aspect ratio and keep awake

contact_sheet.py

@@ -17,7 +17,9 @@ that maximizes each printed photo's area on a fixed 8.5x11 letter sheet
                         a strip has few photos and a wide page fills better
                         (e.g. 1 photo per shoot, or one big photo).
 
-The date is printed only inside the footer.
+The date is printed only inside the footer. Sheets are saved with 300-DPI
+metadata so their physical print size is well-defined (a full letter page)
+for CUPS or any other program they are printed from.
 """
 
 import subprocess
@@ -229,13 +231,24 @@ def build_sheet(photo_paths, out_path, footer_cfg=None,
     for (x, y, w, h) in layout["footer_rects"]:
         sheet.paste(footer_img, (x, y))
 
-    sheet.save(out_path, quality=95)
+    # Stamp 300 DPI into the file: without it the sheet has no defined
+    # physical size and the print pipeline has to guess (and may stretch).
+    sheet.save(out_path, quality=95, dpi=(300, 300))
     return out_path
 
 
 def print_sheet(path, printer=""):
-    """Send the sheet to the printer via lp. Returns (ok, message)."""
-    cmd = ["lp", "-o", "media=letter", "-o", "fit-to-page"]
+    """Send the sheet to the printer via lp. Returns (ok, message).
+
+    fit-to-page scales uniformly, so the sheet's aspect ratio is preserved.
+    Passing the sheet's orientation explicitly keeps a landscape sheet from
+    being rotated or squeezed by driver defaults.
+    """
+    with Image.open(path) as im:
+        sheet_w, sheet_h = im.size
+    orientation = "landscape" if sheet_w > sheet_h else "portrait"
+    cmd = ["lp", "-o", "media=letter", "-o", "fit-to-page",
+           "-o", orientation]
     if printer:
         cmd += ["-d", printer]
     cmd.append(str(path))