fix: a picture prints as the picture, where and as large as the teacher asks

The black bars above and below a printed picture were neither in the
picture nor in the layout. A PNG or JPEG in a tab of its own is shown by
the engine on a viewer page whose background is rgb(14, 14, 14), and Qt
prints page backgrounds by default; 0.2.6.1 printed that page.

A picture is now taken as itself — drawn onto white, capped at 300 dpi
across Letter — and laid out on the printer's paper by the preview, with
Size (as large as fits down to 25%), Position (nine squares), Paper
(automatic, upright, sideways) and Fill the whole page, which says it
cuts the edges off and is never the default. The preview is the PDF that
prints, laid out again after each change. Worksheet PDFs and lesson pages
get no picture controls and are unchanged.

Page backgrounds are no longer printed for lesson pages either, as in
Chrome; Blink darkens light text when it does that, so a white-on-black
page prints readable grey on white. "Colour" is "Color" in the print
interface and the manual.

Lab: 135 print checks, now including eight picture layouts measured on
the job file CUPS kept — byte-identical to the preview, within 1 point of
where asked, no ink anywhere else. Printed from the real session with the
pointer: 50% top left, Elmer-66, the same. Not yet on paper.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

tests/test-classroom-printing.sh

@@ -177,6 +177,59 @@ ok, answer = printing.submit(pdf, "Elmer")
 check("a refused job is a failure, in CUPS's own words",
       not ok and answer == 'Destination "Elmer" is not accepting jobs.', answer)
 
+print()
+print("4b. where a picture goes on the paper")
+LETTER, MARGIN = (612, 792), 28.35
+area = (MARGIN, MARGIN, 612 - 2 * MARGIN, 792 - 2 * MARGIN)
+close = lambda a, b: all(abs(x - y) < 0.01 for x, y in zip(a, b))
+
+x, y, w, h = printing.picture_rect(LETTER, MARGIN, (1600, 1000))
+check("a wide picture fits the width and keeps its shape",
+      abs(w - area[2]) < 0.01 and abs(w / h - 1.6) < 0.001, (x, y, w, h))
+check("and is centred on the page", abs(y + h / 2 - 396) < 0.01)
+check("nothing of it falls outside the margins",
+      x >= MARGIN - 0.01 and y >= MARGIN - 0.01
+      and x + w <= 612 - MARGIN + 0.01 and y + h <= 792 - MARGIN + 0.01)
+
+x, y, w, h = printing.picture_rect(LETTER, MARGIN, (1600, 1000), 50, "top left")
+check("50% in the top left is half the fitted size, at the margin",
+      close((x, y, w), (MARGIN, MARGIN, area[2] / 2)), (x, y, w, h))
+x, y, w, h = printing.picture_rect(LETTER, MARGIN, (1600, 1000), 25, "bottom right")
+check("25% in the bottom right ends at the margin",
+      close((x + w, y + h), (612 - MARGIN, 792 - MARGIN)), (x, y, w, h))
+x, y, w, h = printing.picture_rect(LETTER, MARGIN, (900, 1400), 100, "right")
+check("a tall picture fits the height and sits at the right",
+      abs(h - area[3]) < 0.01 and abs(x + w - (612 - MARGIN)) < 0.01, (x, y, w, h))
+
+x, y, w, h = printing.picture_rect(LETTER, MARGIN, (1600, 1000), fill=True)
+check("fill covers the whole printable area",
+      x <= MARGIN and y <= MARGIN + 0.01 and x + w >= 612 - MARGIN and y + h >= 792 - MARGIN - 0.01)
+check("and keeps the picture's shape, cutting off what overhangs",
+      abs(w / h - 1.6) < 0.001 and w > area[2])
+check("fill ignores size and position",
+      printing.picture_rect(LETTER, MARGIN, (1600, 1000), 25, "top left", True)
+      == printing.picture_rect(LETTER, MARGIN, (1600, 1000), fill=True))
+check("an unknown position is the centre",
+      printing.picture_rect(LETTER, MARGIN, (1600, 1000), 50, "upside down")
+      == printing.picture_rect(LETTER, MARGIN, (1600, 1000), 50, "centre"))
+try:
+    printing.picture_rect(LETTER, MARGIN, (0, 10))
+    check("a picture with no size is refused", False)
+except ValueError:
+    check("a picture with no size is refused", True)
+
+check("automatic paper is landscape for a wide picture",
+      printing.picture_landscape("auto", 1600, 1000))
+check("and portrait for a tall or square one",
+      not printing.picture_landscape("auto", 900, 1400)
+      and not printing.picture_landscape("auto", 1000, 1000))
+check("and the teacher's choice overrides it",
+      not printing.picture_landscape("portrait", 1600, 1000)
+      and printing.picture_landscape("landscape", 900, 1400))
+check("every position is on a three by three grid",
+      len(printing.PICTURE_POSITIONS) == 9
+      and sorted(printing.PICTURE_POSITIONS.values()) == [(c, r) for c in range(3) for r in range(3)])
+
 print()
 print("5. Wonder Ink prints through this, and only this")
 ink = (root / "scripts/classroom-wonderink").read_text()
@@ -193,6 +246,13 @@ check("there is no file chooser in it", "QFileDialog" not in code)
 check("a worksheet tab is printed from its file",
       "self._pdf_views[view] = resolved" in code)
 check("an inline PDF is fetched as a file first", "view.page().download(view.url())" in code)
+check("a page's background is not printed, as in Chrome",
+      "PrintElementBackgrounds, False" in code)
+check("a picture on its own is laid out, not printed as the viewer page",
+      "dialog.show_picture(image)" in code and "document.contentType" in ink)
+preview_code = code[code.index("class PrintPreview"):code.index("\ndef notify")]
+check("the print interface says Color, not Colour",
+      '"Color"' in preview_code and '"Colour"' not in preview_code)
 check("an exception in a handler is logged, not fatal",
       "sys.excepthook = keep_going" in code)
 check("the module is packaged",