<s>
Volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
,	O
sometimes	O
called	O
volumetric	O
ray	B-Algorithm
casting	I-Algorithm
,	O
volumetric	O
ray	B-Algorithm
tracing	I-Algorithm
,	O
or	O
volume	O
ray	O
marching	O
,	O
is	O
an	O
image-based	O
volume	O
rendering	O
technique	O
.	O
</s>
<s>
Volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
,	O
which	O
processes	O
volume	O
data	O
,	O
must	O
not	O
be	O
mistaken	O
with	O
ray	B-Algorithm
casting	I-Algorithm
in	O
the	O
sense	O
used	O
in	O
ray	B-Algorithm
tracing	I-Algorithm
,	O
which	O
processes	O
surface	O
data	O
.	O
</s>
<s>
Unlike	O
ray	B-Algorithm
tracing	I-Algorithm
,	O
volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
does	O
not	O
spawn	O
secondary	O
rays	B-Device
.	O
</s>
<s>
When	O
the	O
context/application	O
is	O
clear	O
,	O
some	O
authors	O
simply	O
call	O
it	O
ray	B-Algorithm
casting	I-Algorithm
.	O
</s>
<s>
Because	O
ray	O
marching	O
does	O
not	O
necessarily	O
require	O
an	O
exact	O
solution	O
to	O
ray	O
intersection	O
and	O
collisions	O
,	O
it	O
is	O
suitable	O
for	O
real	B-General_Concept
time	I-General_Concept
computing	I-General_Concept
for	O
many	O
applications	O
for	O
which	O
ray	B-Algorithm
tracing	I-Algorithm
is	O
unsuitable	O
.	O
</s>
<s>
The	O
technique	O
of	O
volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
can	O
be	O
derived	O
directly	O
from	O
the	O
rendering	O
equation	O
.	O
</s>
<s>
Volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
is	O
classified	O
as	O
an	O
image-based	O
volume	O
rendering	O
technique	O
,	O
as	O
the	O
computation	O
emanates	O
from	O
the	O
output	O
image	O
and	O
not	O
the	O
input	O
volume	O
data	O
,	O
as	O
is	O
the	O
case	O
with	O
object-based	O
techniques	O
.	O
</s>
<s>
In	O
its	O
basic	O
form	O
,	O
the	O
volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
algorithm	O
comprises	O
four	O
steps	O
:	O
</s>
<s>
Ray	B-Algorithm
casting	I-Algorithm
.	O
</s>
<s>
At	O
this	O
stage	O
it	O
is	O
useful	O
to	O
consider	O
the	O
volume	O
being	O
touched	O
and	O
enclosed	O
within	O
a	O
bounding	B-Algorithm
primitive	I-Algorithm
,	O
a	O
simple	O
geometric	O
object	O
—	O
usually	O
a	O
cuboid	B-General_Concept
—	O
that	O
is	O
used	O
to	O
intersect	O
the	O
ray	O
of	O
sight	O
and	O
the	O
volume	O
.	O
</s>
<s>
In	O
general	O
,	O
the	O
volume	O
is	O
not	O
aligned	O
with	O
the	O
ray	O
of	O
sight	O
,	O
and	O
sampling	O
points	O
will	O
usually	O
be	O
located	O
in	O
between	O
voxels	B-Algorithm
.	O
</s>
<s>
Because	O
of	O
that	O
,	O
it	O
is	O
necessary	O
to	O
interpolate	O
the	O
values	O
of	O
the	O
samples	O
from	O
its	O
surrounding	O
voxels	B-Algorithm
(	O
commonly	O
using	O
trilinear	O
interpolation	O
)	O
.	O
</s>
<s>
However	O
,	O
adaptive	O
ray	B-Algorithm
casting	I-Algorithm
upon	O
a	O
projection	O
plane	O
and	O
adaptive	O
sampling	O
along	O
each	O
individual	O
ray	O
do	O
not	O
map	O
well	O
to	O
the	O
SIMD	B-Device
architecture	O
of	O
modern	O
GPU	O
.	O
</s>
<s>
This	O
gallery	O
represents	O
a	O
collection	O
of	O
images	O
rendered	O
using	O
high	O
quality	O
volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
.	O
</s>
<s>
Commonly	O
the	O
crisp	O
appearance	O
of	O
volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
images	O
distinguishes	O
them	O
from	O
output	O
of	O
texture	O
mapping	O
VR	O
due	O
to	O
higher	O
accuracy	O
of	O
volume	B-Algorithm
ray	I-Algorithm
casting	I-Algorithm
renderings	O
.	O
</s>
<s>
The	O
term	O
ray	O
marching	O
is	O
more	O
broad	O
and	O
refers	O
to	O
methods	O
in	O
which	O
simulated	O
rays	B-Device
are	O
traversed	O
iteratively	O
,	O
effectively	O
dividing	O
each	O
ray	O
into	O
smaller	O
ray	O
segments	O
,	O
sampling	O
some	O
function	O
at	O
each	O
step	O
.	O
</s>
<s>
When	O
rendering	O
screen	O
space	O
effects	O
,	O
such	O
as	O
screen	O
space	O
reflection	O
(	O
SSR	O
)	O
and	O
screen	O
space	O
shadows	O
,	O
rays	B-Device
are	O
traced	O
using	O
G-buffers	O
,	O
where	O
depth	O
and	O
surface	O
normal	O
data	O
is	O
stored	O
per	O
each	O
2D	O
pixel	O
.	O
</s>
