From 78c5623632670822b26bac2f2790928db7becdc6 Mon Sep 17 00:00:00 2001
From: Antonio Ospite <ao2@ao2.it>
Date: Wed, 27 Jun 2018 20:39:36 +0200
Subject: [PATCH] svg_tetraflexagon_editor: draw also the backface

One of the faces also have a backface, draw it to make it easier to
visualize how it will look like when folding the flexagon.
---
 src/flexagon/tetraflexagon_diagram.py | 13 +++++++++++++
 src/svg_tetraflexagon_editor.py       | 14 ++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/src/flexagon/tetraflexagon_diagram.py b/src/flexagon/tetraflexagon_diagram.py
index ab3da17..a79a7a0 100755
--- a/src/flexagon/tetraflexagon_diagram.py
+++ b/src/flexagon/tetraflexagon_diagram.py
@@ -39,6 +39,9 @@ class TetraflexagonDiagram(object):
         self.plan_origin = ((self.backend.width - self.tile_side * 5) / 2,
                             self.x_border)
 
+        self.backfaces_origin = (self.squares_centers[0][0] - self.tile_side,
+                                 self.squares_centers[0][1] + self.x_border * 2 + self.tile_side)
+
         self.squares_color_map = [(1, 0, 0), (0, 1, 0), (0, 0, 1)]
 
     def _init_centers(self):
@@ -69,6 +72,16 @@ class TetraflexagonDiagram(object):
         x, y = tile.calc_plan_coordinates(self.tile_side, i, j)
         return x0 + x, y0 + y
 
+    def get_backface_tile_transform(self, tile):
+        src_x, src_y = self.get_tile_center(tile)
+        tile_xoffset, tile_yoffset = tile.calc_offset_in_square(self.tile_side)
+        # When calculating dest_x the minus in the formula switches the columns.
+        dest_x = self.backfaces_origin[0] + self.tile_side - tile_xoffset
+        dest_y = self.backfaces_origin[1] + self.tile_side + tile_yoffset
+
+        return self.backend.calc_rotate_translate_transform(src_x, src_y,
+                                                            dest_x, dest_y, 0)
+
     def get_tile_transform(self, tile):
         """Calculate the transformation matrix from a tile in an square to
         the correspondent tile in the plan.
diff --git a/src/svg_tetraflexagon_editor.py b/src/svg_tetraflexagon_editor.py
index 6b57160..58209c7 100755
--- a/src/svg_tetraflexagon_editor.py
+++ b/src/svg_tetraflexagon_editor.py
@@ -32,6 +32,7 @@ class SvgwriteTetraflexagonDiagram(TetraflexagonDiagram):
         # create some layers and groups
         layers = {
             "Squares": svg.layer(label="Squares"),
+            "Backfaces": svg.layer(label="Backfaces"),
             "Tetraflexagon": svg.layer(label="Tetraflexagon"),
             "Template": svg.layer(label="Template")
         }
@@ -90,6 +91,19 @@ class SvgwriteTetraflexagonDiagram(TetraflexagonDiagram):
                 ref['clip-path'] = "url(%s)" % (tile_href + '-clip-path')
                 group.add(ref)
 
+        # A tri-tetraflexagon only have one visible backface, the first one.
+        group = self.groups["Backfaces"]
+        for tile in self.tetraflexagon.squares[0].tiles:
+            m = self.get_backface_tile_transform(tile)
+            svg_matrix = "matrix(%f, %f, %f, %f, %f, %f)" % (m[0], m[3],
+                                                             m[1], m[4],
+                                                             m[2], m[5])
+            tile_href = "#square0-tile%d" % tile.index
+            ref = self.backend.svg.use("#square0-content")
+            ref['transform'] = svg_matrix
+            ref['clip-path'] = "url(%s)" % (tile_href + '-clip-path')
+            group.add(ref)
+
     def draw_tile_template(self, tile, cx, cy, theta):
         old_active_group = self.backend.active_group
         group_name = "square%d-tile%d" % (tile.square.index, tile.index)
-- 
2.1.4