Updated error message when saving WebP with invalid width or height

Tests/test_file_webp.py

@@ -157,6 +157,17 @@ def test_write_encoding_error_message(self, tmp_path: Path) -> None:
             im.save(temp_file, method=0)
         assert str(e.value) == "encoding error 6"
 
+    @pytest.mark.skipif(sys.maxsize <= 2**32, reason="Requires 64-bit system")
+    def test_write_encoding_error_bad_dimension(self, tmp_path: Path) -> None:
+        temp_file = str(tmp_path / "temp.webp")
+        im = Image.new("L", (16384, 16384))
+        with pytest.raises(ValueError) as e:
+            im.save(temp_file)
+        assert (
+            str(e.value)
+            == "encoding error 5: Image size exceeds WebP limit of 16383 pixels"
+        )
+
     def test_WebPEncode_with_invalid_args(self) -> None:
         """
         Calling encoder functions with no arguments should result in an error.

src/_webp.c

@@ -672,7 +672,16 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
 
     WebPPictureFree(&pic);
     if (!ok) {
-        PyErr_Format(PyExc_ValueError, "encoding error %d", (&pic)->error_code);
+        int error_code = (&pic)->error_code;
+        char message[50] = "";
+        if (error_code == VP8_ENC_ERROR_BAD_DIMENSION) {
+            sprintf(
+                message,
+                ": Image size exceeds WebP limit of %d pixels",
+                WEBP_MAX_DIMENSION
+            );
+        }
+        PyErr_Format(PyExc_ValueError, "encoding error %d%s", error_code, message);
         return NULL;
     }
     output = writer.mem;