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There are many types of shaders, all of which can be substituted by
user-written shaders:
-
material shaders describe the visible material of an object. They
are the only mandatory part of any material description. Material
shaders are called whenever a visible ray (eye ray, reflected ray,
refracted ray, or transparency ray) hits an object. Material shaders
have a central function in mental ray.
-
volume shaders are called to account for atmospheric effects
encountered by a ray. The state (see below) distinguishes two types of
volume shaders: the standard volume shader that is called in most
cases, and the refraction volume shader that is taken from the
object material at the current intersection point, and becomes the
standard volume shader if a refraction
or transparency ray is cast. Many material shaders substitute a new
standard volume shader based on inside/outside calculations.
Volume shaders, unlike other shaders, accept an input color (such as
the one calculated by the material shader at the last intersection
point) that they are expected to modify.
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light shaders implement the characteristics of a light source.
For example, a spot light shader would use the illumination direction to
attenuate the amount of light emitted. A light shader is called whenever
a material shader uses a built-in function to evaluate a light. Light
shaders normally cast shadow rays if shadows are enabled to detect
obscuring objects between the light source and the illuminated point.
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shadow shaders are called instead of material shaders when a
shadow ray intersects with an object. Shadow rays are cast by light
sources to determine visibility of an illuminated object. Shadow shaders
are basically light-weight material shaders that calculate the
transmitted color of an object without casting secondary or shadow rays.
Frequently, material shaders are written such that they can also be
used as shadow shaders.
-
environment shaders are called instead of a material shader when
a visible ray leaves the scene entirely without intersecting an object.
Typical environment shaders evaluate a texture mapped on a virtual
infinite sphere enclosing the scene (virtual because it is not part of
the scene geometry).
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photon shaders are used to propagate photons through the model in
order to simulate caustics and global illumination2.1.
Photon shaders are used in a preprocessing step in which photons are
emitted from the light sources into the model (just as a real light
source emits photons into the world). Each photon is traced through the
scene using a technique called photon tracing which is similar to
ray tracing. As with ray tracing a photon is reflected of a specular
mirror surface in the mirror direction. The most important difference
is the fact that the photon shader modifies the photon energy before reflecting the photon unlike ray tracing which traces a ray and
then modifies the result accordingly (for example multiplies it with
the specular reflection coefficients). Photon shaders also store
information about the incoming photon in a global photon map which
contains all photons stored in the model. This photon map is then used
by the material shaders during the rendering step (ray tracing step) to
simulate caustics and global illumination2.1. Frequently, material
shaders are written such that they can also be used as photon shaders
(and also shadow shaders).
-
photon emitter shaders are used to control the emission of
photons from a light source. Combined with the light shaders it is
possible to simulate complex light sources with complex emission
characteristics. Photon emitters are only used if caustics or
global illumination2.1are enabled, to construct a photon map
before the actual rendering takes place.
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texture shaders come in three flavors: color, scalar, and vector.
Each calculates and returns the respective type. Typical texture shaders
return a color from a texture image after some appropriate coordinate
transformation, or compute a color at a location in 3D space using some
sort of noise function. Their main purpose is to relieve other shaders,
such as material or environment shaders, from performing color and other
computations. For example, if a marble surface were needed, it should
be written as a texture shader and not a material shader because a
texture shader does not have to calculate illumination by light sources,
reflections, and so on. It is much easier to write a texture shader
than a material shader. mental ray never calls a texture shader directly,
it is always called from one of the other types of shaders.
-
displacement shaders are called during tessellation of polygonal
or free-form surface geometry, a procedure that creates triangles to
be rendered. Displacement shaders are called to shift the created
vertices along their normals by a scalar distance returned by the
shader. mental ray supports approximation controls that allow
adjusting the tessellation to better resolve curvature introduced by
displacement shaders.
-
geometry shaders are run before rendering begins. They create
geometry procedurally by using a function call library that closely
follows the .mi2 scene description language. Unlike displacement
shaders, which are called once per vertex, geometry shaders are
responsible for creating an entire object or object hierarchy (each
of which, when tessellated later, can cause displacement shader calls).
-
contour shaders come in four different flavors: contour store
functions, contour contrast shaders, contour shaders, and contour
output shaders. For details see chapter
.
-
lens shaders are called when a primary ray is cast by
the camera. They may modify the eye ray's origin and direction to
implement cameras other than the standard pinhole camera, and may
modify the result of the primary ray to implement effects such as
lens flares.
-
output shaders are different from all other shaders and receive
different parameters. They are called when the entire scene has been
completely rendered and the output image resides in memory. Output
shaders operate on the output image to implement special filtering or
compositing operations. Output shaders are not associated with any
particular ray because they are called after the last ray is completed.
The following diagram illustrates the path of a ray cast by the camera.
It first intersects with a sphere at point A. The sphere's material
shader first casts a reflection ray that hits a box, then a refraction
ray that intersects the sphere at its other side T, and finally it
casts a transparency ray that also intersects the sphere, at D. (This
example is contrived, it is very unusual for a material shader to
cast both a refraction and a transparency ray.) The same material shader
is called at points A, T, and D. In this example, the reflection trace
depth may have prevented further reflection rays to be cast at T and D.
The annotations set in italics are numbered; the events described
happen in the sequence given by the numbers.
Since material shaders may do inside/outside calculations based on
the surface normal or the parent state chain (see below), the volume
shaders are marked (1) and (2), depending on whether the volume shader
left by A or by T/D in the refraction volume field of the state.
The default refraction volume shader is the one found in the material
definition, or the standard volume shader if the material defines no
volume shader. For details on choosing volume shaders, see the section
on writing material and volume shaders. Note that the volume shaders
in this diagram are called immediately after the material shader
returns.
The next two diagrams depict the situation when the material shader at
the intersection point M requests a light ray from the light source at
L, by calling a function such as mi_sample_light. This results
in the light shader of L to be called. No intersection testing is done
at this point. Intersection testing takes place when shadows are enabled
and the light shader casts shadow rays by calling mi_trace_shadow. This function is called only once but may result in
more than one shadow shader call. There are four different modes for shadow
casting, listed in the order of increased computational cost:
- shadow off
No shadows are computed, and no shadow shaders are called. Call to mi_trace_shadow return immediately without modifying the result color.
- shadow on
For each obscuring object (A and B), a shadow ray is generated with
the origin L and the intersection point A or B, and the shadow shaders
of objects A and B are called to modify the light emitted by the light
source based on the transparency attributes of the obscuring object. No
shadow ray is generated for the segment from B to M because no other
obscuring object whose shadow shader could be called exists. Although
shadow rays always go from the light source towards the illuminated point
in this mode, the order in which the shadow shaders are called is undefined.
If an object without shadow shader is found, or if a shadow shader returns
miFALSE, it is assumed that no light reaches the illuminated point
and the search for more obscuring objects is stopped (although the light
shader has the option of ignoring this result and supplying some light
anyway). See the first diagram below. The volume shader of the illuminated
object M is applied to the entire distance between M and L.
- shadow sort
Same as the previous method, but shadow shaders are called in distance
order, object closest to the light source first. In the first diagram,
steps 4 and 5 may be reversed.
- shadow segments
This mode is more sophisticated than the others. Shadow rays become similar
to visible rays; they travel in segments from the illuminated point to the
first obscuring object, then from there to the next obscuring object, and
so on until the light source is reached. This means that shadow rays travel
in the opposite direction, and one shadow ray's end point becomes the next
shadow ray's origin. Volume shaders are called for each of these segments,
and every shadow shader must perform inside/outside calculations to store
the correct volume shader in state - > volume much like material
shaders to. This mode is preferred if volume effects should cast shadows.
Note that the shadow segment mode requires complex shadow shaders to behave
differently. Every shadow shader must be able to work with all these modes,
so shadow shaders that deal with volumes or depend on the ray direction must
test state - > options - > shadow to determine the mode. In
case an incorrectly implemented shadow shader fails to call mi_trace_shadow_seg to evaluate other shadows, mental ray will call
mi_trace_shadow_seg and then call the shadow shader again, thus
simulating the effect.
The first diagram shows the ray casting order and the ray directions for
the shadow on and shadow sort modes:
The next diagram shows the same situation in shadow
segments mode:
The following diagram illustrates the path of a photon shot from the
light source in the caustics or global illumination2.1preprocessing phase. First a photon is traced from the light source.
It hits object A, and the photon material shader of object A is
called. The photon material shader stores energy at the intersection
point and determines how much energy is reflected and how much is
refracted, and the directions of reflection and transmission. It then
traces a new photon from A, in the reflection direction, or in the
transmission direction, or both. The reflected photon hits object B,
and the photon material shader of object B is called. The photon
material shader of object B stores energy at the intersection point and
shoots a new photon.
The remainder of this chapter describes how to write all types of shaders.
First, the concepts of ray tracing state parameter passing common to all
shaders are presented, followed by a detailed discussion of each type of
shader.
Next: 3.4 State Variables
Up: 3. Using and Writing
Previous: 3.2 Coordinate Systems
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