fix: use measured page capacity instead of derived CPI/LPI geometry

Real-world testing (pagetest.sh) showed the printer's actual per-page
line/column capacity didn't match what the A4-geometry math predicted,
and the redundant blank second page was traced to the printer
auto-advancing to a new page once content hits the bottom margin,
with our own unconditional trailing form-feed then ejecting that
already-started blank page on top of it.

Replace PRINTER_CPI/PRINTER_LPI with directly-measured
PRINTER_MAX_LINES/PRINTER_MAX_COLS in printer.conf (read via
pagetest.sh), and have txt2printr.sh skip the trailing form-feed
whenever the print job already reaches that line capacity.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

photobooth.py

@@ -81,8 +81,8 @@ class Config:
         self.flip_y = as_bool(ascii_.get("FLIP_Y"))
 
         self.printer_a4 = as_bool(printer.get("PRINTER_A4"))
-        self.printer_cpi = float(printer.get("PRINTER_CPI", "10") or 10)
-        self.printer_lpi = float(printer.get("PRINTER_LPI", "6") or 6)
+        self.printer_max_lines = int(printer.get("PRINTER_MAX_LINES", "64") or 64)
+        self.printer_max_cols = int(printer.get("PRINTER_MAX_COLS", "54") or 54)
 
         self.key_capture = keys.get("KEY_CAPTURE", " ")
         self.key_brightness_up = keys.get("KEY_BRIGHTNESS_UP", "+")
@@ -173,34 +173,21 @@ def capture_label(cfg):
     return "space" if cfg.key_capture == " " else cfg.key_capture
 
 
-_A4_LONG_MM = 297.0
-_A4_SHORT_MM = 210.0
-_MM_PER_IN = 25.4
-_PRINT_MARGIN_IN = 0.5
-
-
-def a4_landscape_chars(cpi, lpi):
+def a4_landscape_chars(max_lines, max_cols):
     """(cols, rows) to capture/preview for an A4 landscape printout.
 
     txt2printr.sh prints on a normally-fed (portrait) page and rotates the
     ascii art 90 degrees in software; the reader turns the finished sheet
     90 degrees to view it. That second turn cancels the software rotation,
     so this buffer's own dimensions equal the final visual result - but its
-    width becomes the printed *line count* after rotation, which must fit
-    within one page's line capacity along the long (feed) edge. Height is
-    then derived from the target landscape aspect ratio, corrected for
-    non-square character cells (cells are cpi/lpi times taller than wide),
-    so the final printout keeps true A4 proportions instead of just filling
-    however many character cells happen to fit.
+    width becomes the printed *line count* after rotation (bounded by
+    max_lines, the page's real per-page line capacity) and its height
+    becomes each printed line's length (bounded by max_cols, the real
+    per-line character capacity). Both are measured directly against the
+    printer with pagetest.sh rather than derived from paper/pitch math,
+    which doesn't reliably match real-world margins.
     """
-    long_in = _A4_LONG_MM / _MM_PER_IN
-    short_in = _A4_SHORT_MM / _MM_PER_IN
-    max_lines = max(1, round((long_in - 2 * _PRINT_MARGIN_IN) * lpi))
-    max_chars = max(1, round((short_in - 2 * _PRINT_MARGIN_IN) * cpi))
-    aspect = (long_in / short_in) * (cpi / lpi)
-    cols = max_lines
-    rows = max(1, min(max_chars, round(cols / aspect)))
-    return cols, rows
+    return max_lines, max_cols
 
 
 def box_dims_for(term_rows, term_cols, a4_target):
@@ -216,7 +203,7 @@ def main(stdscr):
     cfg = Config()
     conv_flags = cfg.converter_flags()
     a4_target = (
-        a4_landscape_chars(cfg.printer_cpi, cfg.printer_lpi)
+        a4_landscape_chars(cfg.printer_max_lines, cfg.printer_max_cols)
         if cfg.printer_a4 else None
     )
 

printer.conf

@@ -13,7 +13,13 @@
 #    see in the preview matches the shape of what gets printed
 PRINTER_A4=false
 
-# Printer pitch, used only to size the A4 viewfinder above (characters per
-# inch / lines per inch). Defaults match common Epson ESC/P draft settings.
-PRINTER_CPI=10
-PRINTER_LPI=6
+# Your printer's real per-page capacity in the normal (portrait, unrotated)
+# orientation, used to size the A4 viewfinder above and to know when a print
+# job already fills the page (see txt2printr.sh). Paper geometry and stated
+# pitch don't reliably predict this - measure it directly with pagetest.sh:
+#   ./pagetest.sh && PRINTER_A4=false ./txt2printr.sh pagetest.txt
+# then read off the last line number on page 1 (-> PRINTER_MAX_LINES) and
+# how many characters of the 64-wide test lines actually printed before
+# being cut off (-> PRINTER_MAX_COLS).
+PRINTER_MAX_LINES=64
+PRINTER_MAX_COLS=54

txt2printr.sh

@@ -4,11 +4,13 @@
 #
 # Usage: ./txt2printr.sh [ascii-art.txt]
 # Env:   PRINTER_DEV - printer device node (default: /dev/lp0)
-# Reads printer.conf for PRINTER_A4 (landscape rotation).
+# Reads printer.conf for PRINTER_A4 (landscape rotation) and PRINTER_MAX_LINES
+# (per-page line capacity, measured with pagetest.sh).
 
 dir="$(dirname "$0")"
 [ -f "$dir/printer.conf" ] && source "$dir/printer.conf"
 : "${PRINTER_A4:=false}"
+: "${PRINTER_MAX_LINES:=64}"
 
 infile="${1:-ascii-img.txt}"
 # Source - https://stackoverflow.com/a/638980
@@ -57,12 +59,23 @@ rotate90() {
     ' "$infile"
 }
 
-{
-    printf '\x1b\x40'  # ESC @ - reset printer (ESC/P and IBM Proprinter compatible)
+body() {
     if [ "$PRINTER_A4" = "true" ]; then
-        rotate90 | sed 's/$/\r/'  # CRLF line endings, needed for raw writes to avoid staircasing
+        rotate90
     else
-        sed 's/$/\r/' "$infile"
+        cat "$infile"
+    fi
+}
+
+line_count=$(body | wc -l)
+
+{
+    printf '\x1b\x40'                # ESC @ - reset printer (ESC/P and IBM Proprinter compatible)
+    body | sed 's/$/\r/'             # CRLF line endings, needed for raw writes to avoid staircasing
+    # Skip the form feed once the page is already full: printers commonly
+    # auto-advance to a new page as soon as content hits the bottom margin,
+    # so an explicit form feed on top of that just ejects a blank extra page.
+    if [ "$line_count" -lt "$PRINTER_MAX_LINES" ]; then
+        printf '\x0c'                # form feed - eject the page
     fi
-    printf '\x0c'  # form feed - eject the page
 } > "$dev"