diff --git a/assets.h b/assets.h
index 431ed2d..3e2bd84 100644
--- a/assets.h
+++ b/assets.h
@@ -60,7 +60,7 @@ static const char *LCR_internalDataFile =
   ":^s1A0 :fk110" // ramps before wall
   ":=s0B0 :fd910" // concrete wall
 
-  ":nu0j1 :ly0j1 :AA0j1 "
+  ":nu0j1 :uy0j1 :AA0j1 "
 
   ":vw0m0"
   ":vx0m0J"
diff --git a/make.sh b/make.sh
index c291270..aebfd75 100755
--- a/make.sh
+++ b/make.sh
@@ -3,5 +3,5 @@
 clear
 clear
 
-gcc -std=c99 -g -Wall -Wextra -pedantic -O1 -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers `sdl2-config --libs --static-libs` -o game frontend_sdl.c && ./game
+gcc -std=c99 -g -Wall -Wextra -pedantic -O1 -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers `sdl2-config --libs --static-libs` -o game frontend_sdl.c && ./game $@
 
diff --git a/map.h b/map.h
index 3125b50..73543dc 100644
--- a/map.h
+++ b/map.h
@@ -105,7 +105,7 @@
 #define LCR_BLOCK_HILL              '('  ///< curved "hill"
 #define LCR_BLOCK_BUMP              '~'  ///< small bump on the road
 #define LCR_BLOCK_CORNER_CONVEX     'n'  ///< curved corner (convex)
-#define LCR_BLOCK_CORNER_CONCAVE    'l'  ///< curved corner (concave)
+#define LCR_BLOCK_CORNER_CONCAVE    'u'  ///< curved corner (concave)
 
 #define LCR_BLOCK_FULL_ACCEL        '>'  ///< full block with accelerator
 #define LCR_BLOCK_BOTTOM_ACCEL      'z'  ///< bottom half block with accelerator
@@ -450,7 +450,7 @@ uint8_t LCR_mapLoadFromStr(char (*getNextCharFunc)(void), const char *name)
 
       c = getNextCharFunc();
 
-      if (c > '0' && c <= '3')
+      if (c >= '0' && c <= '3')
       {
         mat = c - '0';
         c = getNextCharFunc();
diff --git a/renderer.h b/renderer.h
index 5af6304..05b358a 100644
--- a/renderer.h
+++ b/renderer.h
@@ -252,6 +252,31 @@ void LCR_rendererDrawText(const char *text, int x, int y, uint16_t color,
   }
 }
 
+/**
+  Guesses (not 100% accurately) if a quad is convex or not.
+*/
+int _LCR_rendererQuadLooksConvex(S3L_Unit quad[8])
+{
+  S3L_Unit cx = (quad[0] + quad[2] + quad[4] + quad[6]) / 4;
+  S3L_Unit cy = (quad[1] + quad[3] + quad[5] + quad[7]) / 4;
+
+  S3L_Unit r = 0;
+
+  for (int i = 0; i < 8; i += 2)
+  {
+    r += S3L_abs(cx - quad[i]);
+    r += S3L_abs(cy - quad[i + 1]);
+  }
+
+  r = (3 * r) / 32;
+
+  for (int i = 0; i < 8; i += 2)
+    if (S3L_abs(cx - quad[i]) <= r && S3L_abs(cy - quad[i + 1]) <= r)
+      return 0;
+
+  return 1;
+}
+
 uint16_t _LCR_pixelColor = 0; /**< Holds pixel color for _LCR_pixelFunc3D. This
                                    is needed is texture subsampling is on. */
 
@@ -631,7 +656,8 @@ uint8_t _LCR_rendererCheckMapTriCover(const S3L_Index *t1,
       points2D[13] = (4 * points2D[7] + 3 * points2D[9] + points2D[11]) / 8;
 
       // first: does the triangle alone cover the other one?
-      if (_LCR_rendererQuadCoversTri(points2D + 6,points2D))
+      if (_LCR_rendererQuadLooksConvex(points2D +6) &&
+        _LCR_rendererQuadCoversTri(points2D + 6,points2D))
         result |= 1 << j;
       else
       {