diff --git a/include/window.h b/include/window.h
index 58943b6..46d61d7 100644
--- a/include/window.h
+++ b/include/window.h
@@ -10,6 +10,7 @@
 typedef struct point point_t;
 typedef struct line line_t;
 typedef struct triangle triangle_t;
+typedef struct gquad gquad_t;
 typedef struct rect rect_t;
 typedef struct window window_t;
 
@@ -29,6 +30,13 @@ struct triangle {
   point_t p2;
 };
 
+struct gquad {
+  point_t p0;
+  point_t p1;
+  point_t p2;
+  point_t p3;
+};
+
 struct rect {
   point_t topleft;
   i32 w;
@@ -63,6 +71,8 @@ void draw_point(const window_t *wnd, point_t p, colour_t colour);
 void draw_line(const window_t *wnd, const line_t *ln, colour_t colour);
 void draw_triangle(const window_t *wnd, triangle_t triangle, colour_t colour);
 void fill_triangle(const window_t *wnd, triangle_t triangle, colour_t colour);
+void draw_gquad(const window_t *wnd, gquad_t gquad, colour_t colour);
+void fill_gquad(const window_t *wnd, gquad_t gquad, colour_t colour);
 void draw_rect(const window_t *wnd, rect_t rect, colour_t colour);
 void fill_rect(const window_t *wnd, rect_t rect, colour_t colour);
 
diff --git a/src/window.c b/src/window.c
index 9f299fe..bfc2aa9 100644
--- a/src/window.c
+++ b/src/window.c
@@ -122,15 +122,35 @@ void fill_triangle(const window_t *wnd, triangle_t triangle, colour_t colour) {
 
   for (i32 y = min_y; y < max_y; ++y) {
     for (i32 x = min_x; x < max_x; ++x) {
-      if (half_space(x2, x1, y2, y1, x, y) > 0 &&
-          half_space(x3, x2, y3, y2, x, y) > 0 &&
-          half_space(x1, x3, y1, y3, x, y) > 0) {
+      if (half_space(x2, x1, y2, y1, x, y) >= 0 &&
+          half_space(x3, x2, y3, y2, x, y) >= 0 &&
+          half_space(x1, x3, y1, y3, x, y) >= 0) {
         draw_point(wnd, (point_t){x, y}, colour);
       }
     }
   }
 }
 
+void draw_gquad(const window_t *wnd, gquad_t gquad, colour_t colour) {
+  line_t l0 = (line_t){gquad.p0, gquad.p1};
+  line_t l1 = (line_t){gquad.p1, gquad.p3};
+  line_t l2 = (line_t){gquad.p3, gquad.p2};
+  line_t l3 = (line_t){gquad.p2, gquad.p0};
+
+  draw_line(wnd, &l0, colour);
+  draw_line(wnd, &l1, colour);
+  draw_line(wnd, &l2, colour);
+  draw_line(wnd, &l3, colour);
+}
+
+void fill_gquad(const window_t *wnd, gquad_t gquad, colour_t colour) {
+  triangle_t t0 = (triangle_t){gquad.p3, gquad.p1, gquad.p0};
+  triangle_t t1 = (triangle_t){gquad.p0, gquad.p2, gquad.p3};
+
+  fill_triangle(wnd, t0, colour);
+  fill_triangle(wnd, t1, colour);
+}
+
 void draw_rect(const window_t *wnd, rect_t rect, colour_t colour) {
   set_colour(wnd, colour);