calculate length of tape

print_tape.py

@@ -16,15 +16,22 @@ from escpos.printer import Dummy
 
 from glyphs import price_to_fraction, upload_fractions
 
-# TM-T88V with 90° rotation: Font A chars are 24 dots wide (instead of 12),
-# so 512 printable dots / 24 = 21 chars fit across the 80mm paper width.
-LINE_WIDTH = 21
+# TM-T88V print area and font metrics
+PRINT_WIDTH_DOTS = 512  # 80mm paper printable width
+CHAR_WIDTH_DOTS = 24    # Font A rotated 90°: 24 dots wide
 NUM_COLUMNS = 4
-COL_WIDTH = 4
-EDGE = 1  # 1-char margin on each edge
-GAP = 1   # 1-char gap between columns
-# Layout: edge(1) + col(4) + gap(1) + col(4) + gap(1) + col(4) + gap(1) + col(4) + edge(1) = 21
-COL_STARTS = [EDGE + i * (COL_WIDTH + GAP) for i in range(NUM_COLUMNS)]
+
+# The TM-T88V has ~4mm (~28 dot) non-printable margins on each side.
+# To make columns look evenly spaced on the paper, we treat the margin
+# as part of the edge gap. Setting inter-column gap = margin gives perfect
+# symmetry: 5 gaps(28) + 4 cols(107) = 568 paper dots, 512 printable.
+MARGIN_DOTS = 28
+PAPER_DOTS = PRINT_WIDTH_DOTS + 2 * MARGIN_DOTS
+GAP_DOTS = MARGIN_DOTS
+COL_SPAN_DOTS = (PAPER_DOTS - (NUM_COLUMNS + 1) * GAP_DOTS) // NUM_COLUMNS  # 107
+
+COL_BOTTOM = [(i + 1) * GAP_DOTS + i * COL_SPAN_DOTS - MARGIN_DOTS for i in range(NUM_COLUMNS)]
+COL_TOP = [b + COL_SPAN_DOTS - CHAR_WIDTH_DOTS for b in COL_BOTTOM]
 
 
 def format_price(lots, price):
@@ -71,6 +78,11 @@ def build_column_chars(trades_in_column):
     return chars
 
 
+def _set_pos(p, dots):
+    """ESC $ nL nH: set absolute horizontal print position in dots."""
+    p._raw(b"\x1b\x24" + bytes([dots % 256, dots // 256]))
+
+
 def print_all(p, trades):
     """Print all trades across multiple columns, one row at a time."""
     columns = assign_to_columns(trades)
@@ -78,16 +90,18 @@ def print_all(p, trades):
     max_len = max(len(cc) for cc in col_chars)
 
     for row_idx in range(max_len):
-        row = [" "] * LINE_WIDTH
         for col_idx in range(NUM_COLUMNS):
-            col_start = COL_STARTS[col_idx]
             if row_idx < len(col_chars[col_idx]):
                 entry = col_chars[col_idx][row_idx]
                 if entry[0] == "top":
-                    row[col_start + COL_WIDTH - 1] = entry[1]
+                    _set_pos(p, COL_TOP[col_idx])
+                    p._raw(entry[1].encode())
                 elif entry[0] == "bottom":
-                    row[col_start] = entry[1]
-        p.text("".join(row) + "\n")
+                    _set_pos(p, COL_BOTTOM[col_idx])
+                    p._raw(entry[1].encode())
+        p._raw(b"\n")
+
+    return max_len
 
 
 def main():
@@ -112,15 +126,22 @@ def main():
     # Upload custom fraction glyphs (⅛ ¼ ⅜ ½ ⅝ ¾ ⅞)
     upload_fractions(p)
 
-    print_all(p, trades)
+    num_rows = print_all(p, trades)
 
     # Feed past the cutter (~20mm above print head)
-    p.text("\n" * 16)
+    feed_rows = 16
+    p.text("\n" * feed_rows)
     p.cut()
 
     with open(output_file, "wb") as f:
         f.write(p.output)
+
+    # Calculate print length: each row is 12 dots at 180 DPI
+    total_rows = num_rows + feed_rows
+    length_mm = total_rows * 12 / 180 * 25.4
+    length_in = total_rows * 12 / 180
     print(f"Wrote {len(p.output)} bytes to {output_file}", file=sys.stderr)
+    print(f"{num_rows} rows, {length_mm:.0f}mm ({length_in:.1f}in) of tape", file=sys.stderr)
 
 
 if __name__ == "__main__":