VSE: Python API, allow creation of VSE Movie strips with missing file

It was already possible to create Sound and Image strips that reference
non-existing files. Now it's also possible to create Movie strips
referencing missing files via the Python API call
`Sequences.new_movie()`. In this case, the duration of the strip will be
set to 1 frame.

Note that this commit does not change anything in the user interface.

The Python API of the `MovieStrip` class is extended with a function
`reload_if_needed()`. This function only performs disk I/O if the movie
strip cannot produce frames, that is either when its filepath points to
a non-existing file, or when the video sequence editor has not been
shown yet (for example because it is in an inactive workspace).

This allows for the following:

```
import bpy

scene = bpy.context.scene
vse = scene.sequence_editor_create()

filepath = bpy.path.abspath('//demo.mkv')
strip = vse.sequences.new_movie("movie", filepath,
    channel=2,
    frame_start=47,
    file_must_exist=False)
strip.frame_final_end = 327
```

This will create a new movie strip, even when `demo.mkv` does not exist.

Once `demo.mkv` has appeared at the expected location, either
`strip.reload_if_needed()` or `strip.filepath = strip.filepath` will
load it.

Differential Revision: https://developer.blender.org/D8257

Reviewed By: Sergey, ISS

2 files changed, 16 insertions, 0 deletions

diff --git a/source/blender/imbuf/IMB_imbuf.h b/source/blender/imbuf/IMB_imbuf.h
index 8fb6c1ba1e1..fc7e03c3073 100644
--- a/source/blender/imbuf/IMB_imbuf.h
+++ b/source/blender/imbuf/IMB_imbuf.h
@@ -367,6 +367,7 @@ struct anim *IMB_open_anim(const char *name,
 void IMB_suffix_anim(struct anim *anim, const char *suffix);
 void IMB_close_anim(struct anim *anim);
 void IMB_close_anim_proxies(struct anim *anim);
+bool IMB_anim_can_produce_frames(const struct anim *anim);
 
 /**
  *
diff --git a/source/blender/imbuf/intern/anim_movie.c b/source/blender/imbuf/intern/anim_movie.c
index 11b30a24cde..220801137f5 100644
--- a/source/blender/imbuf/intern/anim_movie.c
+++ b/source/blender/imbuf/intern/anim_movie.c
@@ -304,6 +304,21 @@ struct anim *IMB_open_anim(const char *name,
   return (anim);
 }
 
+bool IMB_anim_can_produce_frames(const struct anim *anim)
+{
+#ifdef WITH_AVI
+  if (anim->avi != NULL) {
+    return true;
+  }
+#endif
+#ifdef WITH_FFMPEG
+  if (anim->pCodecCtx != NULL) {
+    return true;
+  }
+#endif
+  return false;
+}
+
 void IMB_suffix_anim(struct anim *anim, const char *suffix)
 {
   BLI_strncpy(anim->suffix, suffix, sizeof(anim->suffix));