display: Add display_line API for vector strokes.

display/display.c

@@ -809,6 +809,72 @@ display_point(int x,        /* 0..xpixels (unscaled) */
     return lx*lx + ly*ly <= scaled_pen_radius_squared;
 } /* display_point */
 
+#define ABS(_X) ((_X) >= 0 ? (_X) : -(_X))
+#define SIGN(_X) ((_X) >= 0 ? 1 : -1)
+
+static void
+xline (int x, int y, int x2, int dx, int dy, int level)
+{
+    int ix = SIGN(dx);
+    int iy = SIGN(dy);
+    int ay;
+
+    dx = ABS(dx);
+    dy = ABS(dy);
+
+    ay = dy/2;
+    for (;;) {
+        display_point (x, y, level, 0);
+        if (x == x2)
+            break;
+        if (ay > 0) {
+            y += iy;
+            ay -= dx;
+        }
+        ay += dy;
+        x += ix;
+    }
+}
+  
+static void
+yline (int x, int y, int y2, int dx, int dy, int level)
+{
+    int ix = SIGN(dx);
+    int iy = SIGN(dy);
+    int ax;
+
+    dx = ABS(dx);
+    dy = ABS(dy);
+
+    ax = dx/2;
+    for (;;) {
+        display_point (x, y, level, 0);
+        if (y == y2)
+            break;
+        if (ax > 0) {
+            x += ix;
+            ax -= dy;
+        }
+        ax += dx;
+        y += iy;
+    }
+}
+
+void
+display_line(int x1,        /* 0..xpixels (unscaled) */
+          int y1,           /* 0..ypixels (unscaled) */
+          int x2,           /* 0..xpixels (unscaled) */
+          int y2,           /* 0..ypixels (unscaled) */
+          int level)        /* DISPLAY_INT_xxx */
+{
+    int dx = x2 - x1;
+    int dy = y2 - y1;
+    if (ABS (dx) > ABS(dy))
+        xline (x1, y1, x2, dx, dy, level);
+    else
+        yline (x1, y1, y2, dx, dy, level);
+} /* display_line */
+
 /*
  * calculate decay color table for a phosphor mixture
  * must be called AFTER refresh_rate initialized!

display/display.h

@@ -112,6 +112,13 @@ extern int display_is_blank(void);
  */
 extern int display_point(int,int,int,int);
 
+/*
+ * plot a line; arguments are start and end x, y, intensity
+ *
+ * Display initialized on first call.
+ */
+extern void display_line(int,int,int,int,int);
+
 /*
  * force window system to output bits to screen;
  * call after adding points, or aging the screen