sub: do not copy the target image if there is no OSD/subs

It's not easy to tell whether the OSD/subs are empty, or if something is
drawn. In general you have to use osd_draw() with a custom callback. If
nothing is visible, the callback is never invoked. (The actual reason
why this is so "hard" is the implementation of osd_libass.c, which
doesn't allow separating rendering and drawing of OSD elements, because
all OSD elements share the same ASS_Renderer.)

To simplify avoiding copies, make osd_draw_on_image() instead of the
caller use mp_image_make_writeable(). Introduce osd_draw_on_image_p(),
which works like osd_draw_on_image(), but gets the new image allocation
from an image pool. This is supposed to be an optimization, because it
reduces the frequency of large allocations/deallocations for image data.

The result of this is that the frequency of copies needed in conjunction
with vf_sub, screenshots, and vo_lavc (encoding) should be reduced.
vf_sub now always does true pass-through if no subs are shown.

Drop the pts check from vf_sub. This didn't make much sense.

1 files changed, 5 insertions, 0 deletions

diff --git a/sub/sub.h b/sub/sub.h
index ed4401cacf..38b7b29f34 100644
--- a/sub/sub.h
+++ b/sub/sub.h
@@ -222,6 +222,11 @@ struct mp_image;
 bool osd_draw_on_image(struct osd_state *osd, struct mp_osd_res res,
                        double video_pts, int draw_flags, struct mp_image *dest);
 
+struct mp_image_pool;
+void osd_draw_on_image_p(struct osd_state *osd, struct mp_osd_res res,
+                         double video_pts, int draw_flags,
+                         struct mp_image_pool *pool, struct mp_image *dest);
+
 struct mp_draw_sub_backup;
 void osd_draw_on_image_bk(struct osd_state *osd, struct mp_osd_res res,
                           double video_pts, int draw_flags,