add information about badly formatted .mov files

blog/_posts/2016-08-15-videos-in-google-slides-gifs.md

@@ -34,9 +34,79 @@ Gifs](https://addons.mozilla.org/en-US/firefox/addon/toggle-animated-gifs/)
 extension for Firefox (I'm sure there's something comparable for Chrome),
 which lets you do just that.
 
+## Addendum 17-04-28
+
+If you get an error that looks like the following with some weirdly
+formatted `.mov` files:
+
+```
+avconv version 12, Copyright (c) 2000-2016 the Libav developers
+  built on Mar  6 2017 22:35:59 with Apple LLVM version 8.0.0 (clang-800.0.42.1)
+[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc78b000000] stream 0, offset 0x30: partial file
+[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc78b000000] Could not find codec parameters (Video: mpeg4 (Simple Profile) [mp4v / 0x7634706D]
+      none, 1958 kb/s)
+Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'pipe:':
+  Metadata:
+    major_brand     : qt
+    minor_version   : 537199360
+    compatible_brands: qt
+    creation_time   : 2015-05-26 19:04:15
+  Duration: 00:00:10.00, bitrate: N/A
+    Stream #0:0(eng): Video: mpeg4 (Simple Profile) [mp4v / 0x7634706D]
+      none, 1958 kb/s
+      600 tbn (default)
+    Metadata:
+      creation_time   : 2015-05-26 19:04:15
+      handler_name    : Apple Alias Data Handler
+      encoder         : MPEG-4 Video
+Output #0, image2pipe, to 'pipe:':
+Output file #0 does not contain any stream
+convert: no decode delegate for this image format `' @ error/constitute.c/ReadImage/509.
+convert: no images defined `gif:-' @ error/convert.c/ConvertImageCommand/3254.
+convert: no decode delegate for this image format `' @ error/constitute.c/ReadImage/509.
+convert: no images defined `output.gif' @
+error/convert.c/ConvertImageCommand/3254. 
+```
+
+Then installing the `qtfaststart`[^3] Python package via `pip install
+qtfaststart` and then running
+
+```
+qtfaststart bad_movie.mov good_movie.mov
+```
+
+should fix the problem and now you can use `good_movie.mov` with the
+previous commands to create all the gifs. So
+[apparently](https://superuser.com/questions/479063/ffmpeg-pipe-input-error/479064#479064)
+what is happening is that some recording software puts the `mdat` block
+prior to the `moov` block (the structural metadata). This is more
+convenient for recording since the structure isn't known till the
+recording is finished, however for playback it isn't as nice.
+`qtfaststart` fixes this by swapping the two blocks:
+
+```
+$ qtfaststart -l bad_movie.mov
+ftyp (32 bytes)
+wide (8 bytes)
+mdat (2448358 bytes)
+moov (998 bytes)
+```
+
+while
+
+```
+qtfaststart -l good_movie.mov
+ftyp (32 bytes)
+moov (998 bytes)
+wide (8 bytes)
+mdat (2448358 bytes)
+```
 
 [^1]: I like Google Slides for a variety of reasons, but the main ones are
 its simplicity, portability, and collaboration features. Hard to beat for
 presentations.
 
 [^2]: OS X dammit
+
+[^3]: So apparently this Python package just repackages ffmpeg's own
+qtfaststart.c file. Why ffmpeg can't do this on its own? Who knows.