add 16-bits grayscale support for jpeg xl images

Tests/test_file_jxl.py

@@ -20,6 +20,7 @@
 
 # cjxl v0.9.2 41b8cdab
 # hopper.jxl: cjxl hopper.png hopper.jxl -q 75 -e 8
+# 16_bit_binary.jxl: cjxl 16_bit_binary.pgm 16_bit_binary.jxl -q 100 -e 9
 
 
 class TestUnsupportedJpegXl:
@@ -40,6 +41,7 @@ def test_unsupported(self) -> None:
 class TestFileJpegXl:
     def setup_method(self) -> None:
         self.rgb_mode = "RGB"
+        self.i16_mode = "I;16"
 
     def test_version(self) -> None:
         _jpegxl.JpegXlDecoderVersion()
@@ -62,6 +64,20 @@ def test_read_rgb(self) -> None:
             # djxl hopper.jxl hopper_jxl_bits.ppm
             assert_image_similar_tofile(image, "Tests/images/hopper_jxl_bits.ppm", 1.0)
 
+    def test_read_i16(self) -> None:
+        """
+        Can we read 16-bit Grayscale Jpeg XL image?
+        """
+
+        with Image.open("Tests/images/jxl/16bit_subcutaneous.cropped.jxl") as image:
+            assert image.mode == self.i16_mode
+            assert image.size == (128, 64)
+            assert image.format == "JPEG XL"
+            image.load()
+            image.getdata()
+
+            assert_image_similar_tofile(image, "Tests/images/jxl/16bit_subcutaneous.cropped.png", 1.0)
+
     def test_JpegXlDecode_with_invalid_args(self) -> None:
         """
         Calling decoder functions with no arguments should result in an error.

src/_jpegxl.c

@@ -33,6 +33,11 @@ void _pil_jxl_get_pixel_format(JxlPixelFormat *pf, const JxlBasicInfo *bi) {
 // TODO: floating point mode
 char* _pil_jxl_get_mode(const JxlBasicInfo *bi) {
 
+    // 16-bit single channel images are supported
+    if (bi->bits_per_sample == 16 && bi->num_color_channels == 1 &&
+        bi->alpha_bits == 0 && !bi->alpha_premultiplied
+    ) return "I;16";
+
     // PIL doesn't support high bit depth images
     // it will throw an exception but that's for your own good
     // you wouldn't want to see distorted image
@@ -261,7 +266,9 @@ _jxl_decoder_new(PyObject *self, PyObject *args) {
             _PIL_JXL_CHECK("JxlDecoderGetBasicInfo");
 
             _pil_jxl_get_pixel_format(&decp->pixel_format, &decp->basic_info);
-            if (decp->pixel_format.data_type != JXL_TYPE_UINT8) {
+            if (decp->pixel_format.data_type != JXL_TYPE_UINT8 &&
+                decp->pixel_format.data_type != JXL_TYPE_UINT16) {
+
                 // only 8 bit integer value images are supported for now
                 PyErr_SetString(PyExc_NotImplementedError,
                     "unsupported pixel data type");