feat(color): derive primaries for color spaces without a known ID

src/doc/pythonbindings.md

@@ -4044,6 +4044,9 @@ is provided for minimal color support.
 Get the RGB primaries and white point of a color space with a known Color
 Interop ID, as `get_color_interop_id()` finds it: a sequence of eight floats
 (Rx, Ry, Gx, Gy, Bx, By, Wx, Wy, CIE 1931 xy), or None if they are unknown.
+A color space of the config without such an ID, or whose ID OpenImageIO has no
+primaries for, has them derived by analyzing its transforms, unless the
+`colorconfig:analyze_transforms` attribute is 0.
 
 Example:
 
@@ -4059,7 +4062,10 @@ This function was added in OpenImageIO 3.2.
 :::{py:method} get_transfer_function_gamma (colorspace)
 Get the exponent of a color space's transfer function if it decodes as a pure
 power, 1.0 if it is linear, or 0 if it is neither or is unknown. Piecewise
-curves such as sRGB's report 0.
+curves such as sRGB's report 0. A color space whose config declares an
+encoding is linear only if that encoding is linear for its reference type.
+Otherwise it answers for a known Color Interop ID, or, like
+`get_chromaticities()`, by analyzing transforms.
 
 Example:
 

src/include/OpenImageIO/color.h

@@ -453,18 +453,21 @@ class OIIO_API ColorConfig {
     /// Color Interop ID
     /// (https://github.com/AcademySoftwareFoundation/ColorInterop), as
     /// `get_color_interop_id()` finds it: eight floats (Rx, Ry, Gx, Gy, Bx,
-    /// By, Wx, Wy, CIE 1931 xy), or an empty span if they are unknown. The
-    /// span is valid until this ColorConfig is reset or destroyed.
+    /// By, Wx, Wy, CIE 1931 xy), or an empty span if they are unknown. A
+    /// color space of the config without such an ID, or whose ID OpenImageIO
+    /// has no primaries for, has them derived by analyzing its transforms,
+    /// unless the `colorconfig:analyze_transforms` attribute is 0. The span
+    /// is valid until this ColorConfig is reset or destroyed.
     ///
     /// @version 3.2
     OIIO_NODISCARD cspan<float> get_chromaticities(string_view colorspace) const;
 
     /// Get the exponent of a color space's transfer function if it decodes
     /// as a pure power, 1.0 if it is linear, or 0 if it is neither or is
-    /// unknown. Piecewise curves such as sRGB's report 0. Like
-    /// `get_chromaticities()`, it answers for a known Color Interop ID, and
-    /// it reports 1.0 for any color space whose config declares a linear
-    /// encoding.
+    /// unknown. Piecewise curves such as sRGB's report 0. A color space whose
+    /// config declares an encoding is linear only if that encoding is linear
+    /// for its reference type. Otherwise it answers for a known Color Interop
+    /// ID, or, like `get_chromaticities()`, by analyzing transforms.
     ///
     /// @version 3.2
     OIIO_NODISCARD float

src/include/OpenImageIO/imageio.h

@@ -4064,6 +4064,14 @@ OIIO_API std::string geterror(bool clear = true);
 ///   enable globally in an environment where security is a higher priority
 ///   than being tolerant of partially broken image files.
 ///
+/// - `colorconfig:analyze_transforms` (int: 1)
+///
+///   If nonzero (the default), `ColorConfig::get_chromaticities()` and
+///   `get_transfer_function_gamma()` may analyze the transforms of a color
+///   space with no known Color Interop ID to derive its primaries and
+///   whether it is linear. If zero, they answer only from what the config
+///   declares and the names OpenImageIO knows.
+///
 /// - `ustring:cleanup` (int: 0)
 ///
 ///    If nonzero, upon exit, do a thorough (and possibly expensive) teardown

src/include/imageio_pvt.h

@@ -50,6 +50,7 @@ extern int limit_resolution;
 extern int imagebuf_print_uncaught_errors;
 extern int imagebuf_use_imagecache;
 extern int imageinput_strict;
+extern int colorconfig_analyze_transforms;
 extern atomic_ll IB_local_mem_current;
 extern atomic_ll IB_local_mem_peak;
 extern std::atomic<float> IB_total_open_time;

src/libOpenImageIO/CMakeLists.txt

@@ -58,7 +58,7 @@ set (libOpenImageIO_srcs
                           imageinput.cpp imageio.cpp imageioplugin.cpp
                           imageoutput.cpp
                           iptc.cpp xmp.cpp
-                          color_ocio.cpp
+                          color_ocio.cpp color_derive.cpp
                           maketexture.cpp
                           bluenoise.cpp
                           printinfo.cpp
@@ -209,7 +209,8 @@ if (CMAKE_UNITY_BUILD)
         # ${libOpenImageIO_srcs}
             deepdata.cpp exif.cpp exif-canon.cpp formatspec.cpp imagebuf.cpp
             imageinput.cpp imageio.cpp imageioplugin.cpp imageoutput.cpp
-            iptc.cpp xmp.cpp color_ocio.cpp maketexture.cpp bluenoise.cpp
+            iptc.cpp xmp.cpp color_ocio.cpp color_derive.cpp maketexture.cpp
+            bluenoise.cpp
         PROPERTIES
             UNITY_GROUP oiiolib)
     foreach (plugin_dir ${all_format_plugin_dirs} ../libtexture)