Materials

Materials define the surface properties of the geometry. For the standard Phong lighting model that OpenGL uses these are ambient, diffuse and specular color as well as shininess. However, this is an area where extensions are added at an amazing pace. The purpose of materials is to add a level of abstraction and give the user an easy to use interface to define surface properties without having to worry about the details of realizing them (e.g. how to use multiple passes, when and how to calculate derived information like tangents and binormals etc.).

This area is quickly expanding, so what we have right now is just the beginning.

Ext: To use a Material the geometry will call the Material's osg::Material::draw() method, which decides what to do. Right now all it can do is directly draw itself or pass itself to the Rendering Backend to be rendered later. To actually be used by the renderer Materials have a method to give out a osg::State to be used to render their associated geometry. For efficiency reasons this State should be kept ready to use in the Material and returned when needed.

This whole interface is a bit convoluted, unclear, can't handle all the cases that it should and thus will change pretty soon. Stay tuned.

Material types

ChunkMaterial

The osg::ChunkMaterial is a material that is just a collection of osg::StateChunks. This allows adding and using new extensions in the form of state chunks pretty easily.

It is also the base class for the simple materials given below.

SimpleMaterial

The osg::SimpleMaterial is a pretty direct mapping of the OpenGL light model. It has colors for ambient, diffuse, specular and emission properties, and a shininess value. In addition to that it has a transparency setting, ranging from 0 for opaque to 1 for fully transparent.

There are two other attributes in a SimpleMaterial that control the appearance of an object. One is the lit attribute, which defines if the material is influenced by light sources at all. If it isn't, the color is directly taken from the diffuseColor component and other color attributes are ignored.

The other attribute is the colorMaterial field, which defines how colors that are given in the geometry influence the lighting calculation. By default they replace the diffuse color only. Possible values are taken from the glColorMaterial() call, the most useful being GL_DIFFUSE_AND_SPECULAR. One possible value that is not used by glColorMaterial() is GL_NONE, which switches off the color material handling and thus ignores colors that are given in the geometry.

As SimpleMaterial is derived from osg::ChunkMaterial, other attributes can be added in the form of StateChunks.

TexturedSimpleMaterial

osg::SimpleTexturedMaterial is derived from osg::SimpleMaterial and adds a texture. The texture is defined by an image (see Image for details on how to define or load an image).

Additionally there are some parameters to define the behavior of a texture. magFilter and minFilter define how to scale the texture image up or down, legal values taken from glTexParameter(). The most useful ones are GL_NEAREST or GL_LINEAR for magFilter, and additionally GL_LINEAR_MIPMAP_LINEAR for minFilter.

envMode defines how a color from the texture is combined with a color from the lighting calculation. The default is GL_REPLACE which completely ignores the lighting color. Other useful values are GL_MODULATE, which just multiplies the two, and GL_DECAL, which interpolates between lighting and texture based on the texture's alpha channel.

Finally, a texture can be used as a spherical environment map to simulate a reflective object by setting the envMap field to true. Spherical environment maps need to display the image of a reflective sphere in the middle of the environment that is being reflected.

As TexturedSimpleMaterial is derived from osg::ChunkMaterial, other attributes can be added in the form of StateChunks.

---