Handle UI display color space in MaterialX Viewer (#2394)

This PR fixes discrepancies between the rendered preview material in MaterialXView and the NanoGUI color picker. In both the `Color3` and `Color4` cases, it completes the following conversions:

1. Uses `Color3::linearToSrgb()` to convert the stored value in "lin_rec709" color space to sRGB space for use in the color picker.
2. After user selects a color in the GUI and presses the "Pick" button, converts the selected RGB values to linear space using the `Color3::srgbToLinear()` conversion function. The converted linear color value is what is ultimately written to the material.

In the `Color4` case, the alpha channel is left untouched, both during reading and writing.

Author: mialana

source/MaterialXView/Editor.cpp

@@ -327,8 +327,11 @@ void PropertyEditor::addItemToForm(const mx::UIPropertyItem& item, const std::st
         }
         else
         {
+            // Transform stored linear space color3 value to sRGBA for nanogui color picker
             mx::Color3 v = value->asA<mx::Color3>();
-            ng::Color c(v[0], v[1], v[2], 1.0);
+            mx::Color3 displayCol = v.linearToSrgb();
+
+            ng::Color c(displayCol[0], displayCol[1], displayCol[2], 1.0);
 
             new ng::Label(twoColumns, label);
             auto colorVar = new EditorColorPicker(twoColumns, c);
@@ -339,7 +342,10 @@ void PropertyEditor::addItemToForm(const mx::UIPropertyItem& item, const std::st
                 mx::MaterialPtr material = viewer->getSelectedMaterial();
                 if (material)
                 {
-                    mx::Vector3 v(c.r(), c.g(), c.b());
+                    // Transform sRGBA color picker value to linear space for writing to material
+                    mx::Color3 linearCol = mx::Color3(c.r(), c.g(), c.b()).srgbToLinear();
+                    mx::Vector3 v(linearCol[0], linearCol[1], linearCol[2]);
+
                     material->modifyUniform(path, mx::Value::createValue(v));
                 }
             });
@@ -354,7 +360,10 @@ void PropertyEditor::addItemToForm(const mx::UIPropertyItem& item, const std::st
 
         new ng::Label(twoColumns, label);
         mx::Color4 v = value->asA<mx::Color4>();
-        ng::Color c(v[0], v[1], v[2], v[3]);
+        mx::Color3 displayCol = mx::Color3(v[0], v[1], v[2]).linearToSrgb();
+
+        ng::Color c(displayCol[0], displayCol[1], displayCol[2], v[3]);
+
         auto colorVar = new EditorColorPicker(twoColumns, c);
         colorVar->set_fixed_size({ 100, 20 });
         colorVar->set_font_size(15);
@@ -363,7 +372,10 @@ void PropertyEditor::addItemToForm(const mx::UIPropertyItem& item, const std::st
             mx::MaterialPtr material = viewer->getSelectedMaterial();
             if (material)
             {
-                mx::Vector4 v(c.r(), c.g(), c.b(), c.w());
+                // Transform sRGBA color picker value to linear space for writing to material
+                mx::Color3 linearCol = mx::Color3(c.r(), c.g(), c.b()).srgbToLinear();
+                mx::Vector3 v(linearCol[0], linearCol[1], linearCol[2]);
+
                 material->modifyUniform(path, mx::Value::createValue(v));
             }
         });