Warn when webp is asked to decode into grayscale

Author: scotts

test/test_image.py

@@ -934,6 +934,20 @@ def test_decode_webp(decode_fun, scripted):
     img += 123  # make sure image buffer wasn't freed by underlying decoding lib
 
 
+@pytest.mark.parametrize("decode_fun", (decode_webp, decode_image))
+def test_decode_webp_grayscale(decode_fun):
+    encoded_bytes = read_file(next(get_images(FAKEDATA_DIR, ".webp")))
+
+    # Note that we warn at the C++ layer because dispatching for decode_image
+    # doesn't happen until we hit C++. The C++ layer does not propagate
+    # warnings up to Python, so we can't test for them.
+    img = decode_fun(encoded_bytes, mode=ImageReadMode.GRAY)
+
+    # Note that because we do not support grayscale conversions, we expect
+    # that the number of color channels is still 3.
+    assert img.shape == (3, 100, 100)
+
+
 # This test is skipped by default because it requires webp images that we're not
 # including within the repo. The test images were downloaded manually from the
 # different pages of https://developers.google.com/speed/webp/gallery

torchvision/csrc/io/image/cpu/decode_webp.cpp

@@ -32,6 +32,11 @@ torch::Tensor decode_webp(
       res == VP8_STATUS_OK, "WebPGetFeatures failed with error code ", res);
   TORCH_CHECK(
       !features.has_animation, "Animated webp files are not supported.");
+  TORCH_WARN_ONCE(
+      mode == IMAGE_READ_MODE_GRAY ||
+      mode == IMAGE_READ_MODE_GRAY_ALPHA,
+      "Webp does not support grayscale conversions. "
+      "The returned tensor will be in the colorspace of the original image.");
 
   auto return_rgb =
       should_this_return_rgb_or_rgba_let_me_know_in_the_comments_down_below_guys_see_you_in_the_next_video(