Fix for #2729 (#2731)

* fixes #2729

* don't commit vscode stuff

* Update ffmpeg_utils.h

typo

---------

Co-authored-by: pf <pf@me>

.gitignore

@@ -16,3 +16,4 @@ docs/docs/git-logs.xml
 docs/docs/python/classes.txt
 docs/docs/python/functions.txt
 docs/docs/python/constants.txt
+**/.vscode

dlib/media/ffmpeg_utils.h

@@ -802,7 +802,14 @@ namespace dlib
             f->format           = h > 0 && w > 0 ? (int)pixfmt : (int)samplefmt;
             timestamp           = timestamp_;
 
-            int ret = av_frame_get_buffer(f.get(), 0); //use default alignment, which is likely 32
+            // The ffmpeg documentation recommends you always use align==0.
+            // However, in ffmpeg 3.2, there is a bug where if you do that, data buffers don't get allocated.
+            // So workaround is to manually set align==32
+            // Not ideal, but i've checked the source code in ffmpeg 3.3, 4.4 and 5.0 libavutil/frame.c
+            // and in every case, if align==0, then align=32
+            const int align = 32;
+
+            const int ret = av_frame_get_buffer(f.get(), align);
             if (ret < 0)
             {
                 f = nullptr;
@@ -1135,7 +1142,7 @@ namespace dlib
                                                     f.height(), 
                                                     1);    
             
-            DLIB_ASSERT(ret == expsize, "av_image_copy_to_buffer()  error : " << details::get_av_error(ret));
+            DLIB_ASSERT(ret == (int)expsize, "av_image_copy_to_buffer()  error : " << details::get_av_error(ret));
             (void)ret;
         }
 

examples/ffmpeg_video_decoding_ex.cpp

@@ -63,7 +63,7 @@ try
     }
 
     const std::string filepath = get_option(parser, "i", "");
-    const std::string codec    = get_option(parser, "codec", "");
+    const std::string codec    = get_option(parser, "codec", "h264");
 
     image_window win;