avcodec/{ituh263,mpeg4video}enc: Simplify out-of-slice DC prediction

Now that every slice (and therefore every thread) uses its own
DC array, one can simply set the out-of-slice DC values to its
expected values (this is not possible for the decoders due
to error resilience).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/h263enc.h

@@ -27,6 +27,7 @@
 const uint8_t (*ff_h263_get_mv_penalty(void))[MAX_DMV*2+1];
 
 void ff_h263_encode_init(MPVMainEncContext *m);
+void ff_h263_mpeg4_reset_dc(MPVEncContext *s);
 void ff_h263_encode_gob_header(MPVEncContext *s, int mb_line);
 void ff_h263_encode_mba(MPVEncContext *s);
 

libavcodec/ituh263enc.c

@@ -361,6 +361,22 @@ static int h263_encode_picture_header(MPVMainEncContext *const m)
     return 0;
 }
 
+void ff_h263_mpeg4_reset_dc(MPVEncContext *s)
+{
+    int16_t *dc = s->c.dc_val;
+
+    // The "- 1" is for the top-left entry
+    const int l_xy = s->c.block_index[2];
+    for (int i = l_xy - 2 * s->c.b8_stride - 1; i < l_xy; i += 2)
+        AV_WN32A(dc + i, 1024 << 16 | 1024);
+
+    const int u_xy = s->c.block_index[4];
+    const int v_xy = s->c.block_index[5];
+    int16_t *dc2 = dc + v_xy - u_xy;
+    for (int i = u_xy - s->c.mb_stride - 1; i < u_xy; ++i)
+        dc[i] = dc2[i] = 1024;
+}
+
 /**
  * Encode a group of blocks header.
  */
@@ -606,11 +622,6 @@ static int h263_pred_dc(MPVEncContext *const s, int n, int16_t **dc_val_ptr)
     int a = dc_val[-1];
     int c = dc_val[-wrap];
 
-    /* No prediction outside GOB boundary */
-    if (s->c.first_slice_line && n != 3) {
-        if (n != 2) c = 1024;
-        if (n != 1 && s->c.mb_x == s->c.resync_mb_x) a = 1024;
-    }
     /* just DC prediction */
     if (a != 1024 && c != 1024)
         pred_dc = (a + c) >> 1;

libavcodec/mpeg4videoenc.c

@@ -145,6 +145,36 @@ static inline void restore_ac_coeffs(MPVEncContext *const s, int16_t block[6][64
     }
 }
 
+/**
+ * Predict the dc.
+ * @param n block index (0-3 are luma, 4-5 are chroma)
+ * @param dir_ptr pointer to an integer where the prediction direction will be stored
+ */
+static int mpeg4_pred_dc(MpegEncContext *s, int n, int *dir_ptr)
+{
+    const int16_t *const dc_val = s->dc_val + s->block_index[n];
+    const int wrap = s->block_wrap[n];
+
+    /* B C
+     * A X
+     */
+    const int a = dc_val[-1];
+    const int b = dc_val[-1 - wrap];
+    const int c = dc_val[-wrap];
+    int pred;
+
+    // There is no need for out-of-slice handling here, as all values are set
+    // appropriately when a new slice is opened.
+    if (abs(a - b) < abs(b - c)) {
+        pred     = c;
+        *dir_ptr = 1; /* top */
+    } else {
+        pred     = a;
+        *dir_ptr = 0; /* left */
+    }
+    return pred;
+}
+
 /**
  * Return the optimal value (0 or 1) for the ac_pred element for the given MB in MPEG-4.
  * This function will also update s->c.block_last_index and s->c.ac_val.
@@ -737,7 +767,7 @@ static void mpeg4_encode_mb(MPVEncContext *const s, int16_t block[][64],
         int i;
 
         for (int i = 0; i < 6; i++) {
-            int pred  = ff_mpeg4_pred_dc(&s->c, i, &dir[i]);
+            int pred  = mpeg4_pred_dc(&s->c, i, &dir[i]);
             int scale = i < 4 ? s->c.y_dc_scale : s->c.c_dc_scale;
 
             pred = FASTDIV((pred + (scale >> 1)), scale);

libavcodec/mpegvideo_enc.c

@@ -3140,6 +3140,7 @@ static int encode_thread(AVCodecContext *c, void *arg){
                         if (CONFIG_MPEG4_ENCODER) {
                             ff_mpeg4_encode_video_packet_header(s);
                             ff_mpeg4_clean_buffers(&s->c);
+                            ff_h263_mpeg4_reset_dc(s);
                         }
                     break;
                     case AV_CODEC_ID_MPEG1VIDEO:
@@ -3149,8 +3150,13 @@ static int encode_thread(AVCodecContext *c, void *arg){
                             ff_mpeg1_clean_buffers(&s->c);
                         }
                     break;
-                    case AV_CODEC_ID_H263:
+#if CONFIG_H263P_ENCODER
                     case AV_CODEC_ID_H263P:
+                        if (s->c.dc_val)
+                            ff_h263_mpeg4_reset_dc(s);
+                        // fallthrough
+#endif
+                    case AV_CODEC_ID_H263:
                         if (CONFIG_H263_ENCODER) {
                             update_mb_info(s, 1);
                             ff_h263_encode_gob_header(s, mb_y);