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VP9_resizing: add limitation to the downsacling resolution.

Width and height of downscaling resolution should not be lower
than min_width and min_height which can be set as needed, both
are 180 for now.

Change-Id: I34d06704ea51affbdd814246e22ee8d41d991f00

--- a/vp9/encoder/vp9_ratectrl.c

+++ b/vp9/encoder/vp9_ratectrl.c

@@ -1835,6 +1835,9 @@

   RESIZE_ACTION resize_action = NO_RESIZE;

   int avg_qp_thr1 = 70;

   int avg_qp_thr2 = 50;

+  int min_width = 180;

+  int min_height = 180;

+  int down_size_on = 1;

   cpi->resize_scale_num = 1;

   cpi->resize_scale_den = 1;

   // Don't resize on key frame; reset the counters on key frame.

@@ -1843,6 +1846,21 @@

     cpi->resize_count = 0;

     return 0;

   }

+  // Check current frame reslution to avoid generating frames smaller than

+  // the minimum resolution.

+  if (ONEHALFONLY_RESIZE) {

+    if ((cm->width >> 1) < min_width || (cm->height >> 1) < min_height)

+      down_size_on = 0;

+  } else {

+    if (cpi->resize_state == ORIG &&

+        (cm->width * 3 / 4 < min_width ||

+         cm->height * 3 / 4 < min_height))

+      return 0;

+    else if (cpi->resize_state == THREE_QUARTER &&

+             ((cpi->oxcf.width >> 1) < min_width ||

+              (cpi->oxcf.height >> 1) < min_height))

+      down_size_on = 0;

+  }

 #if CONFIG_VP9_TEMPORAL_DENOISING

   // If denoiser is on, apply a smaller qp threshold.

@@ -1869,7 +1887,7 @@

       // down state, i.e. 1/2 or 3/4 of original resolution.

       // Currently, use a flag to turn 3/4 resizing feature on/off.

       if (cpi->resize_buffer_underflow > (cpi->resize_count >> 2)) {

-        if (cpi->resize_state == THREE_QUARTER) {

+        if (cpi->resize_state == THREE_QUARTER && down_size_on) {

           resize_action = DOWN_ONEHALF;

           cpi->resize_state = ONE_HALF;

         } else if (cpi->resize_state == ORIG) {