Wrapping up "shapes from corner" option

Works well, or at least well enough, for our current batch of
shapes.

docs/CHANGES.txt

@@ -8,7 +8,7 @@ http://www.tuxpaint.org/
 
 $Id$
 
-2020.August.14 (0.9.25)
+2020.August.15 (0.9.25)
   * New Features
     ------------
     * Export drawings:
@@ -27,7 +27,7 @@ $Id$
       * The "--exportdir" option can be used to override the default
         location (but a "TuxPaint" subdir. will NOT be placed there).
 
-    * Shape controls: -- WORK IN PROGRESS
+    * Shape controls:
       * Draw shapes from the center (as in previous versions of Tux Paint)
         or from a corner (similar to the default mode of most other
         graphics tools).

docs/en/README.txt

@@ -6,7 +6,7 @@
           Copyright 2002-2020 by various contributors; see AUTHORS.txt
                             http://www.tuxpaint.org/
 
-                         June 14, 2002 - July 27, 2020
+                        June 14, 2002 - August 15, 2020
 
      ----------------------------------------------------------------------
 
@@ -285,10 +285,31 @@ Available Tools
                  Select a shape from the selector on the right (circle,
                  square, oval, etc.).
 
+                 Use the options at the bottom right to choose the shape
+                 tool's behavior:
+
+                      Shapes from center
+                              The shape will expand from where you initially
+                              clicked, and will be centered around that
+                              position. (This was Tux Paint's only behavior
+                              through version 0.9.24.)
+
+                      Shapes from corner
+                              The shape will extend with one corner starting
+                              from where you initially clicked. This is the
+                              default method of most other traditional
+                              drawing software. (This option was added
+                              starting with Tux Paint version 0.9.25.)
+
+                 Note: If shape controls are disabled (e.g., with the
+                 "--noshapecontrols" option), the controls will not be
+                 presented, and the "shapes from center" method will be used.
+
                  In the canvas, click the mouse and hold it to stretch the
                  shape out from where you clicked. Some shapes can change
-                 proportion (e.g., rectangle and oval), others cannot (e.g.,
+                 proportion (e.g., rectangle and oval may be wider than tall,
-                 square and circle).
+                 or taller than wide), others cannot (e.g., square and
+                 circle).
 
                  Let go of the mouse when you're done stretching.
 

docs/en/html/README.html

@@ -35,7 +35,7 @@
       </p>
 
       <p>
-        June 14, 2002 - July 27, 2020
+        June 14, 2002 - August 15, 2020
       </p>
     </center>
 
@@ -707,11 +707,47 @@
                 (circle, square, oval, etc.).
               </p>
 
+              <p>
+                Use the options at the bottom right to choose
+                the shape tool's behavior:
+              </p>
+
+              <dl>
+                <dt>
+                  Shapes from center
+                </dt>
+                <dd>
+                  The shape will expand from where you initially
+                  clicked, and will be centered around that position.
+                  (This was Tux&nbsp;Paint's only behavior through
+                  version 0.9.24.)
+                </dd>
+
+                <dt>
+                  Shapes from corner
+                </dt>
+                <dd>
+                  The shape will extend with one corner starting
+                  from where you initially clicked.  This is the
+                  default method of most other traditional drawing
+                  software. (This option was added starting with
+                  Tux&nbsp;Paint version 0.9.25.)
+                </dd>
+              </dl>
+
+              <p>
+                Note: If shape controls are disabled (e.g., with the
+                "<code>--noshapecontrols</code>" option), the controls
+                will not be presented, and the "shapes from center"
+                method will be used.
+              </p>
+
               <p>
                 In the canvas, click the mouse and hold it to
                 stretch the shape out from where you clicked. Some
                 shapes can change proportion (e.g., rectangle and
-                oval), others cannot (e.g., square and circle).
+                oval may be wider than tall, or taller than wide),
+                others cannot (e.g., square and circle).
               </p>
 
               <p>

src/tuxpaint.c

@@ -12825,7 +12825,7 @@ static void do_shape(int sx, int sy, int nx, int ny, int rotn, int use_brush)
 {
   int side, angle_skip, init_ang, rx, ry, rmax, x1, y1, x2, y2, xp, yp, xv, yv, old_brush, step;
   float a1, a2, rotn_rad;
-  int xx, yy, offx, offy;
+  int xx, yy, offx, offy, max_x, max_y;
 
 
   /* Determine radius/shape of the shape to draw: */
@@ -12882,6 +12882,38 @@ static void do_shape(int sx, int sy, int nx, int ny, int rotn, int use_brush)
   init_ang = shape_init_ang[cur_shape];
 
 
+  if (shape_mode == SHAPEMODE_CORNER)
+    {
+      /* Get extent of shape based on it's vertices,
+         and scale up if we need to
+         (e.g., square's points are at 45, 135, 225 & 315 degrees,
+         which do not extend to the full radius).
+
+         This works well for square and rectangle; it mostly
+         works for triangle and 5-pointed star, but it seems
+         sufficient. -bjk 2020.08.15 */
+      max_x = 0;
+      max_y = 0;
+      for (side = 0; side < shape_sides[cur_shape]; side++)
+        {
+          a1 = (angle_skip * side + init_ang) * M_PI / 180;
+          a2 = (angle_skip * (side + 1) + init_ang) * M_PI / 180;
+          x1 = (int)(cos(a1) * rx);
+          y1 = (int)(-sin(a1) * ry);
+
+          if (abs(x1) > max_x)
+            max_x = abs(x1);
+          if (abs(y1) > max_y)
+            max_y = abs(y1);
+        }
+
+      if (max_x < rx)
+        rx = (rx * rx) / max_x;
+      if (max_y < ry)
+        ry = (ry * ry) / max_y;
+    }
+
+
   step = 1;
 
   if (dont_do_xor && !use_brush)