Expose dav1d_apply_grain as part of the public API

This change is motivated by a desire to be able to toggle between CPU
and GPU film gain synthesis in players such as VLC. Because VLC
initializes the codec before the vout (and, indeed, the active vout
module may change in the middle of decoding), it cannot make the
decision of whether to apply film grain in libdav1d as part of codec
initialization. It needs to be decided on a frame-by-frame basis
depending on whether the currently active vout supports film grain
synthesis or not.

Using the new API, users like VLC can simply set `apply_grain` to 0 and
then manually call `dav1d_apply_grain` whenever the vout does not
support GPU film grain synthesis. As a side note, `dav1d_apply_grain`
could also technically be called from dedicated worker threads,
something that libdav1d does not currently do internally.

The alternative to this solution would have been to allow changing
Dav1dSettings at runtime, but that would be more invasive and a proper
API would also need to take other settings into consideration, some of
which can't be changed as easily as `apply_grain`. This commit
represents a stop-gap solution.

Bump the minor version to allow clients to depend on this API.

1 files changed, 21 insertions, 0 deletions

diff --git a/include/dav1d/dav1d.h b/include/dav1d/dav1d.h
index cbb3ed4..e73d79e 100644
--- a/include/dav1d/dav1d.h
+++ b/include/dav1d/dav1d.h
@@ -187,6 +187,27 @@ DAV1D_API int dav1d_send_data(Dav1dContext *c, Dav1dData *in);
 DAV1D_API int dav1d_get_picture(Dav1dContext *c, Dav1dPicture *out);
 
 /**
+ * Apply film grain to a previously decoded picture. If the picture contains no
+ * film grain metadata, then this function merely returns a new reference.
+ *
+ * @param   c Input decoder instance.
+ * @param out Output frame. The caller assumes ownership of the returned
+ *            reference.
+ * @param  in Input frame. No ownership is transferred.
+ *
+ * @return
+ *         0: Success, and a frame is returned.
+ *  other negative DAV1D_ERR codes: Error due to lack of memory or because of
+ *                                  invalid passed-in arguments.
+ *
+ * @note If `Dav1dSettings.apply_grain` is true, film grain was already applied
+ *       by `dav1d_get_picture`, and so calling this function leads to double
+ *       application of film grain. Users should only call this when needed.
+ */
+DAV1D_API int dav1d_apply_grain(Dav1dContext *c, Dav1dPicture *out,
+                                const Dav1dPicture *in);
+
+/**
  * Close a decoder instance and free all associated memory.
  *
  * @param c_out The decoder instance to close. *c_out will be set to NULL.