Rename parameter and add more tests

Tests/test_file_jpeg2k.py

@@ -209,41 +209,82 @@ def test_layers():
         assert_image_similar(im, test_card, 0.4)
 
 
-def test_use_jp2():
+def test_no_jp2():
-    out = BytesIO()
-    test_card.save(out, "JPEG2000", use_jp2=False)
-    out.seek(0)
-    assert out.read(2) == b"\xff\x4f"
-
     out = BytesIO()
     out.name = "foo.j2k"
-    test_card.save(out, "JPEG2000", use_jp2=False)
+    test_card.save(out, "JPEG2000")
     out.seek(0)
     assert out.read(2) == b"\xff\x4f"
 
     out = BytesIO()
     out.name = "foo.jp2"
-    test_card.save(out, "JPEG2000", use_jp2=False)
+    test_card.save(out, "JPEG2000")
     out.seek(4)
-    assert out.read(2) != b"jP"
+    assert out.read(2) == b"jP"
+
+    out = BytesIO()
+    test_card.save(out, "JPEG2000", no_jp2=True)
+    out.seek(0)
+    assert out.read(2) == b"\xff\x4f"
+
+    out = BytesIO()
+    test_card.save(out, "JPEG2000", no_jp2=True)
+    out.seek(0)
+    assert out.read(2) == b"\xff\x4f"
+
+    out = BytesIO()
+    out.name = "foo.j2k"
+    test_card.save(out, "JPEG2000", no_jp2=True)
+    out.seek(0)
+    assert out.read(2) == b"\xff\x4f"
+
+    out = BytesIO()
+    out.name = "foo.jp2"
+    test_card.save(out, "JPEG2000", no_jp2=True)
+    out.seek(0)
+    assert out.read(2) == b"\xff\x4f"
+
+    # Use the filename extension to determine format
+    out = BytesIO()
+    out.name = "foo.j2k"
+    test_card.save(out, "JPEG2000", no_jp2=False)
+    out.seek(0)
+    assert out.read(2) == b"\xff\x4f"
+
+    out = BytesIO()
+    out.name = "foo.jp2"
+    test_card.save(out, "JPEG2000", no_jp2=False)
+    out.seek(4)
+    assert out.read(2) == b"jP"
+
+    # Default to JP2 if no filename
+    out = BytesIO()
+    test_card.save(out, "JPEG2000", no_jp2=False)
+    out.seek(4)
+    assert out.read(2) == b"jP"
+
+    out = BytesIO()
+    test_card.save(out, "JPEG2000", no_jp2=False)
+    out.seek(4)
+    assert out.read(2) == b"jP"
 
 
 def test_mct():
     # Three component
     for val in (0, 1):
         out = BytesIO()
-        test_card.save(out, "JPEG2000", mct=val, use_jp2=False)
+        test_card.save(out, "JPEG2000", mct=val, no_jp2=True)
         out.seek(0)
         with Image.open(out) as im:
             im.load()
             assert_image_similar(im, test_card, 1.0e-3)
             assert out.getvalue()[59] == val
 
-    # Single component
+    # Single component should have MCT disabled
     for val in (0, 1):
         out = BytesIO()
         with Image.open("Tests/images/16bit.cropped.jp2") as jp2:
-            jp2.save(out, "JPEG2000", mct=val, use_jp2=False)
+            jp2.save(out, "JPEG2000", mct=val, no_jp2=True)
 
         out.seek(0)
         with Image.open(out) as im:

docs/handbook/image-file-formats.rst

@@ -506,10 +506,10 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
     Defaults to ``False``, which uses the lossless DWT 5-3.
 
 **mct**
-    If ``0`` then don't use multiple component transformation when encoding,
+    If ``1`` then apply multiple component transformation when encoding,
-    otherwise use ``1`` to apply to components 0, 1 and 2. MCT works best
+    otherwise use ``0`` for no component transformation (default). MCT works
-    with a ``mode`` of ``RGB`` and is only available for 3 component image
+    best with a ``mode`` of ``RGB`` and is only applied when the image data has
-    data.
+    3 components.
 
 **progression**
     Controls the progression order; must be one of ``"LRCP"``, ``"RLCP"``,
@@ -529,9 +529,10 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
     for compliant 4K files, *at least one* of the dimensions must match
     4096 x 2160.
 
-**use_jp2**
+**no_jp2**
-    If ``False`` then don't wrap the codestream in the JP2 file format when
+    If ``True`` then don't wrap the raw codestream in the JP2 file format when
-    saving. Defaults to ``True``.
+    saving, otherwise the extension of the filename will be used to determine
+    the format (default).
 
 .. note::
 

src/PIL/Jpeg2KImagePlugin.py

@@ -293,10 +293,10 @@ def _save(im, fp, filename):
     # Get the keyword arguments
     info = im.encoderinfo
 
-    if filename.endswith(".j2k"):
+    if filename.endswith(".j2k") or info.get("no_jp2", False):
         kind = "j2k"
     else:
-        kind = "jp2" if info.get("use_jp2", True) else "j2k"
+        kind = "jp2"
 
     offset = info.get("offset", None)
     tile_offset = info.get("tile_offset", None)