rendlay.cpp: Add support in the layout color component to specify colors by hex rgb(a) values.

Also uses the Ensoniq VFX family of layouts to show it working,
as well as documenting it.

Author: cbrunschen

docs/source/techspecs/layout_files.rst

@@ -118,8 +118,8 @@ intensity).  Alpha ranges from 0.0 (fully transparent) to 1.0 (opaque).  Colour
 channel values are not pre-multiplied by the alpha value.
 
 Component and view item colour is specified using ``color`` elements.
-Meaningful attributes are ``red``, ``green``, ``blue`` and ``alpha``.  This
-example ``color`` element specifies all channel values:
+Meaningful attributes are ``red``, ``green``, ``blue`` and ``alpha`` and ``hex``.  This
+example ``color`` element specifies all channel values individually:
 
 .. code-block:: XML
 
@@ -129,6 +129,20 @@ Any omitted channel attributes default to 1.0 (full intensity or opaque).  It
 is an error if any channel value falls outside the range of 0.0 to 1.0
 (inclusive).
 
+Alternatively, you can specify a single hex value, for either RGB (six hex digits)
+or RGBA (eight hex digits):
+
+.. code-block:: XML
+
+    <color hex="d9664c" />
+
+(RGB) or
+
+.. code-block:: XML
+
+    <color hex="d9664cff" />
+
+(RGBA).
 
 .. _layfile-concepts-params:
 

src/emu/rendlay.cpp

@@ -887,6 +887,39 @@ class layout_environment
 		if (!node)
 			return render_color{ 1.0F, 1.0F, 1.0F, 1.0F };
 
+		if (node->has_attribute("hex"))
+		{
+			// Use the single hex attribute
+			if (node->has_attribute("red") || node->has_attribute("green") || node->has_attribute("blue") || node->has_attribute("alpha"))
+				throw layout_syntax_error(util::string_format("specify either hex or (red,green,blue) colors"));
+
+			const std::string hex{node->get_attribute_string("hex", "")};
+			auto l = hex.length();
+			if (l == 6 || l == 8)
+			{
+				std::size_t pos;
+				unsigned long val = std::stoul(hex, &pos, 16);
+				if (pos != l)
+					throw layout_syntax_error(util::string_format("illegal hex color %s", hex));
+
+				float alpha = 1.0;
+				size_t offset = 0;
+				if (l == 8)
+				{
+					alpha = BIT(val, 0, 8) / 255.0;
+					offset = 8;
+				}
+				float blue  = BIT(val, offset +  0, 8) / 255.0;
+				float green = BIT(val, offset +  8, 8) / 255.0;
+				float red   = BIT(val, offset + 16, 8) / 255.0;
+				return render_color { alpha, red, green, blue };
+			}
+			else
+			{
+				throw layout_syntax_error(util::string_format("illegal hex color %s", hex));
+			}
+		}
+
 		// parse attributes
 		render_color const result{
 				get_attribute_float(*node, "alpha", 1.0F),