mpeg2_metadata, cbs_mpeg2: Fix handling of colour_description

If a sequence display extension is read with colour_description equal to
zero, but a user wants to add one or more of the colour_description
elements, then the colour_description elements the user did not explicitly
request to be set are set to zero and not to the value equal to
unknown/unspecified (namely 2). A value of zero is not only inappropriate,
but explicitly forbidden. This is fixed by inferring the right default
values during the reading process if the elements are absent; moreover,
changing any of the colour_description elements to zero is now no longer
possible.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/cbs_mpeg2.c

@@ -82,6 +82,10 @@
     (get_bits_left(rw) >= width && \
      (var = show_bits(rw, width)) == (compare))
 
+#define infer(name, value) do { \
+        current->name = value; \
+    } while (0)
+
 #include "cbs_mpeg2_syntax_template.c"
 
 #undef READ
@@ -91,6 +95,7 @@
 #undef xsi
 #undef marker_bit
 #undef nextbits
+#undef infer
 
 
 #define WRITE
@@ -116,6 +121,15 @@
 
 #define nextbits(width, compare, var) (var)
 
+#define infer(name, value) do { \
+        if (current->name != (value)) { \
+            av_log(ctx->log_ctx, AV_LOG_WARNING, "Warning: " \
+                   "%s does not match inferred value: " \
+                   "%"PRId64", but should be %"PRId64".\n", \
+                   #name, (int64_t)current->name, (int64_t)(value)); \
+        } \
+    } while (0)
+
 #include "cbs_mpeg2_syntax_template.c"
 
 #undef WRITE
@@ -125,6 +139,7 @@
 #undef xsi
 #undef marker_bit
 #undef nextbits
+#undef infer
 
 
 static void cbs_mpeg2_free_user_data(void *unit, uint8_t *content)

libavcodec/cbs_mpeg2_syntax_template.c

@@ -144,6 +144,10 @@ static int FUNC(sequence_display_extension)(CodedBitstreamContext *ctx, RWContex
         uir(8, transfer_characteristics);
         uir(8, matrix_coefficients);
 #endif
+    } else {
+        infer(colour_primaries,         2);
+        infer(transfer_characteristics, 2);
+        infer(matrix_coefficients,      2);
     }
 
     ui(14, display_horizontal_size);

libavcodec/mpeg2_metadata_bsf.c

@@ -213,6 +213,18 @@ static int mpeg2_metadata_init(AVBSFContext *bsf)
     CodedBitstreamFragment *frag = &ctx->fragment;
     int err;
 
+#define VALIDITY_CHECK(name) do { \
+        if (!ctx->name) { \
+            av_log(bsf, AV_LOG_ERROR, "The value 0 for %s is " \
+                                      "forbidden.\n", #name); \
+            return AVERROR(EINVAL); \
+        } \
+    } while (0)
+    VALIDITY_CHECK(colour_primaries);
+    VALIDITY_CHECK(transfer_characteristics);
+    VALIDITY_CHECK(matrix_coefficients);
+#undef VALIDITY_CHECK
+
     err = ff_cbs_init(&ctx->cbc, AV_CODEC_ID_MPEG2VIDEO, bsf);
     if (err < 0)
         return err;