<s>
id	O
Tech	O
4	O
,	O
popularly	O
known	O
as	O
the	O
Doom	B-Application
3	I-Application
engine	O
,	O
is	O
a	O
game	O
engine	O
developed	O
by	O
id	O
Software	O
and	O
first	O
used	O
in	O
the	O
video	O
game	O
Doom	B-Application
3	I-Application
.	O
</s>
<s>
The	O
engine	O
was	O
designed	O
by	O
John	O
Carmack	O
,	O
who	O
also	O
created	O
previous	O
game	O
engines	O
,	O
such	O
as	O
those	O
for	O
Doom	B-Application
and	O
Quake	B-Application
,	O
which	O
are	O
widely	O
recognized	O
as	O
significant	O
advances	O
in	O
the	O
field	O
.	O
</s>
<s>
This	O
OpenGL-based	O
game	O
engine	O
has	O
also	O
been	O
used	O
in	O
Quake	B-Application
4	I-Application
,	O
Prey	B-Application
,	O
Enemy	B-Application
Territory	I-Application
:	I-Application
Quake	I-Application
Wars	I-Application
,	O
Wolfenstein	B-Application
,	O
and	O
Brink	B-Application
.	O
</s>
<s>
id	O
Tech	O
4	O
is	O
licensed	O
under	O
the	O
terms	O
of	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
v3.0	I-License
or	I-License
later	I-License
.	O
</s>
<s>
id	O
Tech	O
4	O
began	O
as	O
an	O
enhancement	O
to	O
id	B-Application
Tech	I-Application
3	I-Application
.	O
</s>
<s>
The	O
decision	O
to	O
switch	O
from	O
C	B-Language
to	O
the	O
C++	B-Language
programming	I-Language
language	I-Language
necessitated	O
a	O
restructuring	O
and	O
rewrite	O
of	O
the	O
rest	O
of	O
the	O
engine	O
;	O
today	O
,	O
while	O
id	O
Tech	O
4	O
contains	O
code	O
from	O
id	B-Application
Tech	I-Application
3	I-Application
,	O
much	O
of	O
it	O
has	O
been	O
rewritten	O
.	O
</s>
<s>
"	O
I	O
mean	O
,	O
I	O
wo	O
n't	O
commit	O
to	O
a	O
date	O
,	O
but	O
the	O
Doom	B-Application
3	I-Application
stuff	O
will	O
be	O
open	O
source	O
"	O
.	O
</s>
<s>
And	O
like	O
its	O
predecessors	O
,	O
John	O
Carmack	O
has	O
said	O
that	O
id	O
Tech	O
4	O
will	O
be	O
released	O
as	O
open-source	B-Application
.	O
</s>
<s>
At	O
the	O
QuakeCon	O
2009	O
,	O
Carmack	O
said	O
that	O
he	O
planned	O
to	O
petition	O
ZeniMax	O
Media	O
to	O
release	O
the	O
id	O
Tech	O
4	O
source	O
upon	O
the	O
release	O
of	O
Rage	B-Application
.	O
</s>
<s>
As	O
part	O
of	O
his	O
keynote	O
for	O
QuakeCon	O
2011	O
he	O
confirmed	O
that	O
the	O
source	O
code	O
to	O
Doom	B-Application
3	I-Application
will	O
be	O
out	O
by	O
the	O
end	O
of	O
the	O
year	O
,	O
after	O
the	O
release	O
of	O
Rage	B-Application
.	O
</s>
<s>
On	O
November	O
16	O
,	O
2011	O
,	O
Carmack	O
announced	O
on	O
Twitter	B-Application
that	O
he	O
's	O
writing	O
new	O
code	O
for	O
Doom	B-Application
3	I-Application
's	O
open	O
source	O
release	O
,	O
because	O
"	O
lawyers	O
are	O
still	O
skittish	O
about	O
the	O
patent	O
issue	O
around	O
'	O
Carmack	O
's	O
reverse	O
 '	O
"	O
.	O
</s>
<s>
On	O
November	O
22	O
,	O
2011	O
,	O
Carmack	O
released	O
the	O
Doom	B-Application
3	I-Application
source	O
code	O
under	O
GNU	B-License
GPL-3.0-or-later	I-License
on	O
GitHub	B-Application
.	O
</s>
<s>
The	O
original	O
requirement	O
of	O
id	O
Tech	O
4	O
was	O
that	O
it	O
needed	O
a	O
high-end	O
graphics	O
processing	O
unit	O
(	O
GPU	O
)	O
with	O
fully	O
programmable	O
vertex	O
and	O
pixel	B-Algorithm
shaders	O
,	O
such	O
as	O
the	O
Nvidia	O
GeForce	O
3	O
or	O
ATI	O
Radeon	B-Device
8500	I-Device
,	O
with	O
at	O
least	O
64	O
MB	O
of	O
VRAM	O
.	O
</s>
<s>
By	O
E3	O
2002	O
,	O
the	O
recommended	O
GPU	O
was	O
"	O
100%	O
DirectX	O
9.0b	O
compatible	O
"	O
,	O
such	O
as	O
the	O
Radeon	B-Device
9700	I-Device
with	O
128	O
MB	O
of	O
VRAM	O
.	O
</s>
<s>
While	O
the	O
Radeon	B-Device
9700	I-Device
's	O
DirectX	O
9.0	O
features	O
are	O
not	O
necessary	O
to	O
render	O
the	O
game	O
,	O
its	O
advanced	O
architecture	O
,	O
256-bit	O
memory	O
bus	O
,	O
and	O
efficiency	O
were	O
needed	O
to	O
run	O
Doom	B-Application
3	I-Application
at	O
high	O
detail	O
and	O
playable	O
speed	O
.	O
</s>
<s>
The	O
"	O
Ultra	O
"	O
graphics	O
mode	O
included	O
in	O
Doom	B-Application
3	I-Application
would	O
not	O
even	O
run	O
on	O
high	O
end	O
graphics	O
cards	O
in	O
2004	O
as	O
the	O
engine	O
required	O
at	O
least	O
512	O
MB	O
of	O
video	O
memory	O
to	O
display	O
properly	O
and	O
at	O
playable	O
speeds	O
.	O
</s>
<s>
id	O
Tech	O
4	O
resulted	O
in	O
the	O
obsolescence	O
of	O
DirectX	O
7	O
graphics	O
chips	O
such	O
as	O
the	O
widespread	O
GeForce	O
2	O
and	O
Radeon	B-Device
7200	I-Device
,	O
as	O
well	O
as	O
older	O
chipsets	O
such	O
as	O
RIVA	B-Device
TNT2	I-Device
and	O
Rage	B-Device
128	I-Device
,	O
and	O
software	O
rendering	O
(	O
with	O
an	O
integrated	O
Intel	B-Device
GMA	I-Device
)	O
.	O
</s>
<s>
There	O
have	O
been	O
cases	O
of	O
enthusiasts	O
forcing	O
Doom	B-Application
3	I-Application
to	O
run	O
on	O
unsupported	O
graphics	O
chips	O
,	O
such	O
as	O
the	O
long	O
obsolete	O
Voodoo	O
2	O
,	O
but	O
these	O
are	O
unable	O
to	O
render	O
the	O
per-pixel	O
lighting	O
and	O
bump	O
mapping	O
.	O
</s>
<s>
Id	O
Tech	O
4	O
added	O
several	O
new	O
graphical	O
features	O
absent	O
in	O
its	O
predecessor	O
,	O
id	B-Application
Tech	I-Application
3	I-Application
.	O
</s>
<s>
These	O
included	O
normal	B-Application
mapping	I-Application
and	O
specular	O
highlighting	O
.	O
</s>
<s>
The	O
primary	O
innovation	O
of	O
id	O
Tech	O
4	O
was	O
its	O
use	O
of	O
entirely	O
dynamic	O
per-pixel	O
lighting	O
,	O
whereas	O
previously	O
,	O
3D	O
engines	O
had	O
relied	O
primarily	O
on	O
pre-calculated	O
per-vertex	O
lighting	O
or	O
lightmaps	B-Data_Structure
and	O
Gouraud	B-Algorithm
shading	I-Algorithm
.	O
</s>
<s>
While	O
dynamic	O
effects	O
had	O
been	O
available	O
before	O
(	O
such	O
as	O
dynamic	O
moving	O
lights	O
)	O
,	O
this	O
effect	O
merely	O
changed	O
the	O
brightness	O
of	O
the	O
vertices	O
of	O
the	O
polygon	O
,	O
with	O
the	O
pixel	B-Algorithm
's	O
colors	O
simply	O
being	O
interpolated	O
between	O
the	O
three	O
vertex	O
colors	O
of	O
its	O
polygon	O
.	O
</s>
<s>
This	O
fully	O
realtime	O
approach	O
used	O
in	O
Doom	B-Application
3	I-Application
,	O
combined	O
with	O
the	O
use	O
of	O
shadow	O
volumes	O
permitted	O
more	O
realistic	O
lighting	O
and	O
shadows	O
than	O
in	O
the	O
previous	O
generation	O
of	O
id	O
's	O
engines	O
.	O
</s>
<s>
The	O
method	O
used	O
to	O
create	O
the	O
shadow	O
volumes	O
is	O
the	O
subject	O
of	O
a	O
patent	O
by	O
Creative	O
,	O
which	O
Creative	O
granted	O
id	O
permission	O
to	O
use	O
in	O
the	O
Doom	B-Application
3	I-Application
engine	O
,	O
in	O
exchange	O
for	O
supporting	O
Creative	O
's	O
EAX	O
advanced	O
sound	O
technologies	O
.	O
</s>
<s>
Because	O
this	O
is	O
CPU	O
intensive	O
,	O
id	O
did	O
some	O
work	O
optimizing	O
this	O
by	O
using	O
Intel	O
's	O
Streaming	B-General_Concept
SIMD	I-General_Concept
Extensions	I-General_Concept
(	O
SSE	O
)	O
.	O
</s>
<s>
By	O
painting	O
a	O
single	O
massive	O
texture	O
(	O
32,768	O
×	O
32,768	O
pixels	B-Algorithm
,	O
though	O
it	O
has	O
been	O
extended	O
to	O
larger	O
dimensions	O
in	O
recent	O
versions	O
of	O
the	O
MegaTexture	O
technology	O
)	O
covering	O
the	O
entire	O
polygon	O
map	O
and	O
highly	O
detailed	O
terrain	O
,	O
the	O
desired	O
effects	O
can	O
be	O
achieved	O
.	O
</s>
<s>
The	O
first	O
game	O
utilizing	O
MegaTexture	O
is	O
Enemy	B-Application
Territory	I-Application
:	I-Application
Quake	I-Application
Wars	I-Application
,	O
based	O
on	O
the	O
Tech	O
4	O
engine	O
.	O
</s>
<s>
All	O
id	B-Application
Tech	I-Application
5	I-Application
and	O
id	B-Operating_System
Tech	I-Operating_System
6	I-Operating_System
games	O
use	O
MegaTexture	O
,	O
with	O
the	O
exception	O
of	O
The	B-Application
Evil	I-Application
Within	I-Application
which	O
uses	O
a	O
new	O
renderer	O
.	O
</s>
<s>
Brink	B-Application
,	O
which	O
uses	O
a	O
heavily	O
modified	O
version	O
of	O
id	O
Tech	O
4	O
also	O
uses	O
virtual	O
texturing	O
.	O
</s>
<s>
id	O
Tech	O
4	O
has	O
a	O
comprehensive	O
scripting	B-Language
language	I-Language
that	O
can	O
be	O
used	O
when	O
creating	O
mods	O
,	O
and	O
is	O
used	O
in	O
Doom	B-Application
3	I-Application
to	O
control	O
monsters	O
,	O
weapons	O
,	O
and	O
map	O
events	O
.	O
</s>
<s>
This	O
scripting	B-Language
language	I-Language
is	O
similar	O
to	O
C++	B-Language
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
main	O
scripting	B-Language
language	I-Language
,	O
idTech	O
4	O
also	O
has	O
another	O
scripting	B-Language
language	I-Language
that	O
is	O
used	O
for	O
GUIs	B-Application
—	O
both	O
the	O
menus	O
and	O
HUD	O
,	O
and	O
also	O
for	O
GUIs	B-Application
embedded	O
into	O
the	O
game	O
world	O
.	O
</s>
<s>
These	O
in-game	O
GUIs	B-Application
are	O
sufficiently	O
powerful	O
that	O
you	O
can	O
,	O
for	O
example	O
,	O
run	O
another	O
game	O
such	O
as	O
Doom	B-Application
1	O
within	O
the	O
game-world	O
.	O
</s>
<s>
Despite	O
this	O
additional	O
level	O
of	O
scripting	O
,	O
it	O
is	O
also	O
possible	O
to	O
create	O
mods	O
using	O
C++	B-Language
to	O
build	O
native	O
code	O
.	O
</s>
<s>
As	O
a	O
result	O
of	O
the	O
agreement	O
with	O
Creative	O
regarding	O
the	O
patent	O
on	O
shadow	O
volumes	O
,	O
the	O
id	O
Tech	O
4	O
engine	O
supports	O
OpenAL	B-Application
,	O
including	O
the	O
Creative	O
Technology	O
specific	O
EAX	O
extensions	O
.	O
</s>
<s>
The	O
work	O
to	O
include	O
OpenAL	B-Application
support	O
was	O
done	O
by	O
Creative	O
Technology	O
,	O
not	O
by	O
id	O
themselves	O
.	O
</s>
<s>
The	O
engine	O
uses	O
a	O
traditional	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
.	O
</s>
<s>
This	O
part	O
of	O
the	O
engine	O
works	O
in	O
a	O
fundamentally	O
similar	O
way	O
to	O
the	O
id	B-Application
Tech	I-Application
3	I-Application
equivalent	O
,	O
however	O
id	O
Tech	O
4	O
exposes	O
a	O
lot	O
more	O
of	O
the	O
network	O
protocol	O
to	O
mod	O
developers	O
.	O
</s>
<s>
Although	O
Doom	B-Application
3	I-Application
only	O
supports	O
4	O
players	O
(	O
and	O
8	O
in	O
the	O
Resurrection	B-Application
of	I-Application
Evil	I-Application
expansion	O
pack	O
)	O
,	O
the	O
id	O
Tech	O
4	O
engine	O
can	O
be	O
used	O
with	O
more	O
players	O
than	O
this	O
,	O
with	O
Quake	B-Application
4	I-Application
and	O
Enemy	B-Application
Territory	I-Application
:	I-Application
Quake	I-Application
Wars	I-Application
both	O
supporting	O
significantly	O
more	O
players	O
.	O
</s>
<s>
Compared	O
to	O
the	O
preceding	O
and	O
widely	O
used	O
id	B-Application
Tech	I-Application
3	I-Application
(	O
Quake	B-Application
III	I-Application
Arena	I-Application
engine	O
)	O
and	O
id	B-Application
Tech	I-Application
2	I-Application
(	O
Quake	B-Application
II	I-Application
engine	I-Application
)	O
,	O
id	O
Tech	O
4	O
has	O
had	O
less	O
success	O
in	O
licensing	O
to	O
third	O
parties	O
.	O
</s>
<s>
id	O
Software	O
was	O
unwilling	O
to	O
license	O
their	O
newest	O
engine	O
before	O
its	O
"	O
parent	O
game	O
"	O
Doom	B-Application
3	I-Application
was	O
completed	O
.	O
</s>
<s>
However	O
the	O
unexpected	O
long	O
development	O
time	O
going	O
into	O
Doom	B-Application
3	I-Application
from	O
2002-04	O
meant	O
that	O
they	O
could	O
not	O
put	O
up	O
competition	O
to	O
Epic	O
Games	O
's	O
Unreal	O
Engine	O
2	O
during	O
that	O
period	O
.	O
</s>
<s>
While	O
id	O
Tech	O
4	O
had	O
taken	O
a	O
new	O
direction	O
with	O
its	O
dynamic	O
per-pixel	O
lighting	O
,	O
this	O
unconventional	O
feature	O
had	O
steeper	O
hardware	O
requirements	O
and	O
was	O
initially	O
only	O
useful	O
in	O
"	O
spooky	O
games	O
"	O
(	O
until	O
the	O
MegaTexture	O
addition	O
)	O
,	O
whereas	O
an	O
increasing	O
number	O
of	O
developers	O
preferred	O
conventional	O
engines	O
that	O
could	O
render	O
large	O
outdoor	O
areas	O
.	O
</s>
<s>
The	O
rival	O
Source	B-Application
engine	I-Application
(	O
which	O
was	O
developed	O
from	O
the	O
previous	O
GoldSrc	B-Application
engine	I-Application
)	O
could	O
still	O
run	O
on	O
the	O
older	O
widespread	O
DirectX	O
7	O
GPUs	O
albeit	O
without	O
shaders	O
being	O
used	O
.	O
</s>
<s>
The	B-Application
Dark	I-Application
Mod	I-Application
(	O
2013	O
)	O
began	O
as	O
an	O
effort	O
to	O
recreate	O
Thief	B-Application
:	I-Application
The	I-Application
Dark	I-Application
Project	I-Application
within	O
Doom	B-Application
3	I-Application
.	O
</s>
<s>
It	O
became	O
a	O
standalone	O
game	O
once	O
the	O
Doom	B-Application
3	I-Application
source	O
code	O
was	O
released	O
.	O
</s>
