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>Blender Documentation Volume I - User Guide: Last modified April 29 2004 S68</TH
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>Materials and textures</H1
><P
>Before you can understand how to design effectively with materials, you
must understand how
simulated light and surfaces interact in Blender's rendering engine and
how material settings control those interactions. A deep
understanding of the engine will help you to get the most from it.
</P
><P
>The rendered image you create with Blender is
a projection of the scene onto an imaginary surface called the
<I
CLASS="emphasis"
>viewing plane</I
>.
The viewing plane is analogous to the film in a traditional camera, or the
rods and cones in the human eye, except that it receives simulated light, not
real light.
</P
><P
> To
render an image of a scene we must first determine what light from
the scene is arriving at each point on the viewing plane. The best way to answer
this question is to follow a straight line (the simulated light
ray) backwards through that point on the viewing plane and the focal
point (the location of the camera) until it hits a renderable surface in the
scene, at which point we can determine what light would strike that point. The
surface properties and incident light angle tell us how much of that
light would be reflected back along the incident viewing angle
(<A
HREF="c3943.html#BSG.MAT.F.S68.101"
>Figure 1</A
>).
</P
><DIV
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><IMG
SRC="PartM/materials/gfx/MatGen01.png"></P
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><P
><B
>Figure 1. Rendering engine basic principle.</B
></P
></DIV
><P
>Two basic types of phenomena take place at any
point on a surface when a light ray strikes it: diffusion and specular
reflection.
Diffusion and specular reflection are
distinguished from each other mainly by the relationship between the incident
light angle and the reflected light angle.
</P
><DIV
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><H1
CLASS="section"
><A
NAME="materials_diffusion"
></A
>Diffusion</H1
><P
>Light striking a surface and then re-irradiated via a Diffusion
phenomenon will be scattered, i.e., re-irradiated
in all directions isotropically. This means that the camera will see
the same amount of light from that surface point no matter what the
<I
CLASS="emphasis"
>incident viewing angle</I
> is.
</P
><P
> It is this quality that makes
diffuse light <I
CLASS="emphasis"
>viewpoint independent</I
>. Of course
the amount of light that strikes the surface depends on the incident light
angle. If most of the light striking a surface is reflected diffusely, the
surface will have a matte appearance
(<A
HREF="c3943.html#BSG.MAT.F.S68.102"
>Figure 2</A
>).
</P
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><P
><IMG
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><P
><B
>Figure 2. Light re-irradiated in the diffusion phenomenon.</B
></P
></DIV
><P
> Since version 2.28, Blender has implemented three different mathematical
formulae to compute diffusion. And, even more notably, the diffusion
and specular phenomena, which are usually bound in a single type of
material, have been separated so that it is possible to select diffusion and
specular reflection implementation separately.
</P
><P
> The three Diffusion implementations, or <I
CLASS="emphasis"
>shaders</I
>, use two or
more
parameters each. The first two parameters are shared by all Diffuse Shaders
and are the <I
CLASS="emphasis"
>Diffuse color</I
>,
or simply <I
CLASS="emphasis"
>color</I
>, of the material, and the amount
of incident light energy that is actually diffused. This latter quantity, given
in a [0,1] range, is actually called <TT
CLASS="literal"
>Refl</TT
> in the interface.
</P
><P
> The implemented shaders are:
</P
><P
></P
><UL
><LI
><P
><I
CLASS="emphasis"
>Lambert</I
> -- This was Blender's default
diffuse shader
up to version 2.27. As such, all old tutorials refer to this, and all
pre-2.28 images were created with this. This shader has only the default
parameters.
</P
></LI
><LI
><P
><I
CLASS="emphasis"
>Oren-Nayar</I
> -- This shader was first
introduced in
Blender 2.28. It takes a somewhat more 'physical' approach to the diffusion
phenomena because, besides the two default parameters, it has a third
one which is used to determine the amount of microscopical roughness of the
surface.
</P
></LI
><LI
><P
><I
CLASS="emphasis"
>Toon</I
> -- This shader was first
introduced in
Blender 2.28. It is a very 'un-physical' shader in that it is not
meant to fake reality but to produce 'toonish' rendering, with
clear light-shadow boundaries and uniformly lit/shadowed regions.
Even though it is relatively simple, it still requires two more parameters
which define the size of the lit area and the sharpness of the shadow
boundaries.
</P
></LI
></UL
><P
>A subsequent section, devoted to the actual implementation of the
material,
will analyze all these and their relative settings.
</P
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