Making all Bayes tests testing both dense and sparse

corelib/include/rtabmap/core/BayesFilter.h

@@ -148,6 +148,14 @@ public:
 	 */
 	float getVirtualPlacePrior() const;
 
+	/**
+	 * @brief Whether the prediction is being kept in its sparse form rather than as a matrix.
+	 *
+	 * False when @ref Parameters::kBayesSparsePrediction() is disabled, and while it is enabled
+	 * over a prediction measured as too dense for the sparse form to be worth it.
+	 */
+	bool isPredictionSparse() const;
+
 	/**
 	 * @brief Returns the loop-closure prediction model as a vector of values.
 	 * @return Vector in the format `{Vp, Lc, l1, l2, l3, ...}`.
@@ -167,11 +175,13 @@ public:
 	 * transition probabilities according to @ref getPredictionLC(). When @p ids match the
 	 * current posterior keys, the cached matrix may be returned without recomputation.
 	 *
+	 * When the prediction is being kept sparse, the matrix is expanded from it rather than
+	 * kept: it costs the memory that keeping the prediction sparse is saving, so ask for it to
+	 * read, dump or compare the prediction, not on every iteration.
+	 *
 	 * @param memory Working memory instance (must not be null).
 	 * @param ids Ordered list of signature ids (often includes @ref Memory::kIdVirtual as first element).
-	 * @return Square CV_32FC1 matrix of size ids.size() × ids.size(), or an empty matrix when
-	 *         @ref Parameters::kBayesSparsePrediction() is enabled and the prediction is being
-	 *         kept in its sparse form, in which case no matrix exists to return.
+	 * @return Square CV_32FC1 matrix of size ids.size() × ids.size().
 	 */
 	cv::Mat generatePrediction(const Memory * memory, const std::vector<int> & ids);
 

corelib/src/BayesFilter.cpp

@@ -114,6 +114,11 @@ float BayesFilter::getVirtualPlacePrior() const
 	return _model->virtualPlacePrior();
 }
 
+bool BayesFilter::isPredictionSparse() const
+{
+	return !_sparse->empty();
+}
+
 void BayesFilter::reset()
 {
 	_posteriorIds.clear();
@@ -294,10 +299,11 @@ bool BayesFilter::computePosterior(const Memory * memory, const std::map<int, fl
 
 cv::Mat BayesFilter::generatePrediction(const Memory * memory, const std::vector<int> & ids)
 {
-	if(!_sparse->empty())
+	if(!_sparse->empty() && _sparse->ids() == ids)
 	{
-		// The prediction is being kept sparse, so there is no matrix to return.
-		return cv::Mat();
+		// Expanded from the sparse form, which holds the same prediction. The matrix costs
+		// what keeping it sparse is saving, so it is built to be read and not kept.
+		return _sparse->toMatrix();
 	}
 	if(!_dense->empty() && _dense->ids() == ids)
 	{

corelib/src/Rtabmap.cpp

@@ -5820,15 +5820,6 @@ void Rtabmap::dumpPrediction() const
 			}
 		}
 		cv::Mat prediction = _bayesFilter->generatePrediction(_memory, uListToVector(signaturesToCompare));
-		if(prediction.empty())
-		{
-			UERROR("The prediction matrix cannot be exported while %s is enabled: the "
-				   "prediction is kept in a sparse form and the matrix is never built. Set "
-				   "%s=false to export it.",
-				   Parameters::kBayesSparsePrediction().c_str(),
-				   Parameters::kBayesSparsePrediction().c_str());
-			return;
-		}
 
 		FILE* fout = 0;
 		std::string fileName = this->getWorkingDir() + "/DumpPrediction.txt";

corelib/src/bayes/SparsePrediction.cpp

@@ -42,6 +42,25 @@ void SparsePrediction::clear()
 	used_ = 0;
 }
 
+cv::Mat SparsePrediction::toMatrix() const
+{
+	const int size = (int)columns_.size();
+	if(size == 0)
+	{
+		return cv::Mat();
+	}
+	cv::Mat matrix = cv::Mat::zeros(size, size, CV_32FC1);
+	for(int col=0; col<size; ++col)
+	{
+		const Column & slot = columns_[col];
+		for(size_t i=slot.offset; i<slot.offset+slot.size; ++i)
+		{
+			matrix.at<float>(values_[i].first, col) = values_[i].second;
+		}
+	}
+	return matrix;
+}
+
 unsigned long SparsePrediction::memoryUsed() const
 {
 	return values_.capacity() * sizeof(std::pair<int, float>)

corelib/src/bayes/SparsePrediction.h

@@ -30,6 +30,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "bayes/PredictionModel.h"
 
+#include <opencv2/core/core.hpp>
 #include <utility>
 #include <vector>
 
@@ -87,6 +88,14 @@ public:
 	/// prior = prediction x posterior.
 	void multiply(const std::vector<float> & posterior, std::vector<float> & prior) const;
 
+	/**
+	 * @brief The same prediction as a matrix, for the one caller that wants to look at it.
+	 *
+	 * The matrix costs what keeping the prediction sparse is saving, so this builds one to be
+	 * read, dumped or compared against DensePrediction, and does not keep it.
+	 */
+	cv::Mat toMatrix() const;
+
 	unsigned long memoryUsed() const;
 
 private:

corelib/test/test_bayesfilter.cpp

@@ -261,6 +261,8 @@ protected:
 		ParametersMap params;
 		params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay50Neighbor25_15));
 		params.insert(ParametersPair(Parameters::kBayesFullPredictionUpdate(), fullPredictionUpdate ? "true" : "false"));
+		// The matrix is what this hands back, so the prediction is not kept sparse.
+		params.insert(ParametersPair(Parameters::kBayesSparsePrediction(), "false"));
 		BayesFilter filter(params);
 
 		std::vector<int> ids = getBayesIds();
@@ -286,6 +288,40 @@ protected:
 
 // Constructor Tests
 
+// Bayes/SparsePrediction changes how the prediction is held and multiplied, not what the
+// posterior is, so every test below that reads the posterior runs in both forms. The ones
+// that read the prediction matrix instead keep the dense form, which is the only one that
+// has a matrix.
+class BayesFilterModeTest : public ::testing::TestWithParam<bool>
+{
+protected:
+	ParametersMap modeParams(ParametersMap params = ParametersMap()) const
+	{
+		params[Parameters::kBayesSparsePrediction()] = GetParam() ? "true" : "false";
+		return params;
+	}
+};
+
+class BayesFilterMemoryModeFixture :
+		public BayesFilterMemoryFixture,
+		public ::testing::WithParamInterface<bool>
+{
+protected:
+	ParametersMap modeParams(ParametersMap params = ParametersMap()) const
+	{
+		params[Parameters::kBayesSparsePrediction()] = GetParam() ? "true" : "false";
+		return params;
+	}
+};
+
+static std::string predictionFormName(const ::testing::TestParamInfo<bool> & info)
+{
+	return info.param ? "Sparse" : "Dense";
+}
+
+INSTANTIATE_TEST_SUITE_P(PredictionForm, BayesFilterModeTest, ::testing::Bool(), predictionFormName);
+INSTANTIATE_TEST_SUITE_P(PredictionForm, BayesFilterMemoryModeFixture, ::testing::Bool(), predictionFormName);
+
 TEST(BayesFilterTest, DefaultConstructor)
 {
 	BayesFilter filter;
@@ -376,10 +412,10 @@ TEST(BayesFilterTest, GetPredictionLCStrRoundTrip)
 
 // reset Tests
 
-TEST(BayesFilterTest, Reset)
+TEST_P(BayesFilterModeTest, Reset)
 {
 	Memory memory;
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay90));
 	BayesFilter filter(params);
 
@@ -394,9 +430,9 @@ TEST(BayesFilterTest, Reset)
 
 // getMemoryUsed Tests
 
-TEST(BayesFilterTest, GetMemoryUsed)
+TEST_P(BayesFilterModeTest, GetMemoryUsed)
 {
-	BayesFilter filter;
+	BayesFilter filter(modeParams());
 	EXPECT_GE(filter.getMemoryUsed(), sizeof(BayesFilter));
 
 	Memory memory;
@@ -430,9 +466,11 @@ TEST(BayesFilterTest, ComputePosteriorEmptyLikelihood)
 
 // generatePrediction Tests (virtual place only, no graph)
 
-TEST(BayesFilterTest, GeneratePredictionVirtualPlaceOnly)
+// Nothing has been computed yet, so there is no sparse form to keep and the matrix is built
+// in both cases.
+TEST_P(BayesFilterModeTest, GeneratePredictionVirtualPlaceOnly)
 {
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesVirtualPlacePriorThr(), "0.9"));
 	BayesFilter filter(params);
 
@@ -468,9 +506,9 @@ TEST(BayesFilterTest, GeneratePredictionCachedWhenIdsUnchanged)
 	EXPECT_EQ(prediction1.data, prediction2.data);
 }
 
-TEST(BayesFilterTest, ComputePosteriorVirtualPlaceOnly)
+TEST_P(BayesFilterModeTest, ComputePosteriorVirtualPlaceOnly)
 {
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay90));
 	BayesFilter filter(params);
 
@@ -486,9 +524,9 @@ TEST(BayesFilterTest, ComputePosteriorVirtualPlaceOnly)
 	EXPECT_TRUE(approxEqual(posterior.begin()->second, 1.0f));
 }
 
-TEST(BayesFilterTest, ComputePosteriorNormalizesPosterior)
+TEST_P(BayesFilterModeTest, ComputePosteriorNormalizesPosterior)
 {
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay90));
 	BayesFilter filter(params);
 
@@ -503,9 +541,9 @@ TEST(BayesFilterTest, ComputePosteriorNormalizesPosterior)
 	EXPECT_TRUE(approxEqual(posterior.begin()->second, 1.0f));
 }
 
-TEST(BayesFilterTest, ComputePosteriorUpdatesWithNewLikelihood)
+TEST_P(BayesFilterModeTest, ComputePosteriorUpdatesWithNewLikelihood)
 {
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay90));
 	BayesFilter filter(params);
 
@@ -524,6 +562,10 @@ TEST(BayesFilterTest, ComputePosteriorUpdatesWithNewLikelihood)
 
 // Integration tests with real Memory graph
 
+// The columns of the prediction, read from the matrix. A map this small is never kept sparse,
+// a column of it reaching more than a quarter of the locations, so this is the dense form
+// whatever Bayes/SparsePrediction says; GeneratePredictionExpandsTheSparseFormIntoTheSameMatrix
+// carries these values over to the sparse form on a map large enough to keep it.
 TEST_F(BayesFilterMemoryFixture, GeneratePredictionLinearChain)
 {
 	addChain(5);
@@ -534,6 +576,7 @@ TEST_F(BayesFilterMemoryFixture, GeneratePredictionLinearChain)
 	ParametersMap params;
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay50Neighbor25_15));
 	params.insert(ParametersPair(Parameters::kBayesVirtualPlacePriorThr(), "0.9"));
+	params.insert(ParametersPair(Parameters::kBayesSparsePrediction(), "false"));
 	BayesFilter filter(params);
 
 	const std::vector<int> ids = getBayesIds();
@@ -592,11 +635,14 @@ TEST_F(BayesFilterMemoryFixture, GeneratePredictionLinearChain)
 		(float)predictionLC[1]});
 }
 
-TEST_F(BayesFilterMemoryFixture, GeneratePredictionNormalizesWhenSumBelowOne)
+// A model whose values sum to less than 1 has normalize() spread the difference over every
+// zero of a column, so there is nothing sparse to keep and the matrix is the form used with
+// Bayes/SparsePrediction enabled as well as disabled. The columns below are its values.
+TEST_P(BayesFilterMemoryModeFixture, GeneratePredictionNormalizesWhenSumBelowOne)
 {
 	addChain(4);
 
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionSumBelowOne));
 	BayesFilter filter(params);
 
@@ -604,6 +650,9 @@ TEST_F(BayesFilterMemoryFixture, GeneratePredictionNormalizesWhenSumBelowOne)
 	ASSERT_EQ(ids.size(), 4u); // virtual + 3 visited WM nodes (latest in STM)
 	ASSERT_EQ(ids[0], Memory::kIdVirtual);
 
+	ASSERT_TRUE(filter.computePosterior(memory_, uniformLikelihood(ids)));
+	ASSERT_FALSE(filter.isPredictionSparse());
+
 	const std::vector<double> & predictionLC = filter.getPredictionLC();
 	ASSERT_EQ(predictionLC.size(), 2u);
 
@@ -647,29 +696,54 @@ TEST_F(BayesFilterMemoryFixture, GeneratePredictionNormalizesWhenSumBelowOne)
 		(float)predictionLC[1] * scaleRatio});
 }
 
-TEST_F(BayesFilterMemoryFixture, GeneratePredictionCachedWithGraph)
+// The matrix is kept between iterations and handed over as it is while the ids do not change.
+// The sparse form is expanded into a matrix per call instead: it is the caller asking to read
+// the prediction that pays for it, not every iteration.
+TEST_P(BayesFilterMemoryModeFixture, GeneratePredictionCachedWithGraph)
 {
-	addChain(4);
+	// Large enough for the sparse form to be worth keeping, so that both forms are exercised.
+	addChain(40);
 
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay70Neighbor20));
 	BayesFilter filter(params);
 
 	const std::vector<int> ids = getBayesIds();
 	std::map<int, float> likelihood = uniformLikelihood(ids);
-	filter.computePosterior(memory_, likelihood);
+	ASSERT_TRUE(filter.computePosterior(memory_, likelihood));
+	ASSERT_EQ(filter.isPredictionSparse(), GetParam());
 
 	cv::Mat prediction1 = filter.generatePrediction(memory_, ids);
 	cv::Mat prediction2 = filter.generatePrediction(memory_, ids);
-	ASSERT_FALSE(prediction1.empty());
-	EXPECT_EQ(prediction1.data, prediction2.data);
+	ASSERT_EQ(prediction1.rows, (int)ids.size());
+	ASSERT_EQ(prediction2.rows, (int)ids.size());
+	for(int col = 0; col < prediction1.cols; ++col)
+	{
+		for(int row = 0; row < prediction1.rows; ++row)
+		{
+			ASSERT_FLOAT_EQ(predictionAt(prediction1, row, col),
+			                predictionAt(prediction2, row, col))
+				<< "row=" << row << " col=" << col;
+		}
+	}
+	// The matrix is the one held, handed over as it is. The sparse form has none to hand over
+	// and is expanded into one per call: same values, another buffer. Handing over the same one
+	// would mean keeping the expansion, which is the allocation the sparse form is avoiding.
+	if(filter.isPredictionSparse())
+	{
+		EXPECT_NE(prediction1.data, prediction2.data);
+	}
+	else
+	{
+		EXPECT_EQ(prediction1.data, prediction2.data);
+	}
 }
 
-TEST_F(BayesFilterMemoryFixture, ComputePosteriorNormalizedWithGraph)
+TEST_P(BayesFilterMemoryModeFixture, ComputePosteriorNormalizedWithGraph)
 {
 	addChain(16);
 
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay50Neighbor25_15));
 	BayesFilter filter(params);
 
@@ -709,11 +783,11 @@ TEST_F(BayesFilterMemoryFixture, ComputePosteriorNormalizedWithGraph)
 	EXPECT_NEAR(sum, 1.0f, 1e-4f);
 }
 
-TEST_F(BayesFilterMemoryFixture, ComputePosteriorFavorsHighLikelihood)
+TEST_P(BayesFilterMemoryModeFixture, ComputePosteriorFavorsHighLikelihood)
 {
 	addChain(16);
 
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay70Neighbor20));
 	BayesFilter filter(params);
 
@@ -772,11 +846,11 @@ TEST_F(BayesFilterMemoryFixture, ComputePosteriorFavorsHighLikelihood)
 	}
 }
 
-TEST_F(BayesFilterMemoryFixture, ComputePosteriorSequentialIterations)
+TEST_P(BayesFilterMemoryModeFixture, ComputePosteriorSequentialIterations)
 {
 	initMemory(5);
 
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay50Neighbor25_15));
 	params.insert(ParametersPair(Parameters::kBayesFullPredictionUpdate(), "true"));
 	BayesFilter filter(params);
@@ -826,13 +900,13 @@ TEST_F(BayesFilterMemoryFixture, ComputePosteriorSequentialIterations)
 	}
 }
 
-TEST_F(BayesFilterMemoryFixture, ComputePosteriorSequentialIterationsWithLoopClosures)
+TEST_P(BayesFilterMemoryModeFixture, ComputePosteriorSequentialIterationsWithLoopClosures)
 {
 	// STM size 5: current signature stays in STM, so loop closures target older WM nodes
 	// (not the odometry neighbor link to the node added in the previous iteration).
 	initMemory(5);
 
-	ParametersMap params;
+	ParametersMap params = modeParams();
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay50Neighbor25_15));
 	params.insert(ParametersPair(Parameters::kBayesFullPredictionUpdate(), "true"));
 	BayesFilter filter(params);
@@ -915,6 +989,8 @@ TEST_F(BayesFilterMemoryFixture, CompareFullPredictionUpdateModes)
 	ParametersMap paramsIncremental;
 	paramsIncremental.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay50Neighbor25_15));
 	paramsIncremental.insert(ParametersPair(Parameters::kBayesFullPredictionUpdate(), "false"));
+	// Both modes are compared column by column, so the matrix has to be there to compare.
+	paramsIncremental.insert(ParametersPair(Parameters::kBayesSparsePrediction(), "false"));
 
 	ParametersMap paramsFull = paramsIncremental;
 	paramsFull[Parameters::kBayesFullPredictionUpdate()] = "true";
@@ -989,6 +1065,7 @@ TEST_F(BayesFilterMemoryFixture, FullPredictionUpdateRegeneratesMatrix)
 
 	ParametersMap params;
 	params.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay70Neighbor20));
+	params.insert(ParametersPair(Parameters::kBayesSparsePrediction(), "false"));
 
 	auto expectMatrixGrowsOnNewNode = [&](bool fullPredictionUpdate) {
 		params[Parameters::kBayesFullPredictionUpdate()] = fullPredictionUpdate ? "true" : "false";
@@ -1332,8 +1409,8 @@ TEST_F(BayesFilterMemoryFixture, PredictionCrossesFromSparseBackToTheMatrix)
 				<< "iter=" << iter << " id=" << ids[i];
 		}
 	}
-	// Grown into the sparse form: there is no matrix left to hand over.
-	ASSERT_TRUE(filterSparse.generatePrediction(memory_, ids).empty());
+	// Grown into the sparse form, so the matrix of the first iterations is gone.
+	ASSERT_TRUE(filterSparse.isPredictionSparse());
 
 	// Back to the matrix, which was last built 40 iterations and as many locations ago. It
 	// has to be built again rather than updated against locations it was never built for.
@@ -1351,6 +1428,7 @@ TEST_F(BayesFilterMemoryFixture, PredictionCrossesFromSparseBackToTheMatrix)
 	ASSERT_TRUE(filterDense.computePosterior(memory_, likelihood));
 	ASSERT_TRUE(filterSparse.computePosterior(memory_, likelihood));
 
+	ASSERT_FALSE(filterSparse.isPredictionSparse());
 	const cv::Mat prediction = filterSparse.generatePrediction(memory_, ids);
 	ASSERT_EQ(prediction.rows, (int)ids.size());
 	ASSERT_EQ(prediction.cols, (int)ids.size());
@@ -1383,7 +1461,7 @@ TEST_F(BayesFilterMemoryFixture, PredictionFallsBackToTheMatrixWhenTheGraphBecom
 		ids = getBayesIds();
 		ASSERT_TRUE(filter.computePosterior(memory_, uniformLikelihood(ids)));
 	}
-	ASSERT_TRUE(filter.generatePrediction(memory_, ids).empty());
+	ASSERT_TRUE(filter.isPredictionSparse());
 
 	const cv::Mat infMatrix = cv::Mat::eye(6, 6, CV_64FC1);
 	const size_t third = ids.size()/3;
@@ -1406,6 +1484,7 @@ TEST_F(BayesFilterMemoryFixture, PredictionFallsBackToTheMatrixWhenTheGraphBecom
 
 	// The matrix is built for the locations of this iteration, not updated against the ones it
 	// was built for before the sparse form took over.
+	ASSERT_FALSE(filter.isPredictionSparse());
 	const cv::Mat prediction = filter.generatePrediction(memory_, fewerIds);
 	ASSERT_EQ(prediction.rows, (int)fewerIds.size());
 	ASSERT_EQ(prediction.cols, (int)fewerIds.size());
@@ -1421,14 +1500,14 @@ TEST_F(BayesFilterMemoryFixture, PredictionFallsBackToTheMatrixWhenTheGraphBecom
 	EXPECT_NEAR(sum, 1.0f, 1e-4f);
 }
 
-// generatePrediction() hands over the prediction matrix, which only the dense mode has.
-// With the prediction kept sparse there is none, and it says so with an empty matrix rather
-// than building one: Rtabmap::dumpPrediction(), the one caller outside the filter, reports
-// that the matrix cannot be exported instead of spending the memory and the time that
-// keeping the prediction sparse is saving.
-TEST_F(BayesFilterMemoryFixture, GeneratePredictionIsEmptyWhileThePredictionIsKeptSparse)
+// generatePrediction() hands over the prediction whichever form it is held in, expanding the
+// sparse form into a matrix for the caller. Rtabmap::dumpPrediction() reads it that way, and
+// so does anyone comparing the two forms: they are built through the same column arithmetic,
+// so the matrices have to come out equal, value for value.
+TEST_F(BayesFilterMemoryFixture, GeneratePredictionExpandsTheSparseFormIntoTheSameMatrix)
 {
-	addChain(20);
+	// Large enough for the sparse form to be worth keeping, so that it is the one answering.
+	addChain(40);
 
 	ParametersMap paramsDense;
 	paramsDense.insert(ParametersPair(Parameters::kBayesPredictionLC(), kPredictionNewPlace10Stay50Neighbor25_15));
@@ -1442,12 +1521,29 @@ TEST_F(BayesFilterMemoryFixture, GeneratePredictionIsEmptyWhileThePredictionIsKe
 
 	const std::vector<int> ids = getBayesIds();
 	const std::map<int, float> likelihood = uniformLikelihood(ids);
-	filterDense.computePosterior(memory_, likelihood);
-	filterSparse.computePosterior(memory_, likelihood);
+	ASSERT_TRUE(filterDense.computePosterior(memory_, likelihood));
+	ASSERT_TRUE(filterSparse.computePosterior(memory_, likelihood));
+	ASSERT_FALSE(filterDense.isPredictionSparse());
+	ASSERT_TRUE(filterSparse.isPredictionSparse());
 
 	const cv::Mat predictionDense = filterDense.generatePrediction(memory_, ids);
+	const cv::Mat predictionSparse = filterSparse.generatePrediction(memory_, ids);
 	ASSERT_EQ(predictionDense.rows, (int)ids.size());
 	ASSERT_EQ(predictionDense.cols, (int)ids.size());
+	ASSERT_EQ(predictionSparse.rows, predictionDense.rows);
+	ASSERT_EQ(predictionSparse.cols, predictionDense.cols);
 
-	EXPECT_TRUE(filterSparse.generatePrediction(memory_, ids).empty());
+	for(int col = 0; col < predictionDense.cols; ++col)
+	{
+		for(int row = 0; row < predictionDense.rows; ++row)
+		{
+			EXPECT_FLOAT_EQ(predictionAt(predictionSparse, row, col),
+			                predictionAt(predictionDense, row, col))
+				<< "row=" << row << " col=" << col;
+		}
+	}
+
+	// The expansion is handed over, not kept: the sparse form is still the one held.
+	EXPECT_TRUE(filterSparse.isPredictionSparse());
+	EXPECT_LT(filterSparse.getMemoryUsed(), filterDense.getMemoryUsed());
 }