Support saving APNG float durations

Tests/test_file_apng.py

@@ -518,6 +518,24 @@ def test_apng_save_duration_loop(tmp_path: Path) -> None:
         assert im.info["duration"] == 600
 
 
+def test_apng_save_duration_float(tmp_path: Path) -> None:
+    test_file = tmp_path / "temp.png"
+    im = Image.new("1", (1, 1))
+    im2 = Image.new("1", (1, 1), 1)
+    im.save(test_file, save_all=True, append_images=[im2], duration=0.5)
+
+    with Image.open(test_file) as reloaded:
+        assert reloaded.info["duration"] == 0.5
+
+
+def test_apng_save_large_duration(tmp_path: Path) -> None:
+    test_file = tmp_path / "temp.png"
+    im = Image.new("1", (1, 1))
+    im2 = Image.new("1", (1, 1), 1)
+    with pytest.raises(ValueError, match="cannot write duration"):
+        im.save(test_file, save_all=True, append_images=[im2], duration=65536000)
+
+
 def test_apng_save_disposal(tmp_path: Path) -> None:
     test_file = tmp_path / "temp.png"
     size = (128, 64)

docs/handbook/image-file-formats.rst

@@ -1041,9 +1041,8 @@ following parameters can also be set:
     Defaults to 0.
 
 **duration**
-    Integer (or list or tuple of integers) length of time to display this APNG frame
-    (in milliseconds).
-    Defaults to 0.
+    The length of time (or list or tuple of lengths of time) to display this APNG frame
+    (in milliseconds). Defaults to 0.
 
 **disposal**
     An integer (or list or tuple of integers) specifying the APNG disposal

src/PIL/PngImagePlugin.py

@@ -39,6 +39,7 @@
 import warnings
 import zlib
 from enum import IntEnum
+from fractions import Fraction
 from typing import IO, NamedTuple, cast
 
 from . import Image, ImageChops, ImageFile, ImagePalette, ImageSequence
@@ -1272,7 +1273,11 @@ def _write_multiple_frames(
             im_frame = im_frame.crop(bbox)
         size = im_frame.size
         encoderinfo = frame_data.encoderinfo
-        frame_duration = int(round(encoderinfo.get("duration", 0)))
+        frame_duration = encoderinfo.get("duration", 0)
+        delay = Fraction(frame_duration / 1000).limit_denominator(65535)
+        if delay.numerator > 65535:
+            msg = "cannot write duration"
+            raise ValueError(msg)
         frame_disposal = encoderinfo.get("disposal", disposal)
         frame_blend = encoderinfo.get("blend", blend)
         # frame control
@@ -1284,8 +1289,8 @@ def _write_multiple_frames(
             o32(size[1]),  # height
             o32(bbox[0]),  # x_offset
             o32(bbox[1]),  # y_offset
-            o16(frame_duration),  # delay_numerator
-            o16(1000),  # delay_denominator
+            o16(delay.numerator),  # delay_numerator
+            o16(delay.denominator),  # delay_denominator
             o8(frame_disposal),  # dispose_op
             o8(frame_blend),  # blend_op
         )