mg2d: do not select the first coarser level too close to the finest one

1 files changed, 9 insertions, 1 deletions

diff --git a/mg2d.c b/mg2d.c
index 8b70f40..fd775e4 100644
--- a/mg2d.c
+++ b/mg2d.c
@@ -535,8 +535,16 @@ static int mg_levels_alloc(MG2DContext *ctx, size_t domain_size)
     if (!priv->root)
         return -ENOMEM;
 
-    cur = priv->root;
+    // choose the domain size for the first child
+    // the children all have sizes 2**n + 1
+    // but we skip the closest lower one if it is too close
+    if (domain_size <= 2)
+        return 0;
     domain_size = (1 << log2i(domain_size - 2)) + 1;
+    if ((double)priv->root->solver->domain_size[0] / domain_size  < 1.5)
+        domain_size = (domain_size >> 1) + 1;
+
+    cur = priv->root;
     for (int i = 0; i < LEVELS_MAX && domain_size > 4; i++) {
         cur->child = mg_level_alloc(domain_size);
         if (!cur->child)