Clarify low complexity decode mode support

Change-Id: I1b31ca127b4ee5838e368311f98a188651f7f64d

av1/arg_defs.c

diff --git a/av1/arg_defs.c b/av1/arg_defs.c
index 81d00c4..1086567 100644
--- a/av1/arg_defs.c
+++ b/av1/arg_defs.c
@@ -709,9 +709,11 @@
       ARG_DEF(NULL, "sb-qp-sweep", 1,
               "When set to 1, enable the superblock level qp sweep for a "
               "given lambda to minimize the rdcost."),
-  .enable_low_complexity_decode =
-      ARG_DEF(NULL, "enable-low-complexity-decode", 1,
-              "Enable low complexity decode (0: false (default), 1: true)"),
+  .enable_low_complexity_decode = ARG_DEF(
+      NULL, "enable-low-complexity-decode", 1,
+      "Enable low complexity decode (0: false (default), 1: true). As of now, "
+      "this only supports good-quality encoding (speed 1 to 3) for vertical "
+      "videos between 608p and 720p."),
   .screen_detection_mode =
       ARG_DEF(NULL, "screen-detection-mode", 1,
               "Screen content detection mode (1: standard (default), "

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 07820d6..e414f8a 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -1337,11 +1337,15 @@
     oxcf->speed = 7;
 
   // Now, low complexity decode mode is only supported for good-quality
-  // encoding speed 1 to 3. This can be further modified if needed.
+  // encoding speed 1 to 3 and for vertical videos with a resolution between
+  // 608p and 720p. This can be further modified if needed.
+  const int is_low_complexity_decode_mode_supported =
+      (cfg->g_usage == AOM_USAGE_GOOD_QUALITY) &&
+      (oxcf->speed >= 1 && oxcf->speed <= 3) && (cfg->g_w < cfg->g_h) &&
+      (AOMMIN(cfg->g_w, cfg->g_h) >= 608 && AOMMIN(cfg->g_w, cfg->g_h) <= 720);
   oxcf->enable_low_complexity_decode =
       extra_cfg->enable_low_complexity_decode &&
-      cfg->g_usage == AOM_USAGE_GOOD_QUALITY && oxcf->speed >= 1 &&
-      oxcf->speed <= 3;
+      is_low_complexity_decode_mode_supported;
 
   // Set Color related configuration.
   color_cfg->color_primaries = extra_cfg->color_primaries;