Added ability to convert linear RGB tuple to sRGB

cadquery/occ_impl/assembly.py

@@ -118,6 +118,23 @@ def toTuple(self) -> Tuple[float, float, float, float]:
 
         return (rgb.Red(), rgb.Green(), rgb.Blue(), a)
 
+    def shiftForExport(self):
+        def _srgb_to_linear(c):
+            """Convert a single sRGB channel (0.0–1.0) to linear RGB."""
+            if c <= 0.04045:
+                return c / 12.92
+            else:
+                return ((c + 0.055) / 1.055) ** 2.4
+
+        # Pre-shift the colors so that when they are converted again during export they end back up at the expected values
+        rgba = self.toTuple()
+        return Color(
+            _srgb_to_linear(rgba[0]),
+            _srgb_to_linear(rgba[1]),
+            _srgb_to_linear(rgba[2]),
+            rgba[3],
+        )
+
     def __getstate__(self) -> Tuple[float, float, float, float]:
 
         return self.toTuple()

cadquery/occ_impl/exporters/assembly.py

@@ -176,7 +176,9 @@ def _process_child(child: AssemblyProtocol, assy_label: TDF_Label):
             part_label = shape_tool.AddShape(shape.wrapped, False)
             TDataStd_Name.Set_s(part_label, TCollection_ExtendedString(name))
             if color:
-                color_tool.SetColor(part_label, color.wrapped, XCAFDoc_ColorGen)
+                color_tool.SetColor(
+                    part_label, color.shiftForExport().wrapped, XCAFDoc_ColorGen
+                )
             shape_tool.AddComponent(assy_label, part_label, loc.wrapped)
 
             # If this assembly has shape metadata, add it to the shape
@@ -207,7 +209,9 @@ def _process_child(child: AssemblyProtocol, assy_label: TDF_Label):
                             # Set the individual face color
                             if face in colors:
                                 color_tool.SetColor(
-                                    face_label, colors[face].wrapped, XCAFDoc_ColorGen,
+                                    face_label,
+                                    colors[face].shiftForExport().wrapped,
+                                    XCAFDoc_ColorGen,
                                 )
 
                             # Also add a layer to hold the face label data

tests/test_assembly.py

@@ -560,6 +560,29 @@ def test_color():
         cq.Color(1, 2, 3, 4, 5)
 
 
+def test_step_color(tmp_path_factory):
+    # Use a temporary directory
+    tmpdir = tmp_path_factory.mktemp("out")
+    step_color_path = os.path.join(tmpdir, "step_color.step")
+
+    # Create a simple assembly with color
+    assy = cq.Assembly()
+    assy.add(cq.Workplane().box(10, 10, 10), color=cq.Color(0.47, 0.253, 0.18, 1.0))
+
+    success = exportStepMeta(assy, step_color_path)
+    assert success
+
+    # Read the file as a string and check for the correct colors
+    with open(step_color_path, "r") as f:
+        step_content = f.readlines()
+        # Step through and try to find the COLOUR line
+        for line in step_content:
+            if "COLOUR_RGB(''," in line:
+                assert "0.47" in line
+                assert "0.25" in line
+                assert "0.18" in line
+
+
 def test_assembly(simple_assy, nested_assy):
 
     # basic checks