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M2: a luminosity soft mask in a DeviceCMYK group converts as ISO 32000-2 10.4.2 says, so 100% K masks fully (#4423)

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A luminosity soft mask is drawn into an RGB buffer and its value read
as the luminance of that buffer. A /DeviceCMYK colour reached the
buffer through the CMYK display profile, where 100% K is a dark grey,
so a mask painted 100% K in a DeviceCMYK group let about an eighth of
the masked paint through (mask value 0.125 instead of 0).

ISO 32000-2 11.5.3 converts a device-space group colour to gray "with
no compensation for gamma or other colour calibration", and 10.4.2.3
gives DeviceCMYK to DeviceGray as
gray = 1 - min(1, 0.3 c + 0.59 m + 0.11 y + k).

hayro-interpret patch (63): when the group colour space of a luminosity
soft mask is /DeviceCMYK itself (not an ICC-based or calibrated space),
the mask's context paints /DeviceCMYK fill and stroke colours as that
gray, and a four-component /BC becomes the same gray. A gray reads back
as itself through any luminance weights that sum to one. CMYK on the
page, CMYK in DeviceGray/DeviceRGB mask groups, and CMYK images,
shadings and tiling patterns inside such a mask keep the profile.

Tests: color::device_cmyk_gray_tests (hayro-interpret) and
it::luminosity_group_space (pdfcraft-render); the three DeviceCMYK
render tests fail with the patch reverted.


Claude-Session: https://claude.ai/code/session_0175FWj4xFhMhSH3UpMU8DSC

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
mtlcl410
2026-10-25 10:02:49 -04:00
committed by GitHub
co-authored by Claude Opus 5.5
parent 2be6eb1577
commit 7dd44301db
8 changed files with 230 additions and 9 deletions
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parity/acrobat-features.toml
+2 -2
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@@ -758,8 +758,8 @@ milestone = "M2"
status = "partial"
commands = []
tools = []
evidence = ["crates/render/src/raster.rs::soft_mask_without_group_colour_space_is_applied", "crates/render/src/raster.rs::a_multiply_highlight_inside_a_transparency_group_keeps_the_text"]
notes = "via hayro bootstrap. A transparency group that is neither isolated nor knockout and is entered at full opacity, with no soft mask and Normal blending, is drawn straight onto the page, so a blend mode inside it (the Multiply of a highlight appearance written by other editors) sees the page content (#1968). Missing: a non-isolated group that needs a layer (partial opacity, a soft mask or a blend mode on the group itself) still composites its elements against a blank backdrop, and knockout groups are drawn as ordinary ones"
evidence = ["crates/render/src/raster.rs::soft_mask_without_group_colour_space_is_applied", "crates/render/src/raster.rs::a_multiply_highlight_inside_a_transparency_group_keeps_the_text", "crates/render/tests/it/luminosity_group_space.rs::black_ink_in_a_device_cmyk_group_masks_fully_and_no_ink_shows_fully"]
notes = "via hayro bootstrap. A luminosity soft mask whose group is in DeviceCMYK reads its flat CMYK colours and backdrop as their gray, 1 - min(1, 0.3 c + 0.59 m + 0.11 y + k) (ISO 32000-2 §11.5.3), so 100% K masks fully; CMYK images, shadings and tiling patterns inside such a mask still go through the display profile. A transparency group that is neither isolated nor knockout and is entered at full opacity, with no soft mask and Normal blending, is drawn straight onto the page, so a blend mode inside it (the Multiply of a highlight appearance written by other editors) sees the page content (#1968). Missing: a non-isolated group that needs a layer (partial opacity, a soft mask or a blend mode on the group itself) still composites its elements against a blank backdrop, and knockout groups are drawn as ordinary ones"
[[feature]]
id = "view.render-knockout"