diff --git a/src/cairo-path-stroke-polygon.c b/src/cairo-path-stroke-polygon.c
index e5082bb..ac85db7 100644
--- a/src/cairo-path-stroke-polygon.c
+++ b/src/cairo-path-stroke-polygon.c
@@ -412,8 +412,8 @@ outer_close (struct stroker *stroker,
     case CAIRO_LINE_JOIN_MITER:
     default: {
        /* dot product of incoming slope vector with outgoing slope vector */
-       double  in_dot_out = in->dev_slope.x * out->dev_slope.x +
-                            in->dev_slope.y * out->dev_slope.y;
+       double  in_dot_out = -in->usr_vector.x * out->usr_vector.x +
+                            -in->usr_vector.y * out->usr_vector.y;
        double  ml = stroker->style.miter_limit;

        /* Check the miter limit -- lines meeting at an acute angle
@@ -473,7 +473,7 @@ outer_close (struct stroker *stroker,
         *      2 <= mlČ (1 - in · out)
         *
         */
-       if (2 <= ml * ml * (1 + in_dot_out)) {
+       if (2 <= ml * ml * (1 - in_dot_out)) {
            double              x1, y1, x2, y2;
            double              mx, my;
            double              dx1, dx2, dy1, dy2;
@@ -596,8 +596,8 @@ outer_join (struct stroker *stroker,
     case CAIRO_LINE_JOIN_MITER:
     default: {
        /* dot product of incoming slope vector with outgoing slope vector */
-       double  in_dot_out = in->dev_slope.x * out->dev_slope.x +
-                            in->dev_slope.y * out->dev_slope.y;
+       double  in_dot_out = -in->usr_vector.x * out->usr_vector.x +
+                            -in->usr_vector.y * out->usr_vector.y;
        double  ml = stroker->style.miter_limit;

        /* Check the miter limit -- lines meeting at an acute angle
@@ -657,7 +657,7 @@ outer_join (struct stroker *stroker,
         *      2 <= mlČ (1 - in · out)
         *
         */
-       if (2 <= ml * ml * (1 + in_dot_out)) {
+       if (2 <= ml * ml * (1 - in_dot_out)) {
            double              x1, y1, x2, y2;
            double              mx, my;
            double              dx1, dx2, dy1, dy2;