diff --git a/src/xrt/drivers/ht/ht_image_math.cpp b/src/xrt/drivers/ht/ht_image_math.cpp
index 9825e297e..b00c01a18 100644
--- a/src/xrt/drivers/ht/ht_image_math.cpp
+++ b/src/xrt/drivers/ht/ht_image_math.cpp
@@ -224,7 +224,7 @@ rotatedRectFromJoints(struct ht_view *htv, xrt_vec2 center, xrt_vec2 wrist_to_mi
 }
 
 void
-planarize(cv::Mat &input, uint8_t *output)
+planarize(const cv::Mat &input, uint8_t *output)
 {
 	// output better be the right size, because we are not doing any bounds checking!
 	assert(input.isContinuous());
diff --git a/src/xrt/drivers/ht/ht_image_math.hpp b/src/xrt/drivers/ht/ht_image_math.hpp
index a2d3ce13f..2b3e333c5 100644
--- a/src/xrt/drivers/ht/ht_image_math.hpp
+++ b/src/xrt/drivers/ht/ht_image_math.hpp
@@ -61,4 +61,4 @@ struct DetectionModelOutput
 rotatedRectFromJoints(struct ht_view *htv, xrt_vec2 center, xrt_vec2 wrist_to_middle, DetectionModelOutput *out);
 
 void
-planarize(cv::Mat &input, uint8_t *output);
+planarize(const cv::Mat &input, uint8_t *output);