diff --git a/stim/envi/bil.h b/stim/envi/bil.h
index 7bc1eaf..08af5f2 100644
--- a/stim/envi/bil.h
+++ b/stim/envi/bil.h
@@ -828,16 +828,19 @@ public:
 	/// @param mask_band is the band used to specify the mask
 	/// @param threshold is the threshold used to determine if the mask value is true or false
 	/// @param p is a pointer to a pre-allocated array at least X * Y in size
-	bool build_mask(unsigned char* mask, double mask_band, double threshold, bool PROGRESS = false){
+	bool build_mask(unsigned char* out_mask, double mask_band, double lower, double upper, unsigned char* mask = NULL, bool PROGRESS = false){
 
 		T* temp = (T*)malloc(X() * Y() * sizeof(T));		//allocate memory for the certain band
 		band(temp, mask_band, PROGRESS);
 
 		for (unsigned long long i = 0; i < X() * Y(); i++) {
-			if (temp[i] < threshold)
-				mask[i] = 0;
-			else
-				mask[i] = 255;
+			if(mask == NULL || mask[i] != 0){
+				if(temp[i] > lower && temp[i] < upper){
+					out_mask[i] = 255;
+				}
+				else
+					out_mask[i] = 0;
+			}
 		}
 
 		free(temp);
diff --git a/stim/envi/bip.h b/stim/envi/bip.h
index 19bc1bd..31d706f 100644
--- a/stim/envi/bip.h
+++ b/stim/envi/bip.h
@@ -762,16 +762,19 @@ public:
 	/// @param mask_band is the band used to specify the mask
 	/// @param threshold is the threshold used to determine if the mask value is true or false
 	/// @param p is a pointer to a pre-allocated array at least X * Y in size
-	bool build_mask(unsigned char* mask, double mask_band, double threshold, bool PROGRESS = false){
+	bool build_mask(unsigned char* out_mask, double mask_band, double lower, double upper, unsigned char* mask = NULL, bool PROGRESS = false){
 
 		T* temp = (T*)malloc(X() * Y() * sizeof(T));		//allocate memory for the certain band
 		band(temp, mask_band, PROGRESS);
 
 		for (unsigned long long i = 0; i < X() * Y();i++) {
-			if (temp[i] < threshold)
-				mask[i] = 0;
-			else
-				mask[i] = 255;
+			if(mask == NULL || mask[i] != 0){
+				if(temp[i] > lower && temp[i] < upper){
+					out_mask[i] = 255;
+				}
+				else
+					out_mask[i] = 0;
+			}
 		}
 
 		free(temp);
diff --git a/stim/envi/bsq.h b/stim/envi/bsq.h
index 6d894b0..670c1dd 100644
--- a/stim/envi/bsq.h
+++ b/stim/envi/bsq.h
@@ -914,16 +914,20 @@ public:
 	/// @param mask_band is the band used to specify the mask
 	/// @param threshold is the threshold used to determine if the mask value is true or false
 	/// @param p is a pointer to a pre-allocated array at least X * Y in size
-	bool build_mask(unsigned char* mask, double mask_band, double threshold, bool PROGRESS = false){
+	bool build_mask(unsigned char* out_mask, double mask_band, double lower, double upper, unsigned char* mask = NULL, bool PROGRESS = false){
+		memset(out_mask, 0, X() * Y());							//initialize the mask to zero
 
 		T* temp = (T*)malloc(X() * Y() * sizeof(T));		//allocate memory for the certain band
 		band(temp, mask_band);
 
 		for (unsigned long long i = 0; i < X() * Y(); i++) {
-			if (temp[i] < threshold)
-				mask[i] = 0;
-			else
-				mask[i] = 255;
+			if(mask == NULL || mask[i] != 0){
+				if(temp[i] > lower && temp[i] < upper){
+					out_mask[i] = 255;
+				}
+				else
+					out_mask[i] = 0;
+			}
 
 			if(PROGRESS) progress = (double) (i+1) / (X() * Y()) * 100;
 		}
diff --git a/stim/envi/envi.h b/stim/envi/envi.h
index 5fd0cb1..6687b08 100644
--- a/stim/envi/envi.h
+++ b/stim/envi/envi.h
@@ -718,31 +718,31 @@ public:
 	/// @param mask_band is the label for the band that will be used to build the mask
 	/// @param threshold is a value selected such that all band values greater than threshold will have a mask value of 'true'
 	/// @param p is memory of size X*Y that will store the resulting mask
-	bool build_mask(unsigned char* mask, double mask_band, double threshold, bool PROGRESS = false)	{
+	bool build_mask(unsigned char* out_mask, double mask_band, double lower, double upper, unsigned char* mask = NULL, bool PROGRESS = false)	{
 
 		if(header.interleave == envi_header::BSQ){		//if the infile is bsq file
 			if(header.data_type ==envi_header::float32)
-				return ((bsq<float>*)file)->build_mask(mask, mask_band, threshold, PROGRESS);
+				return ((bsq<float>*)file)->build_mask(out_mask, mask_band, lower, upper, mask, PROGRESS);
 			else if(header.data_type == envi_header::float64)
-				return ((bsq<double>*)file)->build_mask(mask, mask_band, threshold, PROGRESS);
+				return ((bsq<double>*)file)->build_mask(out_mask, mask_band, lower, upper, mask, PROGRESS);
 			else
 				std::cout<<"ERROR: unidentified data type"<<std::endl;
 		}
 
 		else if(header.interleave == envi_header::BIL){		//if the infile is bil file
 			if(header.data_type ==envi_header::float32)
-				return ((bil<float>*)file)->build_mask(mask, mask_band, threshold, PROGRESS);
+				return ((bil<float>*)file)->build_mask(out_mask, mask_band, lower, upper, mask, PROGRESS);
 			else if(header.data_type == envi_header::float64)
-				return ((bil<double>*)file)->build_mask(mask, mask_band, threshold, PROGRESS);
+				return ((bil<double>*)file)->build_mask(out_mask, mask_band, lower, upper, mask, PROGRESS);
 			else
 				std::cout<<"ERROR: unidentified data type"<<std::endl;
 		}
 
 		else if(header.interleave == envi_header::BIP){		//if the infile is bip file
 			if(header.data_type ==envi_header::float32)
-				return ((bip<float>*)file)->build_mask(mask, mask_band, threshold, PROGRESS);
+				return ((bip<float>*)file)->build_mask(out_mask, mask_band, lower, upper, mask, PROGRESS);
 			else if(header.data_type == envi_header::float64)
-				return ((bip<double>*)file)->build_mask(mask, mask_band, threshold, PROGRESS);
+				return ((bip<double>*)file)->build_mask(out_mask, mask_band, lower, upper, mask, PROGRESS);
 			else
 				std::cout<<"ERROR: unidentified data type"<<std::endl;
 		}
@@ -751,30 +751,30 @@ public:
 	}
 
 	/// Creates a mask with a true value for all pixels that contain finite values
-	void mask_finite(unsigned char* mask, bool PROGRESS = false){
+	void mask_finite(unsigned char* out_mask, unsigned char* mask = NULL, bool PROGRESS = false){
 		if(header.interleave == envi_header::BSQ){		//if the infile is bsq file
 			if(header.data_type ==envi_header::float32)
-				((bsq<float>*)file)->mask_finite(mask, PROGRESS);
+				((bsq<float>*)file)->mask_finite(out_mask, mask, PROGRESS);
 			else if(header.data_type == envi_header::float64)
-				((bsq<double>*)file)->mask_finite(mask, PROGRESS);
+				((bsq<double>*)file)->mask_finite(out_mask, mask, PROGRESS);
 			else
 				std::cout<<"ERROR: unidentified data type"<<std::endl;
 		}
 
 		else if(header.interleave == envi_header::BIL){		//if the infile is bil file
 			if(header.data_type ==envi_header::float32)
-				((bil<float>*)file)->mask_finite(mask, PROGRESS);
+				((bil<float>*)file)->mask_finite(out_mask, mask, PROGRESS);
 			else if(header.data_type == envi_header::float64)
-				((bil<double>*)file)->mask_finite(mask, PROGRESS);
+				((bil<double>*)file)->mask_finite(out_mask, mask, PROGRESS);
 			else
 				std::cout<<"ERROR: unidentified data type"<<std::endl;
 		}
 
 		else if(header.interleave == envi_header::BIP){		//if the infile is bip file
 			if(header.data_type ==envi_header::float32)
-				((bip<float>*)file)->mask_finite(mask, PROGRESS);
+				((bip<float>*)file)->mask_finite(out_mask, mask, PROGRESS);
 			else if(header.data_type == envi_header::float64)
-				((bip<double>*)file)->mask_finite(mask, PROGRESS);
+				((bip<double>*)file)->mask_finite(out_mask, mask, PROGRESS);
 			else
 				std::cout<<"ERROR: unidentified data type"<<std::endl;
 		}
diff --git a/stim/envi/hsi.h b/stim/envi/hsi.h
index c9f3bea..3b60602 100644
--- a/stim/envi/hsi.h
+++ b/stim/envi/hsi.h
@@ -139,9 +139,12 @@ protected:
 
 public:
 			/// Get a mask that has all pixels with inf or NaN values masked out (false)
-	void mask_finite(unsigned char* mask, bool PROGRESS = false){
+	void mask_finite(unsigned char* out_mask, unsigned char* mask, bool PROGRESS = false){
 		size_t XY = X() * Y();
-		memset(mask, 255, XY * sizeof(unsigned char));				//initialize the mask to zero (0)
+		if(mask == NULL)												//if no mask is provided
+			memset(mask, 255, XY * sizeof(unsigned char));				//initialize the mask to 255
+		else															//if a mask is provided
+			memcpy(out_mask, mask, XY * sizeof(unsigned char));			//initialize the current mask to that one
 		T* page = (T*)malloc(R[0] * R[1] * sizeof(T));		//allocate space for a page of data
 
 		for(size_t p = 0; p < R[2]; p++){					//for each page
@@ -155,7 +158,7 @@ public:
 #endif
 					size_t x, y, b;
 					xyb(p * R[0] * R[1] + i, x, y, b);		//find the 3D coordinates of the value
-					mask[ y * X() + x ] = 0;				//remove the pixel (it's bad)
+					out_mask[ y * X() + x ] = 0;				//remove the pixel (it's bad)
 				}
 			}
 			if(PROGRESS) progress = (double)(p + 1) / (double)R[2] * 100;
--
libgit2 0.21.4