<s>
The	O
RenderMan	B-Application
Interface	I-Application
Specification	I-Application
,	O
or	O
RISpec	B-Application
in	O
short	O
,	O
is	O
an	O
open	O
API	B-Application
developed	O
by	O
Pixar	O
Animation	O
Studios	O
to	O
describe	O
three-dimensional	O
scenes	O
and	O
turn	O
them	O
into	O
digital	O
photorealistic	O
images	O
.	O
</s>
<s>
It	O
includes	O
the	O
RenderMan	B-Application
Shading	I-Application
Language	I-Application
.	O
</s>
<s>
As	O
Pixar	O
's	O
technical	O
specification	O
for	O
a	O
standard	O
communications	O
protocol	O
(	O
or	O
interface	B-Application
)	O
between	O
modeling	O
programs	O
and	O
rendering	O
programs	O
capable	O
of	O
producing	O
photorealistic-quality	O
images	O
,	O
RISpec	B-Application
is	O
a	O
similar	O
concept	O
to	O
PostScript	B-Language
but	O
for	O
describing	O
3D	O
scenes	O
rather	O
than	O
2D	O
page	O
layouts	O
.	O
</s>
<s>
Thus	O
,	O
modelling	O
programs	O
which	O
understand	O
the	O
RenderMan	B-Application
Interface	I-Application
protocol	O
can	O
send	O
data	O
to	O
rendering	O
software	O
which	O
implements	O
the	O
RenderMan	B-Application
Interface	I-Application
,	O
without	O
caring	O
what	O
rendering	O
algorithms	O
are	O
utilized	O
by	O
the	O
latter	O
.	O
</s>
<s>
The	O
interface	B-Application
was	O
first	O
published	O
in	O
1988	O
(	O
version	O
3.0	O
)	O
and	O
was	O
designed	O
to	O
be	O
sufficiently	O
future	O
proof	O
to	O
encompass	O
advances	O
in	O
technology	O
for	O
a	O
significant	O
number	O
of	O
years	O
.	O
</s>
<s>
What	O
set	O
the	O
RISpec	B-Application
apart	O
from	O
other	O
standards	O
of	O
the	O
time	O
was	O
that	O
it	O
allowed	O
using	O
high-level	O
geometric	B-Algorithm
primitives	I-Algorithm
,	O
like	O
quadrics	O
or	O
bicubic	O
patches	O
,	O
to	O
specify	O
geometric	B-Algorithm
primitives	I-Algorithm
implicitly	O
,	O
rather	O
than	O
relying	O
on	O
a	O
modeling	O
application	O
to	O
generate	O
polygons	O
approximating	O
these	O
shapes	O
explicitly	O
beforehand	O
.	O
</s>
<s>
Another	O
novelty	O
introduced	O
by	O
the	O
RISpec	B-Application
at	O
the	O
time	O
was	O
the	O
specification	O
of	O
a	O
shading	B-Language
language	I-Language
.	O
</s>
<s>
The	O
RenderMan	B-Application
shading	I-Application
language	I-Application
allows	O
material	O
definitions	O
of	O
surfaces	O
to	O
be	O
described	O
not	O
only	O
by	O
adjusting	O
a	O
small	O
set	O
of	O
parameters	O
,	O
but	O
in	O
an	O
arbitrarily	O
complex	O
fashion	O
by	O
using	O
a	O
C-like	O
programming	O
language	O
to	O
write	O
shading	B-General_Concept
procedures	O
commonly	O
known	O
as	O
procedural	O
textures	O
and	O
shaders	O
.	O
</s>
<s>
Lighting	O
,	O
and	O
displacements	O
on	O
the	O
surface	O
,	O
are	O
also	O
programmable	O
using	O
the	O
shading	B-Language
language	I-Language
.	O
</s>
<s>
The	O
shading	B-Language
language	I-Language
allows	O
each	O
statement	O
to	O
be	O
executed	O
in	O
a	O
SIMD	B-Device
manner	O
,	O
but	O
does	O
not	O
insist	O
on	O
it	O
.	O
</s>
<s>
Another	O
feature	O
that	O
sets	O
renderers	O
based	O
on	O
the	O
RISpec	B-Application
apart	O
from	O
many	O
other	O
renderers	O
is	O
the	O
ability	O
to	O
output	O
arbitrary	O
variables	O
as	O
an	O
image	O
:	O
surface	O
normals	O
,	O
separate	O
lighting	O
passes	O
and	O
pretty	O
much	O
anything	O
else	O
can	O
be	O
output	O
from	O
the	O
renderer	O
in	O
a	O
single	O
pass	O
.	O
</s>
<s>
RenderMan	O
has	O
much	O
in	O
common	O
with	O
OpenGL	B-Application
(	O
developed	O
by	O
the	O
now-defunct	O
Silicon	O
Graphics	O
)	O
,	O
despite	O
the	O
two	O
APIs	B-Application
being	O
targeted	O
to	O
different	O
sets	O
of	O
users	O
(	O
OpenGL	B-Application
to	O
real-time	O
hardware-assisted	O
rendering	O
and	O
RenderMan	O
to	O
photorealistic	O
off-line	O
rendering	O
)	O
.	O
</s>
<s>
Both	O
APIs	B-Application
take	O
the	O
form	O
of	O
a	O
stack-based	O
state	O
machine	O
with	O
(	O
conceptually	O
)	O
immediate	O
rendering	O
of	O
geometric	B-Algorithm
primitives	I-Algorithm
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
implement	O
either	O
API	B-Application
in	O
terms	O
of	O
the	O
other	O
.	O
</s>
<s>
Orthographic	O
and	O
perspective	B-Architecture
viewing	O
transformations	O
.	O
</s>
<s>
Depth-based	O
hidden-surface	B-Algorithm
elimination	I-Algorithm
.	O
</s>
<s>
Pixel	B-Algorithm
filtering	I-Algorithm
and	O
spatial	B-Algorithm
anti-aliasing	I-Algorithm
.	O
</s>
<s>
Gamma	O
correction	O
and	O
dithering	O
before	O
quantization	B-Algorithm
.	O
</s>
<s>
All	O
of	O
the	O
geometric	B-Algorithm
primitives	I-Algorithm
described	O
in	O
the	O
specification	O
,	O
and	O
provide	O
all	O
of	O
the	O
standard	O
primitive	B-Algorithm
variables	O
applicable	O
to	O
each	O
primitive	B-Algorithm
.	O
</s>
<s>
Any	O
additional	O
shaders	O
,	O
and	O
any	O
deviations	O
from	O
the	O
standard	O
shaders	O
presented	O
in	O
this	O
specification	O
,	O
must	O
be	O
documented	O
by	O
providing	O
the	O
equivalent	O
shader	O
expressed	O
in	O
the	O
RenderMan	B-Application
shading	I-Application
language	I-Application
.	O
</s>
