AAC encoder: enforce SF delta in PNS and IS SFs

Make sure to properly enforce maximum scalefactor delta in PNS
and I/S, not as an absolute range but relative to the previous
and first scalefactor, the latter important since PNS uses an
implicit inicial scalefactor derived from the global gain, and
encoded in a different way, which makes a big difference in the
allowed range.

libavcodec/aaccoder.c

@@ -195,23 +195,23 @@ typedef struct TrellisPath {
 
 static void set_special_band_scalefactors(AACEncContext *s, SingleChannelElement *sce)
 {
-    int w, g, start = 0;
-    int minscaler_n = sce->sf_idx[0], minscaler_i = sce->sf_idx[0];
+    int w, g;
+    int prevscaler_n = -255, prevscaler_i = 0;
     int bands = 0;
 
     for (w = 0; w < sce->ics.num_windows; w += sce->ics.group_len[w]) {
-        start = 0;
         for (g = 0;  g < sce->ics.num_swb; g++) {
+            if (sce->zeroes[w*16+g])
+                continue;
             if (sce->band_type[w*16+g] == INTENSITY_BT || sce->band_type[w*16+g] == INTENSITY_BT2) {
                 sce->sf_idx[w*16+g] = av_clip(roundf(log2f(sce->is_ener[w*16+g])*2), -155, 100);
-                minscaler_i = FFMIN(minscaler_i, sce->sf_idx[w*16+g]);
                 bands++;
             } else if (sce->band_type[w*16+g] == NOISE_BT) {
                 sce->sf_idx[w*16+g] = av_clip(3+ceilf(log2f(sce->pns_ener[w*16+g])*2), -100, 155);
-                minscaler_n = FFMIN(minscaler_n, sce->sf_idx[w*16+g]);
+                if (prevscaler_n == -255)
+                    prevscaler_n = sce->sf_idx[w*16+g];
                 bands++;
             }
-            start += sce->ics.swb_sizes[g];
         }
     }
 
@@ -221,10 +221,12 @@ static void set_special_band_scalefactors(AACEncContext *s, SingleChannelElement
     /* Clip the scalefactor indices */
     for (w = 0; w < sce->ics.num_windows; w += sce->ics.group_len[w]) {
         for (g = 0;  g < sce->ics.num_swb; g++) {
+            if (sce->zeroes[w*16+g])
+                continue;
             if (sce->band_type[w*16+g] == INTENSITY_BT || sce->band_type[w*16+g] == INTENSITY_BT2) {
-                sce->sf_idx[w*16+g] = av_clip(sce->sf_idx[w*16+g], minscaler_i, minscaler_i + SCALE_MAX_DIFF);
+                sce->sf_idx[w*16+g] = prevscaler_i = av_clip(sce->sf_idx[w*16+g], prevscaler_i - SCALE_MAX_DIFF, prevscaler_i + SCALE_MAX_DIFF);
             } else if (sce->band_type[w*16+g] == NOISE_BT) {
-                sce->sf_idx[w*16+g] = av_clip(sce->sf_idx[w*16+g], minscaler_n, minscaler_n + SCALE_MAX_DIFF);
+                sce->sf_idx[w*16+g] = prevscaler_n = av_clip(sce->sf_idx[w*16+g], prevscaler_n - SCALE_MAX_DIFF, prevscaler_n + SCALE_MAX_DIFF);
             }
         }
     }