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, 3 insertions, 6 deletions

diff --git a/video/filter/vf_sub.c b/video/filter/vf_sub.c
index 8cda9f5e79..c9d22995c1 100644
--- a/video/filter/vf_sub.c
+++ b/video/filter/vf_sub.c
@@ -155,9 +155,7 @@ static struct mp_image *filter(struct vf_instance *vf, struct mp_image *mpi)
     struct vf_priv_s *priv = vf->priv;
     struct osd_state *osd = priv->osd;
 
-    if (vf->priv->opt_top_margin || vf->priv->opt_bottom_margin ||
-        !mp_image_is_writeable(mpi))
-    {
+    if (vf->priv->opt_top_margin || vf->priv->opt_bottom_margin) {
         struct mp_image *dmpi = vf_alloc_out_image(vf);
         mp_image_copy_attributes(dmpi, mpi);
         prepare_image(vf, dmpi, mpi);
@@ -167,8 +165,8 @@ static struct mp_image *filter(struct vf_instance *vf, struct mp_image *mpi)
 
     mp_image_set_colorspace_details(mpi, &priv->csp);
 
-    if (mpi->pts != MP_NOPTS_VALUE)
-        osd_draw_on_image(osd, priv->dim, mpi->pts, OSD_DRAW_SUB_FILTER, mpi);
+    osd_draw_on_image_p(osd, priv->dim, mpi->pts, OSD_DRAW_SUB_FILTER,
+                        vf->out_pool, mpi);
 
     return mpi;
 }
diff --git a/video/out/vo_lavc.c b/video/out/vo_lavc.c
index 35041c7f63..2aced9351a 100644
--- a/video/out/vo_lavc.c
+++ b/video/out/vo_lavc.c
@@ -487,7 +487,6 @@ static void draw_osd(struct vo *vo, struct osd_state *osd)
         };
 
         mp_image_set_colorspace_details(vc->lastimg, &vc->colorspace);
-        mp_image_make_writeable(vc->lastimg);
 
         osd_draw_on_image(osd, dim, osd->vo_pts, OSD_DRAW_SUB_ONLY, vc->lastimg);
     }