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.

2 files changed, 26 insertions, 0 deletions

diff --git a/sub/sub.c b/sub/sub.c
index 1365d3c844..4baf4f1487 100644
--- a/sub/sub.c
+++ b/sub/sub.c
@@ -39,6 +39,8 @@
 #include "draw_bmp.h"
 #include "spudec.h"
 #include "subreader.h"
+#include "video/mp_image.h"
+#include "video/mp_image_pool.h"
 
 
 char * const sub_osd_names[]={
@@ -274,6 +276,7 @@ struct draw_on_image_closure {
     struct osd_state *osd;
     struct mp_image *dest;
     struct mp_draw_sub_backup *bk;
+    struct mp_image_pool *pool;
     bool changed;
 };
 
@@ -283,11 +286,17 @@ static void draw_on_image(void *ctx, struct sub_bitmaps *imgs)
     struct osd_state *osd = closure->osd;
     if (closure->bk)
         mp_draw_sub_backup_add(closure->bk, closure->dest, imgs);
+    if (closure->pool) {
+        mp_image_pool_make_writeable(closure->pool, closure->dest);
+    } else {
+        mp_image_make_writeable(closure->dest);
+    }
     mp_draw_sub_bitmaps(&osd->draw_cache, closure->dest, imgs);
     talloc_steal(osd, osd->draw_cache);
     closure->changed = true;
 }
 
+// Calls mp_image_make_writeable() on the dest image if something is drawn.
 // Returns whether anything was drawn.
 bool osd_draw_on_image(struct osd_state *osd, struct mp_osd_res res,
                        double video_pts, int draw_flags, struct mp_image *dest)
@@ -298,6 +307,18 @@ bool osd_draw_on_image(struct osd_state *osd, struct mp_osd_res res,
     return closure.changed;
 }
 
+// Like osd_draw_on_image(), but if dest needs to be copied to make it
+// writeable, allocate images from the given pool. (This is a minor
+// optimization to reduce "real" image sized memory allocations.)
+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 draw_on_image_closure closure = {osd, dest, .pool = pool};
+    osd_draw(osd, res, video_pts, draw_flags, mp_draw_sub_formats,
+             &draw_on_image, &closure);
+}
+
 void osd_draw_on_image_bk(struct osd_state *osd, struct mp_osd_res res,
                           double video_pts, int draw_flags,
                           struct mp_draw_sub_backup *bk, struct mp_image *dest)
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,