Added tags when saving YCbCr TIFF

Tests/test_file_libtiff.py

@@ -670,6 +670,15 @@ class TestFileLibTiff(LibTiffTestCase):
         TiffImagePlugin.WRITE_LIBTIFF = False
         TiffImagePlugin.READ_LIBTIFF = False
 
+    def test_save_ycbcr(self, tmp_path):
+        im = hopper("YCbCr")
+        outfile = str(tmp_path / "temp.tif")
+        im.save(outfile, compression="jpeg")
+
+        with Image.open(outfile) as reloaded:
+            assert reloaded.tag_v2[530] == (1, 1)
+            assert reloaded.tag_v2[532] == (0, 255, 128, 255, 128, 255)
+
     def test_crashing_metadata(self, tmp_path):
         # issue 1597
         with Image.open("Tests/images/rdf.tif") as im:

src/PIL/TiffImagePlugin.py

@@ -93,6 +93,7 @@ SUBIFD = 330
 EXTRASAMPLES = 338
 SAMPLEFORMAT = 339
 JPEGTABLES = 347
+YCBCRSUBSAMPLING = 530
 REFERENCEBLACKWHITE = 532
 COPYRIGHT = 33432
 IPTC_NAA_CHUNK = 33723  # newsphoto properties
@@ -1593,6 +1594,13 @@ def _save(im, fp, filename):
     # no compression by default:
     ifd[COMPRESSION] = COMPRESSION_INFO_REV.get(compression, 1)
 
+    if im.mode == "YCbCr":
+        for tag, value in {
+            YCBCRSUBSAMPLING: (1, 1),
+            REFERENCEBLACKWHITE: (0, 255, 128, 255, 128, 255),
+        }.items():
+            ifd.setdefault(tag, value)
+
     if libtiff:
         if "quality" in im.encoderinfo:
             quality = im.encoderinfo["quality"]

src/encode.c

@@ -808,6 +808,12 @@ PyImaging_LibTiffEncoderNew(PyObject *self, PyObject *args) {
                         av + stride * 2);
                     free(av);
                 }
+            } else if (key_int == TIFFTAG_YCBCRSUBSAMPLING) {
+                status = ImagingLibTiffSetField(
+                    &encoder->state,
+                    (ttag_t)key_int,
+                    (UINT16)PyLong_AsLong(PyTuple_GetItem(value, 0)),
+                    (UINT16)PyLong_AsLong(PyTuple_GetItem(value, 1)));
             } else if (type == TIFF_SHORT) {
                 UINT16 *av;
                 /* malloc check ok, calloc checks for overflow */