix value="-1 0 0 0 0 1 0 1 0 0 -1 6.8 0 0 0 1"/>
		</transform>

		<sampler type="sobol">
			<integer name="sampleCount" value="1"/>
		</sampler>

		<film type="ldrfilm">
			<integer name="width" value="640"/>
			<integer name="height" value="640"/>
			<string name="fileFormat" value="png"/>
			<string name="pixelFormat" value="rgb"/>
			<float name="gamma" value="2.2"/>
			<boolean name="banner" value="false"/>

			<rfilter type="tent"/>
		</film>
	</sensor>

	<bsdf type="twosided" id="LeftWall">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0.63, 0.065, 0.05"/>
		</bsdf>
	</bsdf>

	<bsdf type="twosided" id="RightWall">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0.14, 0.45, 0.091"/>
		</bsdf>
	</bsdf>

	<bsdf type="twosided" id="Floor">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0.725, 0.71, 0.68"/>
		</bsdf>
	</bsdf>

	<bsdf type="twosided" id="Ceiling">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0.725, 0.71, 0.68"/>
		</bsdf>
	</bsdf>

	<bsdf type="twosided" id="BackWall">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0.725, 0.71, 0.68"/>
		</bsdf>
	</bsdf>

	<bsdf type="twosided" id="ShortBox">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0.725, 0.71, 0.68"/>
		</bsdf>
	</bsdf>

	<bsdf type="twosided" id="TallBox">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0.725, 0.71, 0.68"/>
		</bsdf>
	</bsdf>

	<bsdf type="twosided" id="Light">
		<bsdf type="diffuse">
			<rgb name="reflectance" value="0, 0, 0"/>
		</bsdf>
	</bsdf>

	<shape type="rectangle">
		<transform name="toWorld">
			<matrix value="-4.37114e-008 1 4.37114e-008 0 0 -8.74228e-008 2 0 1 4.37114e-008 1.91069e-015 0 0 0 0 1"/>
		</transform>

		<ref id="Floor"/>
	</shape>

	<shape type="rectangle">
		<transform name="toWorld">
			<matrix value="-1 7.64274e-015 -1.74846e-007 0 8.74228e-008 8.74228e-008 -2 2 0 -1 -4.37114e-008 0 0 0 0 1"/>
		</transform>

		<ref id="Ceiling"/>
	</shape>

	<shape type="rectangle">
		<transform name="toWorld">
			<matrix value="1.91069e-015 1 1.31134e-007 0 1 3.82137e-015 -8.74228e-008 1 -4.37114e-008 1.31134e-007 -2 -1 0 0 0 1"/>
		</transform>

		<ref id="BackWall"/>
	</shape>

	<shape type="rectangle">
		<transform name="toWorld">
			<matrix value="4.37114e-008 -1.74846e-007 2 1 1 3.82137e-015 -8.74228e-008 1 3.82137e-015 1 2.18557e-007 0 0 0 0 1"/>
		</transform>

		<ref id="RightWall"/>
	</shape>

	<shape type="rectangle">
		<transform name="toWorld">
			<matrix value="-4.37114e-008 8.74228e-008 -2 -1 1 3.82137e-015 -8.74228e-008 1 0 -1 -4.37114e-008 0 0 0 0 1"/>
		</transform>

		<ref id="LeftWall"/>
	</shape>

	<shape type="cube">
		<transform name="toWorld">
			<matrix value="0.0851643 0.289542 1.31134e-008 0.328631 3.72265e-009 1.26563e-008 -0.3 0.3 -0.284951 0.0865363 5.73206e-016 0.374592 0 0 0 1"/>
		</transform>

		<ref id="ShortBox"/>
	</shape>

	<shape type="cube">
		<transform name="toWorld">
			<matrix value="0.286776 0.098229 -2.29282e-015 -0.335439 -4.36233e-009 1.23382e-008 -0.6 0.6 -0.0997984 0.282266 2.62268e-008 -0.291415 0 0 0 1"/>
		</transform>

		<ref id="TallBox"/>
	</shape>

	<emitter type="point">
		<transform name="toWorld">
			<translate x="0.5" y="1.4" z="1.0" />
		</transform>
		<spectrum name="intensity" value="1" />
	</emitter>
</scene>