From a7122d7a85b25887d0af60119e2e60036acaf8e6 Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Wed, 16 Jan 2019 21:02:01 +0000
Subject: [PATCH] revise XYZ2CMYK fallback path

better epsilon detect, add a test
---
 libvips/colour/XYZ2CMYK.c      | 36 ++++++++++++++++------------------
 test/test-suite/test_colour.py |  9 ++++++++-
 2 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/libvips/colour/XYZ2CMYK.c b/libvips/colour/XYZ2CMYK.c
index 475f9235..6d03a7f1 100644
--- a/libvips/colour/XYZ2CMYK.c
+++ b/libvips/colour/XYZ2CMYK.c
@@ -150,26 +150,24 @@ vips_XYZ2CMYK_line( VipsColour *colour, VipsPel *out, VipsPel **in, int width )
 		float g = p[1] / VIPS_D65_Y0;
 		float b = p[2] / VIPS_D65_Z0;
 
-		float c, m, y, k;
+		float c = 1.0 - r;
+		float m = 1.0 - g;
+		float y = 1.0 - b;
+		float k = VIPS_MIN( c, VIPS_MIN( m, y ) );
+		float ik = 1.0 - k;
 
-		c = 255.0 - r;
-		m = 255.0 - g;
-		y = 255.0 - b;
-		k = VIPS_MIN( c, VIPS_MIN( m, y ) );
-
-		c = (c - k) / (255.0 - k) * 255.0;
-		m = (m - k) / (255.0 - k) * 255.0;
-		y = (y - k) / (255.0 - k) * 255.0;
-
-		q[0] = (unsigned char) VIPS_CLIP(0, c, 255.0);
-		q[1] = (unsigned char) VIPS_CLIP(0, m, 255.0);
-		q[2] = (unsigned char) VIPS_CLIP(0, y, 255.0);
-		if( r < epsilon && 
-			g < epsilon && 
-			b < epsilon )
-			q[3] = 255.0;
-		else
-			q[3] = VIPS_CLIP( 0, k, 255 );
+		if( ik < epsilon ) {
+			q[0] = 255;
+			q[1] = 255;
+			q[2] = 255;
+			q[3] = 255;
+		}
+		else {
+			q[0] = VIPS_CLIP( 0, 255 * (c - k) / ik, 255 );
+			q[1] = VIPS_CLIP( 0, 255 * (m - k) / ik, 255 );
+			q[2] = VIPS_CLIP( 0, 255 * (y - k) / ik, 255 );
+			q[3] = VIPS_CLIP( 0, 255 * k, 255 );
+		}
 
 		p += 3;
 		q += 4;
diff --git a/test/test-suite/test_colour.py b/test/test-suite/test_colour.py
index 1e2371f2..b8cb0754 100644
--- a/test/test-suite/test_colour.py
+++ b/test/test-suite/test_colour.py
@@ -3,7 +3,7 @@ import pytest
 
 import pyvips
 from helpers import JPEG_FILE, SRGB_FILE, colour_colourspaces, \
-    mono_colourspaces, assert_almost_equal_objects
+    mono_colourspaces, assert_almost_equal_objects, skip_if_no
 
 
 class TestColour:
@@ -124,6 +124,7 @@ class TestColour:
         assert abs(result - 4.97) < 0.5
         assert pytest.approx(alpha, 0.001) == 42.0
 
+    @skip_if_no("icc_import")
     def test_icc(self):
         test = pyvips.Image.new_from_file(JPEG_FILE)
 
@@ -163,6 +164,12 @@ class TestColour:
         im = test.icc_import()
         assert im.interpretation == pyvips.Interpretation.LAB
 
+    # even without lcms, we should have a working approximation
+    def test_cmyk(self):
+        test = pyvips.Image.new_from_file(JPEG_FILE)
+
+        im = test.colourspace("cmyk").colourspace("srgb")
+
 
 if __name__ == '__main__':
     pytest.main()