/*!

\page material_page Material 

<ul>
<li>\ref material_general_description_anchor "General Description"</li>
<li>\ref material_opengl_model_anchor "OpenGL ligthing model" </li>
<li>\ref material_lib_anchor "Materials library"</li>
<li>\ref material_anchor "Material properties"</li>
</ul>

\anchor material_general_description_anchor <h2>General description</h2>

\note The functionality related to the material properties is
\b experimental, so it might work not as expected. The behavior might
be changed in the future versions of SALOME Geometry module.

You can change the material properties of the selected shape(s) in
the context menu dialog. The layout of context menu can be customized via
"Show predefined materials in popup menu" preferences option.
If this option is switched off, only "Material properties" item will 
be shown in the popup menu. If this option is on (by default), "Material 
properties" item in the popup menu will open a sub-menu with a list of predefined 
materials:

\image html hide_predef_material.png
<center><em>"Show predefined materials in popup menu" option is switched off</em></center>

\n\image html show_predef_material.png
<center><em>"Show predefined materials in popup menu" option is switched on</em></center>

The \b Custom item from this list allows defining \ref material_anchor "Material properties",
including the creation of a custom material. 

It is also possible to define custom materials in the
\ref material_lib_anchor "Materials library" dialog 
available from the main menu via <b>Tools > Materials library </b>. 

\note This functionality works in both OCC and VTK 3D
viewers. However, due to the differences between underlying API
of OCC and VTK libraries, the behavior of the functionality related to
the materials is different:
- shape presentation in OCC and VTK viewers is not fully identical;
- some material attributes can affect the presentation in a different way.

\anchor material_opengl_model_anchor <h2>OpenGL lighting model</h2>

The material is specified by several attributes of the lighting
model. More details can be found in the documentation related to the
OpenGL programming, for example, here: http://www.glprogramming.com/red/chapter05.html.

In the OpenGL lighting model, the light in a scene comes from several
light sources; the light sources have an effect only when there are
surfaces that absorb and reflect light. Each surface is assumed to be
composed of a material with various properties. A material might emit
its own light (like headlights of a vehicle), it might scatter
some incoming light in all directions, and it might reflect a
portion of the incoming light in a preferential direction like a
mirror or other shiny surface.

The OpenGL lighting model considers the lighting to be divided into
four independent components: emissive, ambient, diffuse, and
specular. All four components are computed independently and then
added together. 

- The \b Ambient illumination is the light that has been scattered so much by the
environment that its direction is impossible to determine - it seems
to come from all directions. Backlighting in a room has a large
ambient component, since most of the light that reaches your eye has
first bounced off many surfaces. A spotlight outdoors has a tiny
ambient component; most of the light travels in the same direction,
and since you're outdoors, very little of the light reaches your eye
after bouncing off other objects. When ambient light strikes a
surface, it is scattered equally in all directions.

- The \b Diffuse component is the light that comes from one direction, so
it is brighter if it comes squarely down on a surface than if it barely
glances off the surface. Once it hits a surface, however, it's
scattered equally in all directions, so it appears equally bright, no
matter where the eye is located. Any light coming from a particular
position or direction probably has a diffuse component.

- The \b Specular light comes from a particular direction, and it
tends to bounce off the surface in a preferred direction. A
well-collimated laser beam bouncing off a high-quality mirror produces
specular reflection by almost 100 percent. Shiny metal or plastic has a
high specular component, and chalk or carpet has almost none. You can
think of specularity as shininess.

- The \b Emissive color simulates light originating from an object. 
In the OpenGL lighting model, the emissive color of a surface adds
intensity to the object, but is unaffected by any light sources. Also,
the emissive color does not introduce any additional light into the
overall scene.

Although a light source delivers a single distribution of frequencies,
the ambient, diffuse, and specular components might be different. For
example, if you have a white light in a room with red walls, the
scattered light tends to be red, although the light directly striking
objects is white. OpenGL allows you to set the red, green, and blue
values for each component of light independently.  

\anchor material_lib_anchor <h2>Materials library</h2>

\image html materials_library.png

The dialog consists of two parts:

<ul>
<li>The list to the left shows all available material models, both
predefined and custom.
- <b>[Current]</b> item in the list corresponds to the material model
  currently assigned to the selected shape(s). This model can be
  freely modified by the user.
- <b>Global</b> material models are shown in blue color in the list;
  these are the models predefined by the SALOME Geometry module. The user
  is not allowed to modify the global models.
- <b>User</b> materials are shown in black color in the list. These
  models are specified by the user and can be modified at any moment.
</li>

<li>The widgets to the right allow modifying different properties of the material model:
- \b Ambient color and coefficient (floating point value between 0 and 1)
- \b Diffuse color and coefficient (floating point value between 0 and 1)
- \b Specular color and coefficient (floating point value between 0 and 1)
- \b Emissive color and coefficient (floating point value between 0
and 1). Note: this attribute is applicable only for the OCC viewer;
it simulates light originating from an object.
- \b Shininess
- \b Type of material model: \em physical or \em artificial.
</li>

<li>The buttons <b>Add material</b> and <b>Remove material</b> in the
lower part of the dialog box can be used to create or remove custom
material models. The same commands are also available via the context
menu in the materials list. <b>Rename material</b> command can be used
to change the name of material model.</li>
</ul>

\anchor material_anchor <h2>Material properties</h2>

\image html material.png

In addition to the functionality of <b>Materials library</b>, this
dialog provides objects selection mechanism and \b Color property.

If the material model is specified as a \em physical (\em Gold,
for example), the shape color (more precisely its \em ambient color)
cannot be modified. If you assign a physical material model to the
shape, the "Color" menu item will not be available in the popup menu.

If the model is non-physical (\em artificial), the color can be changed
to any appropriate one, only other attributes will be constant. In the
dialog box you will be able to modify the color of the shape via the
"Color" button. "Ambient color" button will be disabled to signalize
that this attribute of the model is ignored. Also, it will be possible
to modify the color of the shape via the
\ref color_page "corresponding popup menu command".

<b>Examples:</b>

\image html material_OCC.png
<center><em>Various materials in OCC viewer</em></center>

\n\image html material_VTK.png
<center><em>Various materials in VTK viewer</em></center>

The default material model is specified via the preferences of Geometry
module.

*/