posts: fix calculation of animated PNG duration.

Fix certain animated PNGs returning NaN as the duration because the
frame rate was being reported as "0/0" by FFMpeg. This happens when the
animation has zero delay between frames. This is supposed to mean a PNG
with an infinitely fast frame rate, but in practice browsers limit it to
around 10FPS. The exact frame rate browsers will use is unknown and
implementation defined.

app/logical/ffmpeg.rb

@@ -59,10 +59,18 @@ class FFmpeg
     end
   end
 
+  # @return [Float, nil] The frame rate of the video or animation, or nil if
+  # unknown. The frame rate can be unknown for animated PNGs that have zero
+  # delay between frames.
   def frame_rate
     rate = video_streams.first[:avg_frame_rate] # "100/57"
     numerator, denominator = rate.split("/")
-    (numerator.to_f / denominator.to_f)
+
+    if numerator.to_f == 0 || denominator.to_f == 0
+      nil
+    else
+      (numerator.to_f / denominator.to_f)
+    end
   end
 
   def video_streams

app/logical/media_file/image.rb

@@ -61,8 +61,9 @@ class MediaFile::Image < MediaFile
     if is_animated_gif?
       frame_count / duration
     elsif is_animated_png?
-      # XXX we have to resort to ffprobe to get the frame rate because libvips and exiftool can't get it.
-      video.frame_rate
+      # XXX As with GIFs, animated PNGs can have an unspecified frame rate.
+      # Assume 10FPS if the frame rate is unspecified.
+      video.frame_rate.presence || 10.0
     else
       nil
     end

test/unit/media_file_test.rb

@@ -263,6 +263,18 @@ class MediaFileTest < ActiveSupport::TestCase
       end
     end
 
+    context "that is animated but with an unspecified frame rate" do
+      should "have an assumed frame rate of 10FPS" do
+        file = MediaFile.open("test/files/test-animated-inf-fps.png")
+
+        assert_equal(false, file.is_corrupt?)
+        assert_equal(true, file.is_animated?)
+        assert_equal(0.2, file.duration)
+        assert_equal(2, file.frame_count)
+        assert_equal(10, file.frame_rate)
+      end
+    end
+
     context "that is animated but malformed" do
       should "be handled correctly" do
         file = MediaFile.open("test/files/apng/iend_missing.png")