M11: monochrome downsampling: say an indexed image's tie goes to entry 0, its palette unread

The README said an indexed image's palette decides which entry is the
dark one on a tie, but dark_is_one never reads the palette: entry 0
wins whatever its colour. The README, the function's comment and the
parity note now say so (from FredApps' review).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/optimize/README.md

@@ -46,8 +46,9 @@ let report = optimize_with_progress(&mut doc, &settings, &mut |stage| keep_going
     `target_ppi`: each new pixel takes the sample covering more of its area, and the dark one on a
     tie, so a dark line half a new pixel wide stays: black in grey and the painted part of an image
     mask (a 0, or a 1 with an inverting decode array), full ink in a separation (a 1, or a 0), and
-    entry 0 of an indexed image, whose palette says which is darker. Not one with a soft mask, whose own size
-    would have to follow. On a 600 ppi text page, Group 4 at 300 ppi is half the size.
+    entry 0 of an indexed image, whatever colour its palette gives it (the palette isn't read yet).
+    Not one with a soft mask, whose own size would have to follow. On a 600 ppi text page, Group 4
+    at 300 ppi is half the size.
   - **Compression:** CCITT Group 4 (`pdfcraft_filters::encode_g4`), or 1-bit Flate when that is
     smaller, which only a resampled image or one packed from eight bits may be: a 1-bit image that
     is only stored again becomes Group 4 or stays. A CCITT image is only decoded to be resampled,

crates/optimize/src/images.rs

@@ -624,7 +624,8 @@ fn one_bit(doc: &Document, s: &Stream, eight_bit: bool) -> Option<(Vec<u8>, u32,
 /// Whether sample 1 is the dark one of the 1-bit image `d` describes, after its decode array
 /// (`[0 1]` when it has none of two numbers): the painted part of an image mask and black in grey
 /// decode to 0, and full ink in a separation (or a DeviceN of one colourant) to 1. An indexed image
-/// says 0, as its palette, not its decode array, says which entry is darker.
+/// says 0: its palette, not its decode array, says which entry is darker, and the palette isn't
+/// read yet, so a tie goes to entry 0 whatever its colour.
 fn dark_is_one(doc: &Document, d: &Dict) -> bool {
     let number = |v: Option<&Object>| v.and_then(|v| doc.resolve(v).as_f64());
     let (d0, d1) = match d.get(b"Decode").map(|v| doc.resolve(v)).as_deref() {

parity/acrobat-features.toml

@@ -7361,7 +7361,7 @@ status = "partial"
 commands = ["optimize.advanced"]
 tools = ["doc_optimize"]
 evidence = ["crates/optimize/tests/optimize.rs::images_are_measured_where_drawn_and_downsampled", "crates/optimize/tests/optimize.rs::settings_choose_what_happens", "crates/automation/tests/it/automation.rs::optimizing_through_tools", "crates/optimize/tests/optimize.rs::a_page_picture_printed_as_one_pixel_strips_is_merged_into_one_image", "crates/optimize/tests/optimize.rs::images_that_do_not_tile_a_picture_are_left_alone", "crates/optimize/tests/optimize.rs::a_deeply_nested_form_is_measured_at_its_largest_use", "crates/optimize/tests/optimize.rs::images_are_not_resampled_when_some_uses_could_not_be_measured", "crates/optimize/tests/optimize.rs::image_resolution_uses_the_pages_physical_unit", "crates/optimize/tests/optimize.rs::optimization_retains_an_image_below_threshold_on_a_scaled_page", "crates/optimize/tests/optimize.rs::shared_forms_are_measured_in_each_pages_physical_units", "crates/optimize/tests/optimize.rs::black_and_white_images_above_450_ppi_are_averaged_down_to_300", "crates/optimize/src/images.rs::it_matches_the_slow_way", "crates/automation/tests/it/automation.rs::doc_optimize_averages_black_and_white_scans_down", "crates/ui-egui/tests/it/create.rs::the_pdf_optimizer_dialog_saves_an_optimized_copy"]
-notes = "colour and grayscale, bicubic, effective resolution measured where drawn (every use, with page-local UserUnit scaling, through nested forms up to 12 deep; when a use can't be measured, nothing is resampled); image fragments stacked edge to edge are merged first (side-by-side tiles not yet); monochrome (1-bit, CCITT fax and eight-bit black-and-white grey) averaged by area to 300 ppi above 450 ppi by default, a tie to the dark sample (black in grey, painted in a mask, ink in a separation, after the decode array), then stored as Group 4 or 1-bit Flate (Monochrome Images ▸ Downsample to; doc_optimize mono); not monochrome images with a soft mask, and Acrobat's bicubic and subsampling choices for monochrome aren't offered"
+notes = "colour and grayscale, bicubic, effective resolution measured where drawn (every use, with page-local UserUnit scaling, through nested forms up to 12 deep; when a use can't be measured, nothing is resampled); image fragments stacked edge to edge are merged first (side-by-side tiles not yet); monochrome (1-bit, CCITT fax and eight-bit black-and-white grey) averaged by area to 300 ppi above 450 ppi by default, a tie to the dark sample (black in grey, painted in a mask, ink in a separation, after the decode array; an indexed image's entry 0, its palette not read yet), then stored as Group 4 or 1-bit Flate (Monochrome Images ▸ Downsample to; doc_optimize mono); not monochrome images with a soft mask, and Acrobat's bicubic and subsampling choices for monochrome aren't offered"
 
 [[feature]]
 id = "optimize.recompress-images"