Handle chroma position in 4:2:0 Y4M output

tools/output/y4m2.c

@@ -49,14 +49,24 @@ static int y4m2_open(Y4m2OutputContext *const c, const char *const file,
         return -1;
     }
 
-    static const char *const ss_name[][2] = {
+    static const char *const ss_names[][2] = {
         [DAV1D_PIXEL_LAYOUT_I400] = { "mono", "mono10" },
         [DAV1D_PIXEL_LAYOUT_I420] = { "420jpeg", "420p10" },
         [DAV1D_PIXEL_LAYOUT_I422] = { "422", "422p10" },
         [DAV1D_PIXEL_LAYOUT_I444] = { "444", "444p10" }
     };
+    
+    static const char *const chr_names_8bpc_i420[] = {
+        [DAV1D_CHR_UNKNOWN] = "420jpeg",
+        [DAV1D_CHR_VERTICAL] = "420mpeg2",
+        [DAV1D_CHR_COLOCATED] = "420"
+    };
+    
+    const char *const ss_name = p->layout == DAV1D_PIXEL_LAYOUT_I420 && p->bpc == 8 ?
+        chr_names_8bpc_i420[p->chr] : ss_names[p->layout][p->bpc > 8];
+    
     fprintf(c->f, "YUV4MPEG2 W%d H%d C%s Ip F%d:%d\n",
-            p->w, p->h, ss_name[p->layout][p->bpc > 8], fps[0], fps[1]);
+            p->w, p->h, ss_name, fps[0], fps[1]);
 
     return 0;
 }