Scale tRNS grey level for 2-bit and 4-bit grayscale PNGs

Tests/test_file_png.py

@@ -355,6 +355,17 @@ def test_save_grayscale_transparency(self, tmp_path: Path) -> None:
             assert colors is not None
             assert colors[0][0] == num_transparent
 
+    def test_low_bit_grayscale_transparency(self) -> None:
+        with Image.open("Tests/images/tbbn0g04.png") as im:
+            assert im.mode == "L"
+            assert im.info["transparency"] == 255
+            assert im.convert("LA").getpixel((0, 0)) == (255, 0)
+
+        for bits in (2, 4):
+            im2 = Image.new("L", (1, 1))
+            with roundtrip(im2, bits=bits, transparency=255) as reloaded:
+                assert reloaded.info["transparency"] == 255
+
     def test_save_1_transparency(self, tmp_path: Path) -> None:
         out = tmp_path / "temp.png"
 

src/PIL/PngImagePlugin.py

@@ -529,7 +529,12 @@ def chunk_tRNS(self, pos: int, length: int) -> bytes:
             if self.im_mode == "1":
                 self.im_info["transparency"] = 255 if i16(s) else 0
             elif self.im_mode in ("L", "I;16"):
-                self.im_info["transparency"] = i16(s)
+                value = i16(s)
+                if self.im_rawmode in {"L;2", "L;4"}:
+                    # Scale the grey level to match the unpacked 8-bit pixels
+                    max_value = (1 << int(self.im_rawmode[-1])) - 1
+                    value = int(min(1, value / max_value) * 255)
+                self.im_info["transparency"] = value
             elif self.im_mode == "RGB":
                 self.im_info["transparency"] = i16(s), i16(s, 2), i16(s, 4)
         return s