<s>
id	B-Application
Tech	I-Application
1	I-Application
,	O
also	O
known	O
as	O
the	O
Doom	B-Application
engine	I-Application
,	O
is	O
the	O
game	O
engine	O
used	O
in	O
the	O
id	O
Software	O
video	O
games	O
Doom	B-Application
and	O
Doom	B-Application
II	I-Application
:	I-Application
Hell	I-Application
on	I-Application
Earth	I-Application
.	O
</s>
<s>
It	O
is	O
also	O
used	O
in	O
Heretic	B-Application
,	O
Hexen	B-Application
:	I-Application
Beyond	I-Application
Heretic	I-Application
,	O
Strife	B-Application
:	I-Application
Quest	I-Application
for	I-Application
the	I-Application
Sigil	I-Application
,	O
Hacx	O
:	O
Twitch	O
'	O
n	O
Kill	O
,	O
Freedoom	O
,	O
and	O
other	O
games	O
produced	O
by	O
licensees	O
.	O
</s>
<s>
Originally	O
developed	O
on	O
NeXT	O
computers	O
,	O
it	O
was	O
ported	O
to	O
MS-DOS	B-Application
and	O
compatible	O
operating	B-General_Concept
systems	I-General_Concept
for	O
Doom	B-Application
's	O
initial	O
release	O
and	O
was	O
later	O
ported	O
to	O
several	O
game	O
consoles	O
and	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
The	O
source	O
code	O
to	O
the	O
Linux	B-Application
version	O
of	O
Doom	B-Application
was	O
released	O
to	O
the	O
public	O
under	O
a	O
license	O
that	O
granted	O
rights	O
to	O
non-commercial	O
use	O
on	O
December	O
23	O
,	O
1997	O
,	O
followed	O
by	O
the	O
Linux	B-Application
version	O
of	O
Doom	B-Application
II	I-Application
about	O
a	O
week	O
later	O
on	O
December	O
29	O
,	O
1997	O
.	O
</s>
<s>
The	O
source	O
code	O
was	O
later	O
re-released	O
under	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
v2.0	I-License
or	I-License
later	I-License
on	O
October	O
3	O
,	O
1999	O
.	O
</s>
<s>
The	O
dozens	O
of	O
unofficial	O
Doom	B-Application
source	O
ports	O
that	O
have	O
been	O
created	O
since	O
then	O
allow	O
Doom	B-Application
to	O
run	O
on	O
previously	O
unsupported	O
operating	B-General_Concept
systems	I-General_Concept
and	O
sometimes	O
radically	O
expand	O
the	O
engine	O
's	O
functionality	O
with	O
new	O
features	O
.	O
</s>
<s>
Although	O
the	O
engine	O
renders	O
a	O
3D	O
space	O
,	O
that	O
space	O
is	O
projected	O
from	O
a	O
two-dimensional	O
floor	B-Application
plan	I-Application
.	O
</s>
<s>
The	O
Doom	B-Application
engine	I-Application
was	O
later	O
renamed	O
to	O
"	O
id	B-Application
Tech	I-Application
1	I-Application
"	O
in	O
order	O
to	O
categorize	O
it	O
in	O
a	O
list	O
of	O
id	B-Application
Software	I-Application
's	I-Application
long	I-Application
line	I-Application
of	I-Application
game	I-Application
engines	I-Application
.	O
</s>
<s>
The	O
Doom	B-Application
engine	I-Application
separates	O
rendering	O
from	O
the	O
rest	O
of	O
the	O
game	O
.	O
</s>
<s>
Viewed	O
from	O
the	O
top	O
down	O
,	O
all	O
Doom	B-Application
levels	O
are	O
actually	O
two-dimensional	O
,	O
demonstrating	O
one	O
of	O
the	O
key	O
limitations	O
of	O
the	O
Doom	B-Application
engine	I-Application
:	O
room-over-room	O
is	O
not	O
possible	O
.	O
</s>
<s>
Sidedefs	O
are	O
then	O
grouped	O
together	O
to	O
form	O
polygons	B-General_Concept
;	O
these	O
are	O
called	O
"	O
sectors	O
"	O
.	O
</s>
<s>
Doom	B-Application
makes	O
use	O
of	O
a	O
system	O
known	O
as	O
binary	O
space	O
partitioning	O
(	O
BSP	O
)	O
.	O
</s>
<s>
It	O
is	O
because	O
of	O
this	O
that	O
it	O
is	O
not	O
possible	O
to	O
move	O
the	O
walls	O
in	O
Doom	B-Application
;	O
while	O
doors	O
and	O
lifts	O
move	O
up	O
and	O
down	O
,	O
none	O
of	O
them	O
ever	O
move	O
sideways	O
.	O
</s>
<s>
At	O
the	O
leaves	O
of	O
the	O
tree	O
are	O
convex	O
polygons	B-General_Concept
,	O
where	O
further	O
division	O
of	O
the	O
level	O
is	O
not	O
needed	O
.	O
</s>
<s>
These	O
convex	O
polygons	B-General_Concept
are	O
referred	O
to	O
as	O
subsectors	O
(	O
or	O
"	O
SSECTORS	O
"	O
)	O
,	O
and	O
are	O
bound	O
to	O
a	O
particular	O
sector	O
.	O
</s>
<s>
The	O
child	O
node	O
closest	O
to	O
the	O
camera	O
is	O
drawn	O
first	O
using	O
a	O
Scanline	B-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
All	O
of	O
the	O
walls	O
in	O
Doom	B-Application
are	O
drawn	O
vertically	O
;	O
it	O
is	O
because	O
of	O
this	O
that	O
it	O
is	O
not	O
possible	O
to	O
properly	O
look	O
up	O
and	O
down	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
perform	O
a	O
form	O
of	O
look	O
up/down	O
via	O
,	O
and	O
many	O
modern	O
Doom	B-Application
source	O
ports	O
do	O
this	O
,	O
as	O
well	O
as	O
later	O
games	O
that	O
use	O
the	O
engine	O
,	O
such	O
as	O
Heretic	B-Application
.	O
</s>
<s>
The	O
Doom	B-Application
engine	I-Application
renders	O
the	O
walls	O
as	O
it	O
traverses	O
the	O
BSP	O
tree	O
,	O
drawing	O
subsectors	O
by	O
order	O
of	O
distance	O
from	O
the	O
camera	O
so	O
that	O
the	O
closest	O
segs	O
are	O
drawn	O
first	O
.	O
</s>
<s>
The	O
Doom	B-Application
graphic	O
format	O
stores	O
the	O
wall	O
textures	O
as	O
sets	B-Data_Structure
of	I-Data_Structure
vertical	I-Data_Structure
columns	I-Data_Structure
;	O
this	O
is	O
useful	O
to	O
the	O
renderer	O
,	O
which	O
essentially	O
renders	O
the	O
walls	O
by	O
drawing	O
many	O
vertical	O
columns	O
of	O
textures	O
.	O
</s>
<s>
This	O
leads	O
to	O
one	O
of	O
Doom	B-Application
's	O
classic	O
limitations	O
which	O
frustrated	O
many	O
mappers	O
for	O
a	O
long	O
time	O
.	O
</s>
<s>
Doom	B-Application
contained	O
a	O
static	O
limit	O
on	O
the	O
number	O
of	O
visplanes	O
;	O
if	O
exceeded	O
,	O
a	O
"	O
visplane	O
overflow	O
"	O
would	O
occur	O
,	O
causing	O
the	O
game	O
to	O
exit	O
to	O
DOS	O
with	O
one	O
of	O
two	O
messages	O
,	O
"	O
No	O
more	O
visplanes	O
!	O
"	O
</s>
<s>
Sprites	O
in	O
Doom	B-Application
are	O
stored	O
in	O
the	O
same	O
column	O
based	O
format	O
as	O
the	O
walls	O
are	O
,	O
which	O
again	O
is	O
useful	O
for	O
the	O
renderer	O
.	O
</s>
<s>
Doom	B-Application
stores	O
a	O
list	O
of	O
sprites	O
to	O
be	O
drawn	O
(	O
"	O
vissprites	O
"	O
)	O
and	O
sorts	O
the	O
list	O
before	O
rendering	O
.	O
</s>
<s>
The	O
Doom	B-Application
engine	I-Application
achieved	O
most	O
of	O
its	O
fame	O
as	O
a	O
result	O
of	O
powering	O
the	O
classic	O
first	O
person	O
shooter	O
Doom	B-Application
,	O
and	O
it	O
was	O
used	O
in	O
several	O
other	O
games	O
.	O
</s>
<s>
It	O
is	O
usually	O
considered	O
that	O
the	O
"	O
Big	O
Four	O
"	O
Doom	B-Application
engine	I-Application
games	O
are	O
Doom	B-Application
,	O
Heretic	B-Application
,	O
Hexen	B-Application
:	I-Application
Beyond	I-Application
Heretic	I-Application
,	O
and	O
Strife	B-Application
:	I-Application
Quest	I-Application
for	I-Application
the	I-Application
Sigil	I-Application
.	O
</s>
