libvpxenc: extend auto-alt-ref range

vp9 now supports [0, 6]

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: James Zern <jzern@google.com>

doc/encoders.texi

@@ -1641,6 +1641,7 @@ means unlimited.
 @table @option
 @item auto-alt-ref
 Enable use of alternate reference frames (2-pass only).
+Values greater than 1 enable multi-layer alternate reference frames (VP9 only).
 @item arnr-max-frames
 Set altref noise reduction max frame count.
 @item arnr-type

libavcodec/libvpxenc.c

@@ -1075,8 +1075,6 @@ static int vpx_encode(AVCodecContext *avctx, AVPacket *pkt,
 #define VE AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_ENCODING_PARAM
 
 #define COMMON_OPTIONS \
-    { "auto-alt-ref",    "Enable use of alternate reference " \
-                         "frames (2-pass only)",                   OFFSET(auto_alt_ref),    AV_OPT_TYPE_INT, {.i64 = -1},      -1,      2,       VE}, \
     { "lag-in-frames",   "Number of frames to look ahead for " \
                          "alternate reference frame selection",    OFFSET(lag_in_frames),   AV_OPT_TYPE_INT, {.i64 = -1},      -1,      INT_MAX, VE}, \
     { "arnr-maxframes",  "altref noise reduction max frame count", OFFSET(arnr_max_frames), AV_OPT_TYPE_INT, {.i64 = -1},      -1,      INT_MAX, VE}, \
@@ -1120,6 +1118,8 @@ static int vpx_encode(AVCodecContext *avctx, AVPacket *pkt,
 #if CONFIG_LIBVPX_VP8_ENCODER
 static const AVOption vp8_options[] = {
     COMMON_OPTIONS
+    { "auto-alt-ref",    "Enable use of alternate reference "
+                         "frames (2-pass only)",                        OFFSET(auto_alt_ref),    AV_OPT_TYPE_INT, {.i64 = -1}, -1,  2, VE},
     { "cpu-used",        "Quality/Speed ratio modifier",                OFFSET(cpu_used),        AV_OPT_TYPE_INT, {.i64 = 1}, -16, 16, VE},
     LEGACY_OPTIONS
     { NULL }
@@ -1129,6 +1129,8 @@ static const AVOption vp8_options[] = {
 #if CONFIG_LIBVPX_VP9_ENCODER
 static const AVOption vp9_options[] = {
     COMMON_OPTIONS
+    { "auto-alt-ref",    "Enable use of alternate reference "
+                         "frames (2-pass only)",                        OFFSET(auto_alt_ref),    AV_OPT_TYPE_INT, {.i64 = -1}, -1, 6, VE},
     { "cpu-used",        "Quality/Speed ratio modifier",                OFFSET(cpu_used),        AV_OPT_TYPE_INT, {.i64 = 1},  -8, 8, VE},
     { "lossless",        "Lossless mode",                               OFFSET(lossless),        AV_OPT_TYPE_INT, {.i64 = -1}, -1, 1, VE},
     { "tile-columns",    "Number of tile columns to use, log2",         OFFSET(tile_columns),    AV_OPT_TYPE_INT, {.i64 = -1}, -1, 6, VE},