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Avoids unnecessary collapse of the HF stereo image in hybrid mode.

SILK now reports an "effective width" that takes into account side prediction.

--- a/silk/enc_API.c

+++ b/silk/enc_API.c

@@ -452,7 +452,7 @@

     encControl->allowBandwidthSwitch = psEnc->allowBandwidthSwitch;

     encControl->inWBmodeWithoutVariableLP = psEnc->state_Fxx[ 0 ].sCmn.fs_kHz == 16 && psEnc->state_Fxx[ 0 ].sCmn.sLP.mode == 0;

     encControl->internalSampleRate = silk_SMULBB( psEnc->state_Fxx[ 0 ].sCmn.fs_kHz, 1000 );

-    encControl->stereoWidth_Q14 = psEnc->sStereo.width_prev_Q14;

+    encControl->stereoWidth_Q14 = psEnc->sStereo.effective_width_prev_Q14;

     if( prefillFlag ) {

         encControl->payloadSize_ms = tmp_payloadSize_ms;

         encControl->complexity = tmp_complexity;

--- a/silk/stereo_LR_to_MS.c

+++ b/silk/stereo_LR_to_MS.c

@@ -53,6 +53,7 @@

     opus_int16 LP_mid[  MAX_FRAME_LENGTH ], HP_mid[  MAX_FRAME_LENGTH ];

     opus_int16 LP_side[ MAX_FRAME_LENGTH ], HP_side[ MAX_FRAME_LENGTH ];

     opus_int16 *mid = &x1[ -2 ];

+    opus_int16 effective_pred=0;

     /* Convert to basic mid/side signals */

     for( n = 0; n < frame_length + 2; n++ ) {

@@ -139,6 +140,7 @@

         silk_stereo_quant_pred( pred_Q13, ix );

         /* Collapse stereo width */

         width_Q14 = 0;

+        effective_pred = silk_abs( pred_Q13[ 0 ] ) + silk_abs( pred_Q13[ 1 ] );

         pred_Q13[ 0 ] = 0;

         pred_Q13[ 1 ] = 0;

         mid_side_rates_bps[ 0 ] = total_rate_bps;

@@ -154,6 +156,7 @@

         silk_stereo_quant_pred( pred_Q13, ix );

         /* Collapse stereo width */

         width_Q14 = 0;

+        effective_pred = silk_abs( pred_Q13[ 0 ] ) + silk_abs( pred_Q13[ 1 ] );

         pred_Q13[ 0 ] = 0;

         pred_Q13[ 1 ] = 0;

     } else if( state->smth_width_Q14 > SILK_FIX_CONST( 0.95, 14 ) ) {

@@ -166,6 +169,7 @@

         pred_Q13[ 1 ] = silk_RSHIFT( silk_SMULBB( state->smth_width_Q14, pred_Q13[ 1 ] ), 14 );

         silk_stereo_quant_pred( pred_Q13, ix );

         width_Q14 = state->smth_width_Q14;

+        effective_pred = silk_abs( pred_Q13[ 0 ] ) + silk_abs( pred_Q13[ 1 ] );

     }

     if (*mid_only_flag == 0 && mid_side_rates_bps[ 1 ] < 1)

@@ -210,4 +214,15 @@

     state->pred_prev_Q13[ 0 ] = (opus_int16)pred_Q13[ 0 ];

     state->pred_prev_Q13[ 1 ] = (opus_int16)pred_Q13[ 1 ];

     state->width_prev_Q14     = (opus_int16)width_Q14;

+    {

+       /* Evaluate the effective width of the SILK stereo so we can report it

+          for the CELT layer to partially collapse the image. We need to take

+          into account the prediction otherwise we risk compeltely collapsing

+          the HF image when doing panned mono */

+       opus_int32 effective_width;

+       effective_width = (opus_int32)state->width_prev_Q14 + silk_LSHIFT32( (opus_int32)effective_pred, 1);

+       effective_width += silk_RSHIFT( effective_width, 2 );

+       effective_width = silk_min_32(effective_width, 16384);

+       state->effective_width_prev_Q14 = effective_width;

+    }

 }

--- a/silk/structs.h

+++ b/silk/structs.h

@@ -101,6 +101,7 @@

     opus_int32                   mid_side_amp_Q0[ 4 ];

     opus_int16                   smth_width_Q14;

     opus_int16                   width_prev_Q14;

+    opus_int16                   effective_width_prev_Q14;

     opus_int8                    predIx[ MAX_FRAMES_PER_PACKET ][ 2 ][ 3 ];

     opus_int8                    mid_only_flags[ MAX_FRAMES_PER_PACKET ];

 } stereo_enc_state;