avcodec/dvbsubdec: Compute default CLUT based on bitmap analysis

Fixes displaying subtitles before any CLUT has been received
Fixes Ticket153

This will of course not display these initial subtitles in the correct
color (as that is not known at that point) but they should look clean
and not corrupted
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dvbsubdec.c

@@ -754,6 +754,63 @@ static int dvbsub_read_8bit_string(AVCodecContext *avctx,
     return pixels_read;
 }
 
+static void compute_default_clut(AVPicture *frame, int w, int h)
+{
+    uint8_t list[256] = {0};
+    uint8_t list_inv[256];
+    int counttab[256] = {0};
+    int count, i, x, y;
+
+#define V(x,y) frame->data[0][(x) + (y)*frame->linesize[0]]
+    for (y = 0; y<h; y++) {
+        for (x = 0; x<w; x++) {
+            int v = V(x,y) + 1;
+            int vl = x     ? V(x-1,y) + 1 : 0;
+            int vr = x+1<w ? V(x+1,y) + 1 : 0;
+            int vt = y     ? V(x,y-1) + 1 : 0;
+            int vb = y+1<h ? V(x,y+1) + 1 : 0;
+            counttab[v-1] += !!((v!=vl) + (v!=vr) + (v!=vt) + (v!=vb));
+        }
+    }
+#define L(x,y) list[ frame->data[0][(x) + (y)*frame->linesize[0]] ]
+
+    for (i = 0; i<256; i++) {
+        int scoretab[256] = {0};
+        int bestscore = 0;
+        int bestv = 0;
+        for (y = 0; y<h; y++) {
+            for (x = 0; x<w; x++) {
+                int v = frame->data[0][x + y*frame->linesize[0]];
+                int l_m = list[v];
+                int l_l = x     ? L(x-1, y) : 1;
+                int l_r = x+1<w ? L(x+1, y) : 1;
+                int l_t = y     ? L(x, y-1) : 1;
+                int l_b = y+1<h ? L(x, y+1) : 1;
+                int score;
+                if (l_m)
+                    continue;
+                scoretab[v] += l_l + l_r + l_t + l_b;
+                score = 1024LL*scoretab[v] / counttab[v];
+                if (score > bestscore) {
+                    bestscore = score;
+                    bestv = v;
+                }
+            }
+        }
+        if (!bestscore)
+            break;
+        list    [ bestv ] = 1;
+        list_inv[     i ] = bestv;
+    }
+
+    count = i - 1;
+    for (i--; i>=0; i--) {
+        int v = i*255/count;
+        AV_WN32(frame->data[1] + 4*list_inv[i], RGBA(v/2,v,v/2,v));
+    }
+}
+
+
 static int save_subtitle_set(AVCodecContext *avctx, AVSubtitle *sub, int *got_output)
 {
     DVBSubContext *ctx = avctx->priv_data;
@@ -855,6 +912,9 @@ static int save_subtitle_set(AVCodecContext *avctx, AVSubtitle *sub, int *got_ou
 
             memcpy(rect->pict.data[0], region->pbuf, region->buf_size);
 
+            if (clut == &default_clut)
+                compute_default_clut(&rect->pict, rect->w, rect->h);
+
             i++;
         }
     }