Document still picture and reduced header

Document how to make the encoder set the still_picture and
reduced_still_picture_header syntax elements to 1 in the sequence header
OBU.

Also explicitly test if cfg->full_still_picture_hdr is nonzero to match
the comment.

Bug: aomedia:2733
Change-Id: Ife54ca8ca581606756337610a184d8a675d170bd

aom/aom_encoder.h

diff --git a/aom/aom_encoder.h b/aom/aom_encoder.h
index 7009018..4d89176 100644
--- a/aom/aom_encoder.h
+++ b/aom/aom_encoder.h
@@ -435,6 +435,11 @@
 
   /*!\brief Max number of frames to encode
    *
+   * If force video mode is off (the default) and g_limit is 1, the encoder
+   * will encode a still picture (still_picture is set to 1 in the sequence
+   * header OBU). If in addition full_still_picture_hdr is 0 (the default),
+   * the encoder will use a reduced header (reduced_still_picture_header is
+   * set to 1 in the sequence header OBU) for the still picture.
    */
   unsigned int g_limit;
 
@@ -817,10 +822,12 @@
 
   /*!\brief full_still_picture_hdr
    *
-   * If this is nonzero, the encoder will generate a full header even for
-   * still picture encoding. if zero, a reduced header is used for still
-   * picture. This flag has no effect when a regular video with more than
-   * a single frame is encoded.
+   * If this is nonzero, the encoder will generate a full header
+   * (reduced_still_picture_header is set to 0 in the sequence header OBU) even
+   * for still picture encoding. If this is zero (the default), a reduced
+   * header (reduced_still_picture_header is set to 1 in the sequence header
+   * OBU) is used for still picture encoding. This flag has no effect when a
+   * regular video with more than a single frame is encoded.
    */
   unsigned int full_still_picture_hdr;
 

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 61e7ed6..a11ee9c 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -1139,7 +1139,7 @@
       extra_cfg->allow_ref_frame_mvs && !cfg->large_scale_tile;
   tool_cfg->superblock_size = extra_cfg->superblock_size;
   tool_cfg->enable_monochrome = cfg->monochrome;
-  tool_cfg->full_still_picture_hdr = cfg->full_still_picture_hdr;
+  tool_cfg->full_still_picture_hdr = cfg->full_still_picture_hdr != 0;
   tool_cfg->enable_dual_filter = extra_cfg->enable_dual_filter;
   tool_cfg->enable_order_hint = extra_cfg->enable_order_hint;
   tool_cfg->enable_interintra_comp = extra_cfg->enable_interintra_comp;