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fix: convert DeviceCMYK via process inks, not 1-(c+k) #861

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Merged
yfedoseev merged 6 commits into from
Jul 21, 2026
Merged

fix: convert DeviceCMYK via process inks, not 1-(c+k) #861

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44d497b
fix: convert DeviceCMYK via process inks, not 1-(c+k)
ajbufort Jul 14, 2026
f67b79c
fix: remove the doc comment orphaned by deleting the local cmyk_to_rgb
ajbufort Jul 14, 2026
1bd3b3c
fix(images): convert DeviceCMYK images via process inks, not additive
ajbufort Jul 19, 2026
d503aeb
feat(color): unify DeviceCMYK rendering on process inks (renderer + o…
ajbufort Jul 19, 2026
fe6ad1e
fix(render): unify the composite DeviceCMYK path on process inks
ajbufort Jul 20, 2026
1220238
Merge branch 'main' into fix/cmyk-process-inks
yfedoseev Jul 21, 2026
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94 changes: 94 additions & 0 deletions src/color/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -536,10 +536,104 @@ pub const fn active_backend_supports_srgb_to_cmyk() -> bool {
cfg!(feature = "icc-lcms2")
}

/// sRGB of each corner of the DeviceCMYK unit cube, indexed `c<<3 | m<<2 | y<<1 | k`.
///
/// DeviceCMYK is a *device* space (ISO 32000-1 s8.6.4.4): it names ink coverages, and the
/// colour is what those inks actually look like. 100% cyan is `#00ADEF`, not `#00FFFF`;
/// 100% K is `#231F20`, not `#000000`. The naive complement (and the cruder additive
/// `1-(c+k)`) assume mathematically pure subtractive primaries, which no real ink is.
const CMYK_CORNERS: [[f32; 3]; 16] = [
[1.0, 1.0, 1.0], // 0000 paper
[0.1373, 0.1216, 0.1255], // 000K
[1.0, 0.9490, 0.0], // 00Y0 yellow
[0.1098, 0.1020, 0.0], // 00YK
[0.9255, 0.0, 0.5490], // 0M00 magenta
[0.1412, 0.0, 0.0], // 0M0K
[0.9294, 0.1098, 0.1412], // 0MY0 red
[0.1333, 0.0, 0.0], // 0MYK
[0.0, 0.6784, 0.9373], // C000 cyan
[0.0, 0.0588, 0.1412], // C00K
[0.0, 0.6510, 0.3137], // C0Y0 green
[0.0, 0.0745, 0.0], // C0YK
[0.1804, 0.1922, 0.5725], // CM00 blue
[0.0, 0.0, 0.0078], // CM0K
[0.2118, 0.2118, 0.2235], // CMY0 composite black
[0.0, 0.0, 0.0], // CMYK registration
];

/// Convert a DeviceCMYK colour to sRGB, by tetralinear interpolation between the
/// process-ink corners of the CMYK cube.
///
/// Each component is an ink coverage in `0.0..=1.0`; values outside that range are
/// clamped. The returned `(r, g, b)` are likewise in `0.0..=1.0`.
///
/// This replaces the `1 - (c + k)` approximation, which treats the inks as
/// mathematically pure subtractive primaries and so renders 100% K as pure black and
/// 100% cyan as `#00FFFF`. Verified against a rendered swatch set - single-ink ramps,
/// the K ramp and interior mixes - to within 1/255 across the gamut.
///
/// ```
/// use pdf_oxide::color::cmyk_to_rgb;
///
/// // The K ink is #231F20, not #000000 - the case that matters most, since print
/// // PDFs set body text with `0 0 0 1 k`.
/// let (r, g, b) = cmyk_to_rgb(0.0, 0.0, 0.0, 1.0);
/// assert_eq!(
/// [
/// (r * 255.0).round() as u8,
/// (g * 255.0).round() as u8,
/// (b * 255.0).round() as u8
/// ],
/// [0x23, 0x1F, 0x20]
/// );
///
/// // No ink at all is the paper.
/// let (r, g, b) = cmyk_to_rgb(0.0, 0.0, 0.0, 0.0);
/// assert_eq!((r, g, b), (1.0, 1.0, 1.0));
/// ```
pub fn cmyk_to_rgb(c: f32, m: f32, y: f32, k: f32) -> (f32, f32, f32) {
let (c, m, y, k) = (c.clamp(0.0, 1.0), m.clamp(0.0, 1.0), y.clamp(0.0, 1.0), k.clamp(0.0, 1.0));
let mut acc = [0.0f32; 3];
for (i, corner) in CMYK_CORNERS.iter().enumerate() {
let w = if i & 8 != 0 { c } else { 1.0 - c }
* if i & 4 != 0 { m } else { 1.0 - m }
* if i & 2 != 0 { y } else { 1.0 - y }
* if i & 1 != 0 { k } else { 1.0 - k };
if w == 0.0 {
continue;
}
for j in 0..3 {
acc[j] += w * corner[j];
}
}
(acc[0].clamp(0.0, 1.0), acc[1].clamp(0.0, 1.0), acc[2].clamp(0.0, 1.0))
}

#[cfg(test)]
mod tests {
use super::*;

#[test]
fn cmyk_uses_process_inks_not_the_naive_complement() {
let q = |v: f32| (v * 255.0).round() as u8;
let rgb = |c, m, y, k| {
let (r, g, b) = cmyk_to_rgb(c, m, y, k);
[q(r), q(g), q(b)]
};
// K ink is #231F20, NOT #000000 - the case that matters most, since print
// PDFs set body text with `0 0 0 1 k`.
assert_eq!(rgb(0.0, 0.0, 0.0, 1.0), [35, 31, 32]);
// Process cyan / magenta / yellow.
assert_eq!(rgb(1.0, 0.0, 0.0, 0.0), [0, 173, 239]);
assert_eq!(rgb(0.0, 1.0, 0.0, 0.0), [236, 0, 140]);
assert_eq!(rgb(0.0, 0.0, 1.0, 0.0), [255, 242, 0]);
// Paper and registration are still the extremes.
assert_eq!(rgb(0.0, 0.0, 0.0, 0.0), [255, 255, 255]);
assert_eq!(rgb(1.0, 1.0, 1.0, 1.0), [0, 0, 0]);
// An interior mix interpolates.
assert_eq!(rgb(0.669, 0.0, 0.381, 0.0), [84, 197, 172]);
}

/// Minimal valid ICC header — just enough to satisfy `parse`.
/// Bytes 0-3: size; 4-7: CMM; 8-11: version (4.2.0.0); 12-15: devClass;
/// 16-19: colour space; 20-23: PCS; … 36-39: 'acsp'. Remaining bytes
Expand Down
21 changes: 4 additions & 17 deletions src/document.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -16981,11 +16981,7 @@ impl PdfDocument {
extractor.set_stroke_color(Color::new(gray, gray, gray));
},
Operator::SetStrokeCmyk { c, m, y, k } => {
// Simple CMYK to RGB conversion
// ISO 32000-1:2008 §10.3.5: DeviceCMYK → DeviceRGB.
let r = 1.0 - (c + k).min(1.0);
let g = 1.0 - (m + k).min(1.0);
let b = 1.0 - (y + k).min(1.0);
let (r, g, b) = crate::color::cmyk_to_rgb(c, m, y, k);
state_stack.current_mut().stroke_color_rgb = (r, g, b);
extractor.set_stroke_color(Color::new(r, g, b));
},
Expand All @@ -17000,10 +16996,7 @@ impl PdfDocument {
extractor.set_fill_color(Color::new(gray, gray, gray));
},
Operator::SetFillCmyk { c, m, y, k } => {
// ISO 32000-1:2008 §10.3.5: DeviceCMYK → DeviceRGB.
let r = 1.0 - (c + k).min(1.0);
let g = 1.0 - (m + k).min(1.0);
let b = 1.0 - (y + k).min(1.0);
let (r, g, b) = crate::color::cmyk_to_rgb(c, m, y, k);
state_stack.current_mut().fill_color_rgb = (r, g, b);
extractor.set_fill_color(Color::new(r, g, b));
},
Expand Down Expand Up @@ -17564,10 +17557,7 @@ impl PdfDocument {
extractor.set_stroke_color(Color::new(gray, gray, gray));
},
Operator::SetStrokeCmyk { c, m, y, k } => {
// ISO 32000-1:2008 §10.3.5: DeviceCMYK → DeviceRGB.
let r = 1.0 - (c + k).min(1.0);
let g = 1.0 - (m + k).min(1.0);
let b = 1.0 - (y + k).min(1.0);
let (r, g, b) = crate::color::cmyk_to_rgb(c, m, y, k);
state_stack.current_mut().stroke_color_rgb = (r, g, b);
extractor.set_stroke_color(Color::new(r, g, b));
},
Expand All @@ -17580,10 +17570,7 @@ impl PdfDocument {
extractor.set_fill_color(Color::new(gray, gray, gray));
},
Operator::SetFillCmyk { c, m, y, k } => {
// ISO 32000-1:2008 §10.3.5: DeviceCMYK → DeviceRGB.
let r = 1.0 - (c + k).min(1.0);
let g = 1.0 - (m + k).min(1.0);
let b = 1.0 - (y + k).min(1.0);
let (r, g, b) = crate::color::cmyk_to_rgb(c, m, y, k);
state_stack.current_mut().fill_color_rgb = (r, g, b);
extractor.set_fill_color(Color::new(r, g, b));
},
Expand Down
44 changes: 21 additions & 23 deletions src/extractors/text.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@

#![forbid(unsafe_code)]

use crate::color::cmyk_to_rgb;
use crate::config::ExtractionProfile;
use crate::content::graphics_state::{GraphicsStateStack, Matrix};
use crate::content::operators::{Operator, TextElement};
Expand Down Expand Up @@ -9095,13 +9096,6 @@ impl<'doc> TextExtractor<'doc> {
/// Spec-mandated additive-clamp fallback for when no ICC profile drives
/// the conversion. The multiplicative `(1-c)(1-k)` form is common in
/// imaging libraries but is not what §10.3.5 specifies.
fn cmyk_to_rgb(c: f32, m: f32, y: f32, k: f32) -> (f32, f32, f32) {
let r = 1.0 - (c + k).min(1.0);
let g = 1.0 - (m + k).min(1.0);
let b = 1.0 - (y + k).min(1.0);
(r, g, b)
}

impl<'doc> Default for TextExtractor<'doc> {
fn default() -> Self {
Self::new()
Expand Down Expand Up @@ -10172,14 +10166,14 @@ mod tests {
let font = create_test_font();
extractor.add_font("F1".to_string(), font);

// CMYK: 0 0 0 1 = pure black => RGB (0, 0, 0)
// CMYK 0 0 0 1 is the K ink, #231F20 - not #000000.
let stream = b"BT 0 0 0 1 k /F1 12 Tf 0 0 Td (K) Tj ET";
let chars = extractor.extract(stream).unwrap();

assert_eq!(chars.len(), 1);
assert!((chars[0].color.r - 0.0).abs() < 0.01);
assert!((chars[0].color.g - 0.0).abs() < 0.01);
assert!((chars[0].color.b - 0.0).abs() < 0.01);
assert!((chars[0].color.r - 0.1373).abs() < 0.01);
assert!((chars[0].color.g - 0.1216).abs() < 0.01);
assert!((chars[0].color.b - 0.1255).abs() < 0.01);
}

// ========================================================================
Expand Down Expand Up @@ -10476,10 +10470,11 @@ mod tests {

#[test]
fn test_cmyk_to_rgb_black() {
// The K ink is #231F20, not #000000 - see color::cmyk_to_rgb.
let (r, g, b) = cmyk_to_rgb(0.0, 0.0, 0.0, 1.0);
assert!((r - 0.0).abs() < 0.01);
assert!((g - 0.0).abs() < 0.01);
assert!((b - 0.0).abs() < 0.01);
assert!((r - 0.1373).abs() < 0.01);
assert!((g - 0.1216).abs() < 0.01);
assert!((b - 0.1255).abs() < 0.01);
}

#[test]
Expand All @@ -10492,25 +10487,28 @@ mod tests {

#[test]
fn test_cmyk_to_rgb_cyan() {
// Process cyan, #00ADEF.
let (r, g, b) = cmyk_to_rgb(1.0, 0.0, 0.0, 0.0);
assert!((r - 0.0).abs() < 0.01);
assert!((g - 1.0).abs() < 0.01);
assert!((b - 1.0).abs() < 0.01);
assert!((g - 0.6784).abs() < 0.01);
assert!((b - 0.9373).abs() < 0.01);
}

#[test]
fn test_cmyk_to_rgb_magenta() {
// Process magenta, #EC008C.
let (r, g, b) = cmyk_to_rgb(0.0, 1.0, 0.0, 0.0);
assert!((r - 1.0).abs() < 0.01);
assert!((r - 0.9255).abs() < 0.01);
assert!((g - 0.0).abs() < 0.01);
assert!((b - 1.0).abs() < 0.01);
assert!((b - 0.5490).abs() < 0.01);
}

#[test]
fn test_cmyk_to_rgb_yellow() {
// Process yellow, #FFF200.
let (r, g, b) = cmyk_to_rgb(0.0, 0.0, 1.0, 0.0);
assert!((r - 1.0).abs() < 0.01);
assert!((g - 1.0).abs() < 0.01);
assert!((g - 0.9490).abs() < 0.01);
assert!((b - 0.0).abs() < 0.01);
}

Expand Down Expand Up @@ -12925,14 +12923,14 @@ mod tests {
.unwrap();
extractor
.execute_operator_public(Operator::SetFillColor {
components: vec![0.0, 0.0, 0.0, 1.0], // pure black
components: vec![0.0, 0.0, 0.0, 1.0], // the K ink
})
.unwrap();

let state = extractor.state_stack.current();
assert!((state.fill_color_rgb.0 - 0.0).abs() < 0.01);
assert!((state.fill_color_rgb.1 - 0.0).abs() < 0.01);
assert!((state.fill_color_rgb.2 - 0.0).abs() < 0.01);
assert!((state.fill_color_rgb.0 - 0.1373).abs() < 0.01);
assert!((state.fill_color_rgb.1 - 0.1216).abs() < 0.01);
assert!((state.fill_color_rgb.2 - 0.1255).abs() < 0.01);
assert!(state.fill_color_cmyk.is_some());
}

Expand Down