Fixed issue #857.

When saving a JPEG and specifying 'keep' for quality or subsampling,
if the source JPEG image is in grayscale mode, don't try to find the
subsampling of the source, because grayscale images don't have any
subsampling (it's only for color components).

For the moment the fix also ignores subsampling of CMYK JPEG because
currently Pillow doesn't support encoding JPEG in YCCK mode (and
subsampling doesn't make sense in CMYK, but Pillow permits saving CMYK
JPEG with subsampling, that's a bug). This fix pass those errors
silently, i.e. it doesn't raise an error when 'keep' is used but it's
not possible to keep the subsampling (because the image is grayscale
or CMYK). I think it's the proper behavior but I'm not sure.

PIL/JpegImagePlugin.py

@@ -545,6 +545,15 @@ def convert_dict_qtables(qtables):
 
 
 def get_sampling(im):
+    # There's no subsampling when image have only 1 layer
+    # (grayscale images) or when they are CMYK (4 layers),
+    # so set subsampling to default value.
+    #
+    # NOTE: currently Pillow can't encode JPEG to YCCK format.
+    # If YCCK support is added in the future, subsampling code will have
+    # to be updated (here and in JpegEncode.c) to deal with 4 layers.
+    if not hasattr(im, 'layers') or im.layers in (1, 4):
+        return -1
     sampling = im.layer[0][1:3] + im.layer[1][1:3] + im.layer[2][1:3]
     return samplings.get(sampling, -1)
 

Tests/test_file_jpeg.py

@@ -224,9 +224,18 @@ class TestFileJpeg(PillowTestCase):
         self.assertIsNone(im._getmp())
 
     def test_quality_keep(self):
+        # RGB
         im = Image.open("Tests/images/lena.jpg")
         f = self.tempfile('temp.jpg')
         im.save(f, quality='keep')
+        # Grayscale
+        im = Image.open("Tests/images/lena_gray.jpg")
+        f = self.tempfile('temp.jpg')
+        im.save(f, quality='keep')
+        # CMYK
+        im = Image.open("Tests/images/pil_sample_cmyk.jpg")
+        f = self.tempfile('temp.jpg')
+        im.save(f, quality='keep')
 
     def test_junk_jpeg_header(self):
         # https://github.com/python-pillow/Pillow/issues/630

encode.c

@@ -540,8 +540,8 @@ static unsigned int** get_qtables_arrays(PyObject* qtables) {
 
     tables = PySequence_Fast(qtables, "expected a sequence");
     num_tables = PySequence_Size(qtables);
-    if (num_tables < 2 || num_tables > NUM_QUANT_TBLS) {
-        PyErr_SetString(PyExc_ValueError, "Not a valid numbers of quantization tables. Should be between 2 and 4.");
+    if (num_tables < 1 || num_tables > NUM_QUANT_TBLS) {
+        PyErr_SetString(PyExc_ValueError, "Not a valid numbers of quantization tables. Should be between 1 and 4.");
         return NULL;
     }
     qarrays = (unsigned int**) PyMem_Malloc(num_tables * sizeof(unsigned int*));
@@ -760,7 +760,7 @@ PyImaging_LibTiffEncoderNew(PyObject* self, PyObject* args)
                                                         (ttag_t) PyInt_AsLong(key),
                                                         intav);
                         free(intav);
-                    }       
+                    }
                 } else {
                     TRACE((" %d elements, setting as floats \n", len));
                     floatav = malloc(sizeof(float)*len);
@@ -903,7 +903,7 @@ PyImaging_Jpeg2KEncoderNew(PyObject *self, PyObject *args)
     j2k_decode_coord_tuple(tile_offset,
                            &context->tile_offset_x,
                            &context->tile_offset_y);
-    j2k_decode_coord_tuple(tile_size, 
+    j2k_decode_coord_tuple(tile_size,
                            &context->tile_size_x,
                            &context->tile_size_y);
 
@@ -918,7 +918,7 @@ PyImaging_Jpeg2KEncoderNew(PyObject *self, PyObject *args)
 
         if (context->tile_offset_x > context->offset_x
             || context->tile_offset_y > context->offset_y) {
-            PyErr_SetString(PyExc_ValueError, 
+            PyErr_SetString(PyExc_ValueError,
                             "JPEG 2000 tile offset too large to cover image area");
             Py_DECREF(encoder);
             return NULL;