Fixed 40 x 15mm drawer labels, 40 x 20mm for a shared drawer

Drawer labels are now a set physical size rather than however long the
text happens to be:

  one part type in the drawer    40 x 15 mm
  two part types                 40 x 20 mm

They lie across the tape, so the 40mm spans the width and each label only
costs its height in tape length - 1.5m per 100 standard labels instead of
4m. That needs 50mm tape, which is now the default; a narrower tape is
detected and warned about rather than silently clipping.

Two part types in one drawer are matched on the location field and
combined onto a single taller label. Nothing is ever dropped: in
"always 40 x 15" a shared drawer simply yields two labels, and a third
part type grows the label by 5mm rather than vanishing.

Part names are wrapped to two lines and the type auto-sized to fit, so
"Plate, Modified with Clip" prints in full instead of truncating.

"Fit the tape width" keeps the previous content-sized behaviour.

app/services/tape_printer.py

@@ -12,11 +12,21 @@ VC-500W facts this is built around:
   * up to about 17 inches (431mm) per label
   * AirPrint, so it speaks IPP on port 631
 
-ORIENTATION: the tape feeds lengthwise, so the raster sent to the printer is
+THE HOUSE LABEL SIZE
-normally rotated 90 degrees - the image is tape-width across and
+--------------------
-label-length tall. That is the default. If your prints come out sideways,
+Drawer labels are a fixed physical size, not "however long the text is":
-untick "Rotate for tape feed"; to make that permanent, turn off the default
+
-orientation in Label Settings.
+    one part in the drawer    40 x 15 mm
+    two parts in the drawer   40 x 20 mm
+
+They are laid ACROSS the tape - the 40mm spans the tape width, so each
+label only eats 15mm (or 20mm) of tape length. That needs 50mm tape, and it
+is about 2.7x cheaper than running the 40mm along the feed: 100 standard
+labels come to 1.5m of tape instead of 4m. Cut across the tape between
+labels and you get a 40 x 15mm sticker.
+
+"Fit tape width" keeps the older behaviour - the label is as long as its
+content needs and fills the tape width - for anyone who wants it.
 
 Created by: Jess Rogerson (yelling commands at Claude.AI)
 """
@@ -40,6 +50,10 @@ def mm_to_px(mm, dpi=DPI):
     return max(1, int(round(mm / 25.4 * dpi)))
 
 
+def px_to_mm(px, dpi=DPI):
+    return px / dpi * 25.4
+
+
 @dataclass
 class TapePreset:
     key: str
@@ -59,29 +73,80 @@ class TapePreset:
 TAPE_PRESETS = {
     p.key: p for p in [
         TapePreset('cz9', 'CZ tape 9mm', 9.0, 8.0,
-                   'Tiny. One line, part number only.'),
+                   'Too narrow for the 40mm house label.'),
         TapePreset('cz12', 'CZ tape 12mm', 12.0, 11.0,
-                   'One bold line, or two small ones.'),
+                   'Too narrow for the 40mm house label.'),
         TapePreset('cz19', 'CZ tape 19mm', 19.0, 17.5,
-                   'Two comfortable lines. Good for bags.'),
+                   'Too narrow for the 40mm house label.'),
         TapePreset('cz25', 'CZ tape 25mm', 25.0, 23.5,
-                   'The default roll. Image plus two lines.'),
+                   'Too narrow for the 40mm house label.'),
         TapePreset('cz50', 'CZ tape 50mm', 50.0, 48.0,
-                   'Wide. Big artwork, drawer-front labels.'),
+                   'The one to use. Fits the 40mm label across the tape.'),
     ]
 }
 
-DEFAULT_TAPE = 'cz25'
+DEFAULT_TAPE = 'cz50'
 
-# What a label shows when you have not said otherwise. Defined once: the
+# ---------------------------------------------------------- label sizing
-# dataclass default and the form parser both read it, so they cannot drift.
-DEFAULT_TAPE_FIELDS = ['image', 'name', 'dimensions', 'part_number', 'accent']
 
-# Hidden marker every options form submits. Its presence is how we tell
+LABEL_WIDTH_MM = 40.0
-# "submitted with the box unticked" from "arrived here with no options at
+ONE_PART_HEIGHT_MM = 15.0
-# all" - an unticked checkbox sends nothing, so without this they look
+TWO_PART_HEIGHT_MM = 20.0
-# identical and a default-on box can never be turned off.
+# Three or more part types in one drawer is not a size you asked for, so
-FORM_MARKER = '_opts'
+# rather than silently dropping parts the label grows by this much each.
+EXTRA_PART_HEIGHT_MM = 5.0
+
+SIZE_MODES = [
+    ('auto', '40 x 15mm, or 40 x 20mm for a shared drawer'),
+    ('standard', 'Always 40 x 15mm'),
+    ('double', 'Always 40 x 20mm'),
+    ('fit', 'Fit the tape width (length follows the content)'),
+]
+DEFAULT_SIZE_MODE = 'auto'
+FIXED_MODES = ('auto', 'standard', 'double')
+
+
+def label_height_mm(part_count, size_mode=DEFAULT_SIZE_MODE):
+    """The physical height of a label holding this many part types."""
+    if size_mode == 'standard':
+        return ONE_PART_HEIGHT_MM
+    if size_mode == 'double':
+        return TWO_PART_HEIGHT_MM
+    if part_count <= 1:
+        return ONE_PART_HEIGHT_MM
+    if part_count == 2:
+        return TWO_PART_HEIGHT_MM
+    return TWO_PART_HEIGHT_MM + EXTRA_PART_HEIGHT_MM * (part_count - 2)
+
+
+def group_by_drawer(labels, enabled=True):
+    """
+    One physical label per drawer.
+
+    Labels sharing a location are the same drawer, so they go on one taller
+    label. A label with no location can't be grouped - it gets its own.
+    Order is preserved: a group sits where its first member was.
+    """
+    if not enabled:
+        return [[label] for label in labels]
+
+    groups = []
+    by_location = {}
+    for label in labels:
+        key = (label.location or '').strip().lower()
+        if not key:
+            groups.append([label])
+            continue
+        if key in by_location:
+            by_location[key].append(label)
+        else:
+            group = [label]
+            by_location[key] = group
+            groups.append(group)
+    return groups
+
+
+# ------------------------------------------------------------------ fonts
 
 FONT_CANDIDATES = {
     False: [  # regular
@@ -121,14 +186,30 @@ def load_font(size_px, bold=False):
     return font
 
 
+# ---------------------------------------------------------------- options
+
+DEFAULT_TAPE_FIELDS = ['image', 'name', 'dimensions', 'part_number', 'accent']
+
+# Hidden marker every options form submits. Its presence is how we tell
+# "submitted with the box unticked" from "arrived here with no options at
+# all" - an unticked checkbox sends nothing, so without this they look
+# identical and a default-on box can never be turned off.
+FORM_MARKER = '_opts'
+
+
 @dataclass
 class TapeOptions:
     tape: str = DEFAULT_TAPE
+    size_mode: str = DEFAULT_SIZE_MODE
+    # Combine labels that share a drawer onto one taller label.
+    group_drawers: bool = True
     fields: List[str] = field(
         default_factory=lambda: list(DEFAULT_TAPE_FIELDS))
+    # Fixed sizes always lie across the tape - that is the point of them.
+    # This only applies to 'fit' mode.
     rotate: bool = True
     copies: int = 1
-    # Fixed length in mm, or None to size each label to its content.
+    # 'fit' mode only: a fixed length in mm instead of sizing to content.
     fixed_length_mm: Optional[float] = None
     padding_mm: float = 1.5
     font_scale: float = 1.0
@@ -136,12 +217,22 @@ class TapeOptions:
     align: str = 'left'          # left | center
     outline: bool = False        # hairline box, for checking alignment
 
+    @property
+    def is_fixed(self):
+        return self.size_mode in FIXED_MODES
+
 
 def get_tape(key):
     return TAPE_PRESETS.get(key or DEFAULT_TAPE, TAPE_PRESETS[DEFAULT_TAPE])
 
 
-def options_from_form(form, rotate_default=True):
+def tape_fits_label(tape):
+    """Can the 40mm label lie across this tape?"""
+    return get_tape(tape).printable_mm >= LABEL_WIDTH_MM
+
+
+def options_from_form(form, rotate_default=True, size_default=None,
+                      tape_default=None):
     """
     Build TapeOptions from request.form / request.args.
 
@@ -152,8 +243,9 @@ def options_from_form(form, rotate_default=True):
     field the options form always sends: when it is present we know every
     checkbox was on screen, so absent means the user unticked it.
 
-    rotate_default applies only when the form was NOT submitted (a fresh
+    The *_default arguments apply only when the form was NOT submitted (a
-    arrival), so the saved site-wide preference is honoured there.
+    fresh arrival), so saved site-wide preferences are honoured there
+    without ever overriding an explicit choice on the page.
     """
     submitted = FORM_MARKER in form
 
@@ -179,8 +271,15 @@ def options_from_form(form, rotate_default=True):
     except ValueError:
         fixed = None
 
+    valid_modes = {key for key, _ in SIZE_MODES}
+    size_mode = form.get('size_mode') or size_default or DEFAULT_SIZE_MODE
+    if size_mode not in valid_modes:
+        size_mode = DEFAULT_SIZE_MODE
+
     return TapeOptions(
-        tape=form.get('tape') or DEFAULT_TAPE,
+        tape=form.get('tape') or tape_default or DEFAULT_TAPE,
+        size_mode=size_mode,
+        group_drawers=flag('group_drawers', True),
         fields=fields,
         rotate=flag('rotate', rotate_default),
         copies=max(1, min(99, number('copies', 1, int))),
@@ -193,6 +292,8 @@ def options_from_form(form, rotate_default=True):
     )
 
 
+# ------------------------------------------------------------ text pieces
+
 def _label_lines(label, fields):
     """The text that goes on the tape, as (text, weight) pairs."""
     lines = []
@@ -232,11 +333,217 @@ def _text_width(draw, text, font):
         return draw.textlength(text, font=font)
 
 
-def render_label(label, options, upload_folder):
+def _fit_text(draw, text, font, available):
-    """
+    """Trim with an ellipsis until it fits."""
-    Render one PartLabel as a PIL image, unrotated (reading left to right).
+    shown = text
+    while _text_width(draw, shown, font) > available and len(shown) > 2:
+        shown = shown[:-2]
+    if shown != text and len(shown) > 1:
+        shown = shown[:-1] + '\u2026'
+    return shown
 
-    Height is the printable tape width; width is the label length.
+
+def wrap_text(draw, text, font, available):
+    """Greedy word wrap. A word longer than the line gets its own line."""
+    words = (text or '').split()
+    if not words:
+        return []
+    lines = [words[0]]
+    for word in words[1:]:
+        trial = lines[-1] + ' ' + word
+        if _text_width(draw, trial, font) <= available:
+            lines[-1] = trial
+        else:
+            lines.append(word)
+    return lines
+
+
+def layout_title(draw, text, available, max_h, max_px, min_px,
+                 bold=True, max_lines=2):
+    """
+    Largest type at which the title fits the box, wrapping if that helps.
+
+    "Plate, Modified with Clip" will not fit one line of a 40mm label at a
+    readable size, but it fits two comfortably. Prefer wrapping over
+    shrinking, and shrinking over truncating - the whole name is the point
+    of the label.
+    """
+    size = int(max_px)
+    floor = max(6, int(min_px))
+    best = None
+    while size >= floor:
+        font = load_font(size, bold=bold)
+        lines = wrap_text(draw, text, font, available)
+        if lines and len(lines) <= max_lines:
+            widest = max(_text_width(draw, line, font) for line in lines)
+            height = len(lines) * size * 1.12
+            if widest <= available and height <= max_h:
+                return font, lines, height, size
+            if best is None:
+                best = (font, lines, height, size)
+        size -= 1
+    font = load_font(floor, bold=bold)
+    return font, [text], floor * 1.12, floor
+
+
+def fit_font(draw, text, available, max_px, min_px, bold=False):
+    """
+    The largest font size at which `text` still fits in `available`.
+
+    A 40mm label has very little room once the artwork is on it, and part
+    names vary from "Tile" to "Plate, Modified with Clip". Shrinking the
+    type keeps the whole name readable; truncating it to "Plate, Mod..."
+    does not. Below min_px it stops shrinking and the caller ellipsises.
+    """
+    size = int(max_px)
+    floor = max(6, int(min_px))
+    while size > floor:
+        font = load_font(size, bold=bold)
+        if _text_width(draw, text, font) <= available:
+            return font, size
+        size -= 1
+    return load_font(floor, bold=bold), floor
+
+
+def _load_art(label, upload_folder, box_px):
+    if not label.image:
+        return None
+    path = os.path.join(upload_folder, label.image.replace('/', os.sep))
+    if not os.path.exists(path):
+        return None
+    try:
+        art = Image.open(path).convert('RGBA')
+        art.thumbnail((box_px, box_px), Image.LANCZOS)
+        return art
+    except Exception as exc:
+        logger.warning('Could not load part image %s: %s', path, exc)
+        return None
+
+
+# -------------------------------------------------------- fixed-size draw
+
+def render_fixed(labels, options, upload_folder):
+    """
+    One drawer label at the house size, drawn flat and reading normally:
+    40mm wide, 15mm tall for one part, 20mm for two, taller beyond that.
+
+    `labels` is every part type in that drawer - one row each.
+    """
+    # Whatever is handed in gets drawn. Deciding how many parts belong on
+    # one label is plan_groups' job - doing it here too once meant a part
+    # was quietly dropped instead of getting a label of its own.
+    labels = list(labels) or []
+    rows = max(1, len(labels))
+
+    width_px = mm_to_px(LABEL_WIDTH_MM)
+    height_mm = label_height_mm(len(labels), options.size_mode)
+    height_px = mm_to_px(height_mm)
+
+    canvas = Image.new('RGB', (width_px, height_px), 'white')
+    draw = ImageDraw.Draw(canvas)
+
+    if options.outline:
+        draw.rectangle([0, 0, width_px - 1, height_px - 1],
+                       outline=(190, 190, 190), width=max(1, mm_to_px(0.2)))
+
+    fields = set(options.fields)
+    # Deliberately tighter than the fit-tape padding: 1.5mm top and bottom
+    # would eat a third of a two-part label's 10mm row.
+    padding = mm_to_px(min(options.padding_mm, 0.8))
+    row_h = height_px / rows
+
+    for index, label in enumerate(labels):
+        top = int(index * row_h)
+        bottom = int((index + 1) * row_h)
+        inner_h = bottom - top
+
+        # A hairline between parts so the two halves read as separate.
+        if index:
+            draw.line([(padding, top), (width_px - padding, top)],
+                      fill=(205, 205, 205), width=max(1, mm_to_px(0.15)))
+
+        cursor_x = padding
+        if 'accent' in fields and label.category:
+            accent_w = mm_to_px(1.2)
+            colour = label.category.accent_colour()
+            try:
+                rgb = tuple(int(colour.lstrip('#')[i:i + 2], 16)
+                            for i in (0, 2, 4))
+            except Exception:
+                rgb = (209, 16, 19)
+            draw.rectangle([cursor_x, top + padding,
+                            cursor_x + accent_w, bottom - padding], fill=rgb)
+            cursor_x += accent_w + padding
+
+        if 'image' in fields and options.image_percent:
+            # Cap the artwork by label width as well as row height, or a
+            # square image eats a third of a 40mm label.
+            box = min(inner_h - 2 * padding,
+                      int(width_px * options.image_percent / 100.0))
+            art = _load_art(label, upload_folder, box)
+            if art is not None:
+                canvas.paste(art, (cursor_x, top + (inner_h - art.height) // 2),
+                             art)
+                cursor_x += art.width + padding
+
+        lines = _label_lines(label, fields)
+        title = next((t for t, w in lines if w == 'title'), None)
+        rest = [t for t, w in lines if w != 'title']
+        meta_text = ' \u00b7 '.join(rest) if rest else None
+
+        available = width_px - cursor_x - padding
+
+        # The meta line is short and fixed, so size it first and give the
+        # title whatever vertical room is left.
+        meta_font = meta_px = None
+        meta_h = 0
+        if meta_text:
+            meta_font, meta_px = fit_font(
+                draw, meta_text, available,
+                inner_h * 0.26 * options.font_scale,
+                inner_h * 0.15 * options.font_scale, bold=False)
+            meta_h = meta_px * 1.15
+
+        title_font = title_lines = None
+        title_h = 0
+        if title:
+            # The floor has to be low enough to reach a size where a long
+            # name wraps to two lines; stopping at 20% of the row meant
+            # "Plate, Modified with Clip" was truncated instead.
+            title_font, title_lines, title_h, title_px = layout_title(
+                draw, title, available,
+                inner_h - 2 * padding - meta_h,
+                inner_h * 0.44 * options.font_scale,
+                inner_h * 0.13 * options.font_scale, bold=True)
+
+        block_h = title_h + meta_h
+        y = top + max(padding, (inner_h - block_h) / 2)
+
+        if title_lines:
+            for line in title_lines:
+                shown = _fit_text(draw, line, title_font, available)
+                x = cursor_x
+                if options.align == 'center':
+                    x += max(0, (available - _text_width(draw, shown, title_font)) / 2)
+                draw.text((x, y), shown, font=title_font, fill=(0, 0, 0))
+                y += title_px * 1.12
+
+        if meta_text:
+            shown = _fit_text(draw, meta_text, meta_font, available)
+            x = cursor_x
+            if options.align == 'center':
+                x += max(0, (available - _text_width(draw, shown, meta_font)) / 2)
+            draw.text((x, y), shown, font=meta_font, fill=(70, 70, 70))
+
+    return canvas
+
+
+# ------------------------------------------------------- fit-tape drawing
+
+def render_fit(label, options, upload_folder):
+    """
+    The older behaviour: the label fills the tape width and runs as long as
+    its content needs. Drawn flat, reading left to right.
     """
     tape = get_tape(options.tape)
     height = tape.printable_px
@@ -245,7 +552,6 @@ def render_label(label, options, upload_folder):
     fields = set(options.fields)
     lines = _label_lines(label, fields)
 
-    # Type sizes scale off tape width so 9mm doesn't get 25mm's typography.
     base = height * 0.34 * options.font_scale
     if len(lines) > 2:
         base *= 0.78
@@ -260,24 +566,15 @@ def render_label(label, options, upload_folder):
         'meta': load_font(sizes['meta'], bold=False),
     }
 
-    # Artwork
     art = None
     art_width = 0
-    if 'image' in fields and label.image and options.image_percent:
+    if 'image' in fields and options.image_percent:
-        path = os.path.join(upload_folder, label.image.replace('/', os.sep))
+        art = _load_art(label, upload_folder, height - 2 * padding)
-        if os.path.exists(path):
+        if art is not None:
-            try:
+            art_width = art.width + padding
-                art = Image.open(path).convert('RGBA')
-                box = height - 2 * padding
-                art.thumbnail((box, box), Image.LANCZOS)
-                art_width = art.width + padding
-            except Exception as exc:
-                logger.warning('Could not load part image %s: %s', path, exc)
-                art = None
 
     accent_width = mm_to_px(1.2) if ('accent' in fields and label.category) else 0
 
-    # Measure to find the natural length
     probe = ImageDraw.Draw(Image.new('RGB', (10, 10)))
     text_width = 0
     for text, weight in lines:
@@ -312,30 +609,129 @@ def render_label(label, options, upload_folder):
         canvas.paste(art, (cursor_x, (height - art.height) // 2), art)
         cursor_x += art.width + padding
 
-    # Vertically centre the text block
     total_text = sum(sizes[w] * 1.22 for _, w in lines)
     y = max(padding, (height - total_text) / 2)
     available = width - cursor_x - padding
 
     for text, weight in lines:
         font = fonts[weight]
-        shown = text
+        shown = _fit_text(draw, text, font, available)
-        while (_text_width(draw, shown, font) > available and len(shown) > 2):
-            shown = shown[:-2]
-        if shown != text and len(shown) > 1:
-            shown = shown[:-1] + '\u2026'
         x = cursor_x
         if options.align == 'center':
             x = cursor_x + max(0, (available - _text_width(draw, shown, font)) / 2)
         draw.text((x, y), shown, font=font, fill=(0, 0, 0))
         y += sizes[weight] * 1.22
 
-    if options.rotate:
-        canvas = canvas.rotate(90, expand=True)
-
     return canvas
 
 
+# --------------------------------------------------------- tape composing
+
+def compose_for_tape(flat, options):
+    """
+    Place a flat label onto the tape raster.
+
+    Convention: the returned image's WIDTH is the across-tape dimension and
+    its HEIGHT is how much tape length the label consumes.
+
+    Fixed sizes always lie across the tape - the 40mm spans the tape width,
+    which is the whole point of them - so the flat image goes on as-is,
+    centred, and each label only costs 15mm of tape. In 'fit' mode the
+    rotate option still chooses between reading along the tape or across it.
+
+    The raster is the full printable tape width even though the label is
+    40mm: AirPrint scales a job to the media width, so sending a 40mm-wide
+    image would be stretched to 48 and print oversize. Full width at 1:1
+    keeps the printed label exactly 40mm.
+    """
+    tape = get_tape(options.tape)
+    tape_px = tape.printable_px
+
+    if options.is_fixed:
+        placed = flat
+        # Too narrow a tape would clip the label; scale to fit and let the
+        # caller warn, rather than printing something cut off.
+        if placed.width > tape_px:
+            ratio = tape_px / placed.width
+            placed = placed.resize(
+                (tape_px, max(1, int(placed.height * ratio))), Image.LANCZOS)
+        canvas = Image.new('RGB', (tape_px, placed.height), 'white')
+        canvas.paste(placed, ((tape_px - placed.width) // 2, 0))
+        return canvas
+
+    # 'fit' mode: rotate so the label reads along the tape as it feeds.
+    return flat.rotate(90, expand=True) if options.rotate else flat
+
+
+def render_group(labels, options, upload_folder):
+    """A finished tape raster for one drawer (one or more part types)."""
+    labels = list(labels)
+    if options.is_fixed:
+        flat = render_fixed(labels, options, upload_folder)
+    else:
+        flat = render_fit(labels[0], options, upload_folder)
+    return compose_for_tape(flat, options)
+
+
+def render_label(label, options, upload_folder):
+    """Single-label convenience wrapper (previews, one-off prints)."""
+    group = label if isinstance(label, (list, tuple)) else [label]
+    return render_group(group, options, upload_folder)
+
+
+def plan_groups(labels, options):
+    """
+    How the selection turns into physical labels.
+
+    Returns a list of dicts: the parts on each label, its size in mm, and
+    whether it will fit the loaded tape.
+    """
+    tape = get_tape(options.tape)
+    groups = group_by_drawer(
+        labels, enabled=options.group_drawers and options.is_fixed)
+
+    # How many part types one label may carry. 'standard' is a one-part
+    # label, so a drawer with two parts gets two labels rather than losing
+    # one; 'double' spills a third part onto a second label.
+    per_label = {'standard': 1, 'double': 2}.get(options.size_mode)
+
+    plan = []
+    for group in groups:
+        chunks = [group]
+        if options.is_fixed and per_label:
+            chunks = [group[i:i + per_label]
+                      for i in range(0, len(group), per_label)] or [[]]
+
+        for chunk in chunks:
+            if not chunk:
+                continue
+            if options.is_fixed:
+                width_mm = LABEL_WIDTH_MM
+                height_mm = label_height_mm(len(chunk), options.size_mode)
+                fits = tape.printable_mm >= width_mm
+            else:
+                width_mm = height_mm = None
+                fits = True
+            plan.append({
+                'labels': chunk,
+                'width_mm': width_mm,
+                'height_mm': height_mm,
+                'fits': fits,
+                'drawer': (chunk[0].location or '').strip(),
+            })
+    return plan
+
+
+def tape_used_mm(plan, copies=1):
+    """Total tape length a print run will consume, in mm."""
+    total = 0.0
+    for item in plan:
+        total += (item['height_mm'] or 0) * copies
+    return total
+
+
+# --------------------------------------------------------------- outputs
+
 def image_to_bytes(image, mime_type):
     """Serialise a rendered label in whatever format the printer accepts."""
     buffer = io.BytesIO()
@@ -352,11 +748,12 @@ def image_to_bytes(image, mime_type):
 
 
 def render_batch_pdf(labels, options, upload_folder):
-    """All selected labels as one multi-page PDF, one tape label per page."""
+    """Every label as one multi-page PDF, one tape label per page."""
+    plan = plan_groups(labels, options)
     images = []
-    for label in labels:
+    for item in plan:
         for _ in range(options.copies):
-            images.append(render_label(label, options, upload_folder)
+            images.append(render_group(item['labels'], options, upload_folder)
                           .convert('RGB'))
     if not images:
         return None
@@ -367,23 +764,30 @@ def render_batch_pdf(labels, options, upload_folder):
     return buffer
 
 
-def preview_png(label, options, upload_folder, scale=2.0):
+def preview_png(labels, options, upload_folder, scale=11.0):
     """
-    A screen-sized preview. Rendered unrotated however the print option is
+    A screen preview of one physical label, shown flat so it is readable.
-    set, because a sideways preview is useless to look at.
+
+    Scale is px per mm on screen, so a 40 x 15mm label previews at roughly
+    440 x 165 - close enough to actual size on a typical monitor to judge.
     """
-    saved = options.rotate
+    group = labels if isinstance(labels, (list, tuple)) else [labels]
-    options.rotate = False
+
-    try:
+    if options.is_fixed:
-        image = render_label(label, options, upload_folder)
+        flat = render_fixed(group, options, upload_folder)
-    finally:
+    else:
-        options.rotate = saved
+        saved = options.rotate
+        options.rotate = False
+        try:
+            flat = render_fit(group[0], options, upload_folder)
+        finally:
+            options.rotate = saved
+
+    target_w = max(1, int(px_to_mm(flat.width) * scale))
+    target_h = max(1, int(px_to_mm(flat.height) * scale))
+    preview = flat.resize((target_w, target_h), Image.LANCZOS)
 
-    target_height = int(get_tape(options.tape).printable_mm * scale * 4)
-    ratio = target_height / image.height
-    image = image.resize((max(1, int(image.width * ratio)), target_height),
-                         Image.LANCZOS)
     buffer = io.BytesIO()
-    image.save(buffer, 'PNG')
+    preview.save(buffer, 'PNG')
     buffer.seek(0)
     return buffer