Fix booklet arranger cropmark, change sheet naming

Sheet naming changed from "p1f" to "p1a" for front sides. While "f" and
"b" are nice mnemonics, "a" and "b" allow for images to be previewed in
file explorers in the right order.

A set of cropmarks is added to p1a to mark the middle of the cover page.
This is useful for final folding and cropping, especially when cover art
makes it hard to manually determine where the middle is.

Comments are added, especially to make sense of the confusingly named
`side_number`.

booklet_arranger/booklet_arranger_extension.py

@@ -45,7 +45,7 @@ class BookletArranger(Extension):
         rown = QHBoxLayout()
         rown.addWidget(stats)
         layout.addLayout(rown)
-        
+
         # Options row. Bleed, cropmarks, outer-page offset.
         self.bleedInput = QLineEdit('24')
         self.cropmarksCheck = QCheckBox('Cropmarks')
@@ -163,27 +163,38 @@ class BookletArranger(Extension):
 
     # Actually arranges the pages into a booklet!
     def arrangePage(self, page):
+        max_sheets = self.page_count / 4
         page_name = page.name()
         match = re.search('[0-9]+', page_name)
         page_number = int(match[0])
 
         # Is the page in the higher or lower half? e.g. p1-16 or p17-32?
+        # Calculation differs based on this.
         high = page_number > (self.page_count / 2)
 
+        # Work out which paper sheet/side a page should be on.
+        # e.g. for 32 pages: p32 p1/p2 p31 will be on sheet 1 sides A/B.
+        #
+        # Side number 1 = sheet 1 side A (front).
+        # Side number 2 = sheet 1 side B (back).
+        # Side number 3 = sheet 2 side A (front).
+        # ...
+        #
         if high:
             side_number = self.page_count - page_number + 1
         else:
             side_number = page_number
-        odd_side = side_number % 2 # Detect odd paper sides, as sides have different page arrangements.
+        current_sheet = ceil(side_number / 2)
+
+        # Work out if a page should be on the left or right side of the sheet.
+        front_side = side_number % 2 # Odds sheet/sides (1, 3, 5...) are front.
         right = True
-        if (high and odd_side) or (not high and not odd_side):
+        if (high and front_side) or (not high and not front_side):
             right = False
-        max_sheets = self.page_count / 4
-        current_sheet = ceil(side_number / 2)
 
-        # Add page to correct group/paper sheet side.
+        # Add page to correct group (which represent paper sheets/sides).
         side_name = "p" + str(current_sheet)
-        side_name += "f" if odd_side else "b"
+        side_name += "a" if front_side else "b"
         sidegroup = self.doc.nodeByName(side_name)
         if not sidegroup:
             sidegroup = self.doc.createGroupLayer(side_name)
@@ -213,7 +224,7 @@ class BookletArranger(Extension):
             x_diff = x_diff - (bds.right() + 1 - bds.left()) - offset
         page.move(page_pos.x() + x_diff, page_pos.y() + y_diff)
 
-        # Keep new bounds for crop marks.
+        # Keep pre-bleed bounds for crop marks.
         bds = page.bounds()
 
         # Bleed by stated amount on outward sides, bleed to middle on inward sides.
@@ -228,24 +239,31 @@ class BookletArranger(Extension):
         self.bleedPage(page, bleed_amounts)
 
         # Add crop marks. Which crop marks are created depends on which side the page is on.
-        black_pixel = b'\x00\x00\x00\xff'
-        crop_mark_width = 2
-        crop_mark_height = 48
-        crop_mark = QByteArray(black_pixel * crop_mark_width * crop_mark_height)
-        ## Vertical cropmarks
-        x = bds.right() if right else bds.left() - 1
-        y = bds.top() - crop_mark_height - bleed_amount
-        page.setPixelData(crop_mark, x, y, crop_mark_width, crop_mark_height)
-        y = bds.bottom() + 1 + bleed_amount
-        page.setPixelData(crop_mark, x, y, crop_mark_width, crop_mark_height)
-        ## Horizontal cropmarks
-        crop_mark_width = 48
-        crop_mark_height = 2
-        x = bds.right() + 1 + bleed_amount if right else bds.left() - bleed_amount - crop_mark_width
-        y = bds.top() - 1
-        page.setPixelData(crop_mark, x, y, crop_mark_width, crop_mark_height)
-        y = bds.bottom()
-        page.setPixelData(crop_mark, x, y, crop_mark_width, crop_mark_height)
+        if self.cropmarksCheck.isChecked():
+            black_pixel = b'\x00\x00\x00\xff'
+            crop_mark_width = 2
+            crop_mark_height = 48
+            crop_mark = QByteArray(black_pixel * crop_mark_width * crop_mark_height)
+            ## Vertical cropmarks
+            x = bds.right() if right else bds.left() - 1
+            ytop = bds.top() - crop_mark_height - bleed_amount
+            page.setPixelData(crop_mark, x, ytop, crop_mark_width, crop_mark_height)
+            ybot = bds.bottom() + 1 + bleed_amount
+            page.setPixelData(crop_mark, x, ybot, crop_mark_width, crop_mark_height)
+            if page_number == 1:
+                # Specially mark centre on first page (always p1a right side)
+                x = bds.left() - 1 - offset
+                page.setPixelData(crop_mark, x, ytop, crop_mark_width, crop_mark_height)
+                page.setPixelData(crop_mark, x, ybot, crop_mark_width, crop_mark_height)
+
+            ## Horizontal cropmarks
+            crop_mark_width = 48
+            crop_mark_height = 2
+            x = bds.right() + 1 + bleed_amount if right else bds.left() - bleed_amount - crop_mark_width
+            y = bds.top() - 1
+            page.setPixelData(crop_mark, x, y, crop_mark_width, crop_mark_height)
+            y = bds.bottom()
+            page.setPixelData(crop_mark, x, y, crop_mark_width, crop_mark_height)
 
     def bleedPage(self, page, amounts):