diff --git a/include/aare/hist/PixelHistogram.hpp b/include/aare/hist/PixelHistogram.hpp
index f4e0812..c5f7b79 100644
--- a/include/aare/hist/PixelHistogram.hpp
+++ b/include/aare/hist/PixelHistogram.hpp
@@ -49,6 +49,8 @@ class PixelHistogram {
 
     // Async producer/consumer pipeline. SPSC queue feeds the coordinator
     // thread, which fans each image out to the worker pool one at a time.
+    // TODO: batch processing?
+    // TODO: FIFO to avoid allocations?
     std::unique_ptr<AsyncQueue> async_queue_;
     std::thread coordinator_;
     std::atomic<bool> stop_coordinator_{false};
@@ -74,7 +76,7 @@ class PixelHistogram {
     // coordinator thread, and returns. Blocks the caller only if the queue
     // is full (single-producer, single-consumer queue with a sleep-poll
     // backpressure loop, matching the convention in ClusterFinderMT).
-    void fill_async(NDArray<AxisType, 2> image);
+    void fill_async(NDArray<AxisType, 2> &&image);
 
     // Wait for all queued async fills to complete. Cheap when the queue
     // is already drained.
@@ -289,7 +291,7 @@ void PixelHistogram<StorageType, AxisType>::dispatch(
 
 template <typename StorageType, typename AxisType>
 void PixelHistogram<StorageType, AxisType>::fill_async(
-    NDArray<AxisType, 2> image) {
+    NDArray<AxisType, 2> &&image) {
     if (image.shape(0) != rows_ || image.shape(1) != cols_) {
         throw std::invalid_argument(
             "PixelHistogram image shape does not match constructor shape");
diff --git a/python/src/bind_PixelHistogram.hpp b/python/src/bind_PixelHistogram.hpp
index 0e61f52..d205bce 100644
--- a/python/src/bind_PixelHistogram.hpp
+++ b/python/src/bind_PixelHistogram.hpp
@@ -40,7 +40,7 @@ void define_pixel_histogram_binding(py::module &m, const char *class_name,
                      asynchronous filling before fill_async() applies
                      backpressure on the caller (default: 16)
              )",
-             py::arg("rows"), py::arg("cols"), py::arg("n_bins"),
+             py::kw_only(), py::arg("rows"), py::arg("cols"), py::arg("n_bins"),
              py::arg("xmin"), py::arg("xmax"), py::arg("n_threads") = 1,
              py::arg("max_pending") = std::size_t{16})