diff --git a/plot.c b/plot.c
index 95d225e..8327504 100644
--- a/plot.c
+++ b/plot.c
@@ -807,56 +807,78 @@ void plot_into_index(float measured[2][101][2])
   markmap_all_markers();
 }
 
+const uint8_t INSIDE = 0b0000;
+const uint8_t LEFT   = 0b0001;
+const uint8_t RIGHT  = 0b0010;
+const uint8_t BOTTOM = 0b0100;
+const uint8_t TOP    = 0b1000;
+
+inline static uint8_t
+_compute_outcode(int w, int h, int x, int y)
+{
+    uint8_t code = 0;
+    if (x < 0) {
+        code |= LEFT;
+    } else
+    if (x > w) {
+        code |= RIGHT;
+    }
+    if (y < 0) {
+        code |= BOTTOM;
+    } else
+    if (y > h) {
+        code |= TOP;
+    }
+    return code;
+}
+
 void
 cell_drawline(int w, int h, int x0, int y0, int x1, int y1, int c)
 {
+  uint8_t outcode0 = _compute_outcode(w, h, x0, y0);
+  uint8_t outcode1 = _compute_outcode(w, h, x1, y1);
+
+  if (outcode0 & outcode1) {
+      // this line is out of requested area. early return
+      return;
+  }
+
   if (x0 > x1) {
     SWAP(x0, x1);
     SWAP(y0, y1);
   }
 
-  while (x0 <= x1) {
-    int dx = x1 - x0 + 1;
-    int dy = y1 - y0;
-    if (dy >= 0) {
-      dy++;
-      if (dy > dx) {
-        dy /= dx; dx = 1;
-      } else {
-        dx /= dy; dy = 1;
-      }
-    } else {
-      dy--;
-      if (-dy > dx) {
-        dy /= dx; dx = 1;
-      } else {
-        dx /= -dy; dy = -1;
-      }
-    }
+  int dx = x1 - x0;
+  int dy = y1 - y0;
+  int sy = dy > 0 ? 1 : -1;
+  int e = 0;
 
-    if (dx == 1) {
-      if (dy > 0) {
-        while (dy-- > 0) {
-          if (y0 >= 0 && y0 < h && x0 >= 0 && x0 < w)
-            spi_buffer[y0*w+x0] |= c;
-          y0++;
-        }
-      } else {
-        while (dy++ < 0) {
-          if (y0 >= 0 && y0 < h && x0 >= 0 && x0 < w)
-            spi_buffer[y0*w+x0] |= c;
-          y0--;
-        }
+  dy *= sy;
+
+  if (dx >= dy) {
+      e = dy * 2 - dx;
+      while (x0 != x1) {
+          if (y0 >= 0 && y0 < h && x0 >= 0 && x0 < w)  spi_buffer[y0*w+x0] |= c;
+          x0++;
+          e += dy * 2;
+          if (e >= 0) {
+              e -= dx * 2;
+              y0 += sy;
+          }
       }
-      x0++;
-    } else {
-      while (dx-- > 0) {
-        if (y0 >= 0 && y0 < h && x0 >= 0 && x0 < w)
-          spi_buffer[y0*w+x0] |= c;
-        x0++;
+      if (y0 >= 0 && y0 < h && x0 >= 0 && x0 < w)  spi_buffer[y0*w+x0] |= c;
+  } else {
+      e = dx * 2 - dy;
+      while (y0 != y1) {
+          if (y0 >= 0 && y0 < h && x0 >= 0 && x0 < w)  spi_buffer[y0*w+x0] |= c;
+          y0 += sy;
+          e += dx * 2;
+          if (e >= 0) {
+              e -= dy * 2;
+              x0++;
+          }
       }
-      y0 += dy;
-    }
+      if (y0 >= 0 && y0 < h && x0 >= 0 && x0 < w)  spi_buffer[y0*w+x0] |= c;
   }
 }