Merge pull request #23 from ramonaoptics/fix-3d-single-channel-monochrome

Fix writing 3D arrays with single channel dimension as monochrome

Author: hmaarrfk

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.5.1] - 2025-12-29
+
+- Fix writing 3D arrays with a single channel dimension (shape `(H, W, 1)`) as monochrome
+  images. The last dimension is now automatically collapsed when `num_components == 1`,
+  restoring compatibility with version 0.4.6 behavior.
+
 ## [0.5.0] - 2025-12-29
 
 - Optimize image reading and writing for multi-threaded workloads by releasing the

ojph/ojph_bindings.cpp

@@ -130,12 +130,17 @@ PYBIND11_MODULE(ojph_bindings, m) {
                  size_t component_stride;
 
                  if (num_components == 1) {
-                     if (buf.ndim != 2) {
-                         throw py::value_error("Image must be 2-dimensional for single component");
+                     if (buf.ndim == 2) {
+                         height = buf.shape[0];
+                         width = buf.shape[1];
+                         component_stride = 0;
+                     } else if (buf.ndim == 3 && buf.shape[2] == 1) {
+                         height = buf.shape[0];
+                         width = buf.shape[1];
+                         component_stride = 0;
+                     } else {
+                         throw py::value_error("Image must be 2-dimensional or 3-dimensional with last dimension of 1 for single component");
                      }
-                     height = buf.shape[0];
-                     width = buf.shape[1];
-                     component_stride = 0;
                  } else {
                      if (buf.ndim != 3) {
                          throw py::value_error("Image must be 3-dimensional for multiple components");
@@ -151,8 +156,17 @@ PYBIND11_MODULE(ojph_bindings, m) {
                      }
                  }
 
-                 size_t row_stride = (num_components == 1 || channel_order == "CHW") ? buf.strides[buf.ndim - 2] : buf.strides[0];
-                 size_t col_stride = (num_components == 1 || channel_order == "CHW") ? buf.strides[buf.ndim - 1] : buf.strides[1];
+                 size_t row_stride, col_stride;
+                 if (num_components == 1) {
+                     row_stride = buf.strides[0];
+                     col_stride = buf.strides[1];
+                 } else if (channel_order == "CHW") {
+                     row_stride = buf.strides[1];
+                     col_stride = buf.strides[2];
+                 } else {
+                     row_stride = buf.strides[0];
+                     col_stride = buf.strides[1];
+                 }
 
                  {
                      py::gil_scoped_release release;

tests/test_memory_compression.py

@@ -115,6 +115,20 @@ def test_imwrite_to_memory_consistency():
     )
 
 
+def test_imwrite_3d_single_channel_as_monochrome(tmp_path):
+    test_image = np.random.randint(0, 256, (100, 150, 1), dtype=np.uint8)
+
+    compressed_data = imwrite_to_memory(test_image)
+
+    filename = tmp_path / 'test.j2c'
+    with open(filename, 'wb') as f:
+        f.write(compressed_data.tobytes())
+
+    loaded_image = imread(filename)
+    expected_image = test_image.squeeze()
+    np.testing.assert_array_equal(loaded_image, expected_image)
+
+
 def test_lossless_vs_lossy_memory_error_increase(tmp_path):
     rng = np.random.default_rng(321)
     test_image = rng.integers(0, 256, (256, 256), dtype=np.uint8)