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

1 files changed, 58 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/sequencer.c b/source/blender/blenkernel/intern/sequencer.c
index bde94f366b9..7339c887151 100644
--- a/source/blender/blenkernel/intern/sequencer.c
+++ b/source/blender/blenkernel/intern/sequencer.c
@@ -1090,6 +1090,64 @@ void BKE_sequence_reload_new_file(Main *bmain, Scene *scene, Sequence *seq, cons
   BKE_sequence_calc(scene, seq);
 }
 
+void BKE_sequence_movie_reload_if_needed(struct Main *bmain,
+                                         struct Scene *scene,
+                                         struct Sequence *seq,
+                                         bool *r_was_reloaded,
+                                         bool *r_can_produce_frames)
+{
+  BLI_assert(seq->type == SEQ_TYPE_MOVIE ||
+             !"This function is only implemented for movie strips.");
+
+  bool must_reload = false;
+
+  /* The Sequence struct allows for multiple anim structs to be associated with one strip. This
+   * function will return true only if there is at least one 'anim' AND all anims can produce
+   * frames. */
+
+  if (BLI_listbase_is_empty(&seq->anims)) {
+    /* No anim present, so reloading is always necessary. */
+    must_reload = true;
+  }
+  else {
+    LISTBASE_FOREACH (StripAnim *, sanim, &seq->anims) {
+      if (!IMB_anim_can_produce_frames(sanim->anim)) {
+        /* Anim cannot produce frames, try reloading. */
+        must_reload = true;
+        break;
+      }
+    };
+  }
+
+  if (!must_reload) {
+    /* There are one or more anims, and all can produce frames. */
+    *r_was_reloaded = false;
+    *r_can_produce_frames = true;
+    return;
+  }
+
+  BKE_sequence_reload_new_file(bmain, scene, seq, true);
+  *r_was_reloaded = true;
+
+  if (BLI_listbase_is_empty(&seq->anims)) {
+    /* No anims present after reloading => no frames can be produced. */
+    *r_can_produce_frames = false;
+    return;
+  }
+
+  /* Check if there are still anims that cannot produce frames. */
+  LISTBASE_FOREACH (StripAnim *, sanim, &seq->anims) {
+    if (!IMB_anim_can_produce_frames(sanim->anim)) {
+      /* There still is an anim that cannot produce frames. */
+      *r_can_produce_frames = false;
+      return;
+    }
+  };
+
+  /* There are one or more anims, and all can produce frames. */
+  *r_can_produce_frames = true;
+}
+
 void BKE_sequencer_sort(Scene *scene)
 {
   /* all strips together per kind, and in order of y location ("machine") */