Fix wrong formula for subsampling=2 while JPEG saving

PIL/JpegImagePlugin.py

@@ -635,7 +635,11 @@ def _save(im, fp, filename):
         subsampling = 0
     elif subsampling == "4:2:2":
         subsampling = 1
+    elif subsampling == "4:2:0":
+        subsampling = 2
     elif subsampling == "4:1:1":
+        # For compatibility. Before Pillow 4.3, 4:1:1 actually meant 4:2:0.
+        # Set 4:2:0 if someone is still using that value.
         subsampling = 2
     elif subsampling == "keep":
         if im.format != "JPEG":

PIL/JpegPresets.py

@@ -30,7 +30,7 @@ for chroma information than for luma information.
 (ref.: https://en.wikipedia.org/wiki/Chroma_subsampling)
 
 Possible subsampling values are 0, 1 and 2 that correspond to 4:4:4, 4:2:2 and
-4:1:1 (or 4:2:0?).
+4:2:0.
 
 You can get the subsampling of a JPEG with the
 `JpegImagePlugin.get_subsampling(im)` function.
@@ -67,7 +67,7 @@ Libjpeg ref.: https://web.archive.org/web/20120328125543/http://www.jpegcameras.
 """
 
 presets = {
-            'web_low':      {'subsampling':  2,  # "4:1:1"
+            'web_low':      {'subsampling':  2,  # "4:2:0"
                              'quantization': [
                                [20, 16, 25, 39, 50, 46, 62, 68,
                                 16, 18, 23, 38, 38, 53, 65, 68,
@@ -86,7 +86,7 @@ presets = {
                                 68, 68, 68, 68, 68, 68, 68, 68,
                                 68, 68, 68, 68, 68, 68, 68, 68]
                               ]},
-            'web_medium':   {'subsampling':  2,  # "4:1:1"
+            'web_medium':   {'subsampling':  2,  # "4:2:0"
                              'quantization': [
                                [16, 11, 11, 16, 23, 27, 31, 30,
                                 11, 12, 12, 15, 20, 23, 23, 30,
@@ -162,7 +162,7 @@ presets = {
                                  3,  3,  3,  3,  3,  3,  3,  3,
                                  3,  3,  3,  3,  3,  3,  3,  3]
                              ]},
-            'low':          {'subsampling':  2,  # "4:1:1"
+            'low':          {'subsampling':  2,  # "4:2:0"
                              'quantization': [
                                [18, 14, 14, 21, 30, 35, 34, 17,
                                 14, 16, 16, 19, 26, 23, 12, 12,
@@ -181,7 +181,7 @@ presets = {
                                 17, 12, 12, 12, 12, 12, 12, 12,
                                 17, 12, 12, 12, 12, 12, 12, 12]
                              ]},
-            'medium':       {'subsampling':  2,  # "4:1:1"
+            'medium':       {'subsampling':  2,  # "4:2:0"
                              'quantization': [
                                [12,  8,  8, 12, 17, 21, 24, 17,
                                  8,  9,  9, 11, 15, 19, 12, 12,

Tests/test_file_jpeg.py

@@ -299,7 +299,7 @@ class TestFileJpeg(PillowTestCase):
         self.assertEqual(getsampling(im), (1, 1, 1, 1, 1, 1))
         im = self.roundtrip(hopper(), subsampling=1)  # 4:2:2
         self.assertEqual(getsampling(im), (2, 1, 1, 1, 1, 1))
-        im = self.roundtrip(hopper(), subsampling=2)  # 4:1:1
+        im = self.roundtrip(hopper(), subsampling=2)  # 4:2:0
         self.assertEqual(getsampling(im), (2, 2, 1, 1, 1, 1))
         im = self.roundtrip(hopper(), subsampling=3)  # default (undefined)
         self.assertEqual(getsampling(im), (2, 2, 1, 1, 1, 1))
@@ -308,6 +308,8 @@ class TestFileJpeg(PillowTestCase):
         self.assertEqual(getsampling(im), (1, 1, 1, 1, 1, 1))
         im = self.roundtrip(hopper(), subsampling="4:2:2")
         self.assertEqual(getsampling(im), (2, 1, 1, 1, 1, 1))
+        im = self.roundtrip(hopper(), subsampling="4:2:0")
+        self.assertEqual(getsampling(im), (2, 2, 1, 1, 1, 1))
         im = self.roundtrip(hopper(), subsampling="4:1:1")
         self.assertEqual(getsampling(im), (2, 2, 1, 1, 1, 1))
 

docs/handbook/image-file-formats.rst

@@ -275,11 +275,11 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
     If present, sets the subsampling for the encoder.
 
     * ``keep``: Only valid for JPEG files, will retain the original image setting.
-    * ``4:4:4``, ``4:2:2``, ``4:1:1``: Specific sampling values
+    * ``4:4:4``, ``4:2:2``, ``4:2:0``: Specific sampling values
     * ``-1``: equivalent to ``keep``
     * ``0``: equivalent to ``4:4:4``
     * ``1``: equivalent to ``4:2:2``
-    * ``2``: equivalent to ``4:1:1``
+    * ``2``: equivalent to ``4:2:0``
 
 **qtables**
     If present, sets the qtables for the encoder. This is listed as an

libImaging/JpegEncode.c

@@ -194,7 +194,7 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8* buf, int bytes)
 		    context->cinfo.comp_info[2].v_samp_factor = 1;
 		    break;
 		}
-	    case 2:  /* 2x2, 1x1, 1x1 (4:1:1) : High */
+	    case 2:  /* 2x2, 1x1, 1x1 (4:2:0) : High */
 		{
 		    context->cinfo.comp_info[0].h_samp_factor = 2;
 		    context->cinfo.comp_info[0].v_samp_factor = 2;