<s>
The	O
original	O
ZX	B-Operating_System
Spectrum	I-Operating_System
computer	O
outputs	O
video	O
through	O
an	O
RF	O
modulator	O
and	O
was	O
designed	O
for	O
use	O
with	O
contemporary	O
1980s	O
television	O
sets	O
,	O
for	O
a	O
simple	O
colour	O
graphic	O
display	O
.	O
</s>
<s>
The	O
image	B-Algorithm
resolution	I-Algorithm
is	O
256×192	O
with	O
colour	O
limitations	O
.	O
</s>
<s>
To	O
conserve	O
memory	O
,	O
colour	O
is	O
stored	O
separate	O
from	O
the	O
pixel	O
bitmap	O
in	O
a	O
low	B-Algorithm
resolution	I-Algorithm
,	O
32×24	O
grid	O
overlay	O
,	O
corresponding	O
to	O
text	O
character	O
cells	O
.	O
</s>
<s>
Usually	O
the	O
border	O
area	O
assumes	O
a	O
single	O
color	O
,	O
but	O
using	O
software	O
tricks	O
it	O
possible	O
to	O
display	O
some	O
low	B-Algorithm
resolution	I-Algorithm
graphics	O
there	O
.	O
</s>
<s>
The	O
ZX	B-Operating_System
Spectrum	I-Operating_System
is	O
generally	O
considered	O
to	O
have	O
limited	O
graphical	O
capabilities	O
in	O
comparison	O
to	O
some	O
other	O
home	O
computers	O
of	O
the	O
same	O
era	O
such	O
as	O
the	O
Commodore	O
64	O
,	O
largely	O
due	O
to	O
its	O
lack	O
of	O
a	O
dedicated	O
graphics	O
chip	O
.	O
</s>
<s>
The	O
ZX	B-Operating_System
Spectrum	I-Operating_System
(	O
and	O
compatibles	O
)	O
computers	O
uses	O
a	O
variation	O
of	O
the	O
4-bit	O
RGBI	O
palette	O
philosophy	O
(	O
also	O
used	O
on	O
CGA	B-Device
,	O
Thomson	B-Device
MO5	I-Device
,	O
Sharp	O
MZ-800	O
,	O
Mattel	B-Device
Aquarius	I-Device
,	O
etc	O
.	O
)	O
</s>
<s>
In	O
the	O
ZX	B-Operating_System
Spectrum	I-Operating_System
encoding	O
the	O
colour	O
components	O
are	O
in	O
GRB	O
(	O
Green	O
,	O
Red	O
and	O
Blue	O
)	O
order	O
(	O
from	O
most	O
to	O
least	O
significant	O
bit	O
)	O
rather	O
than	O
the	O
more	O
common	O
RGB	O
order	O
.	O
</s>
<s>
For	O
any	O
value	O
of	O
n	O
from	O
0	O
to	O
7	O
,	O
the	O
following	O
Sinclair	B-Language
BASIC	I-Language
commands	O
can	O
be	O
used	O
to	O
set	O
or	O
alter	O
the	O
screen	O
's	O
colours	O
:	O
</s>
<s>
Some	O
ZX	B-Architecture
Spectrum	I-Architecture
clones	I-Architecture
or	O
NTSC	O
machines	O
might	O
display	O
"	O
bright	O
black	O
"	O
as	O
dark	O
gray	O
.	O
</s>
<s>
Each	O
ZX	B-Operating_System
Spectrum	I-Operating_System
model	O
used	O
different	O
voltages	O
for	O
colours	O
,	O
so	O
the	O
values	O
here	O
are	O
only	O
indicative	O
.	O
</s>
<s>
sRGB	O
values	O
shown	O
are	O
an	O
approximation	O
taking	O
into	O
account	O
PAL	B-Language
gamma	O
.	O
</s>
<s>
Nevertheless	O
viewing	O
on	O
an	O
actual	O
PAL	B-Language
CRT	O
display	O
would	O
give	O
slightly	O
different	O
results	O
,	O
due	O
to	O
variations	O
on	O
analog	O
hardware	O
setup	O
and	O
components	O
.	O
</s>
<s>
at	O
)	O
to	O
resort	O
to	O
various	O
dithering	O
techniques	O
;	O
the	O
simplest	O
of	O
which	O
employs	O
ordered	B-Algorithm
patterns	I-Algorithm
,	O
as	O
shown	O
exhaustively	O
in	O
the	O
opposite	O
chart	O
.	O
</s>
<s>
Dithering	O
also	O
works	O
very	O
well	O
with	O
the	O
original	O
Composite	O
video	O
connection	O
to	O
an	O
analog	O
PAL	B-Language
TV	O
,	O
generating	O
colour	O
mixing	O
along	O
horizontal	O
pixel	O
lines	O
.	O
</s>
<s>
The	O
original	O
ZX	B-Operating_System
Spectrum	I-Operating_System
has	O
a	O
screen	O
resolution	O
of	O
256×192	O
pixels	O
.	O
</s>
<s>
In	O
many	O
programs	O
this	O
limitation	O
was	O
evident	O
as	O
attribute	B-Algorithm
clash	I-Algorithm
.	O
</s>
<s>
Due	O
to	O
the	O
limitations	O
of	O
the	O
original	O
Composite	O
video	O
connection	O
to	O
an	O
analogue	O
PAL	B-Language
TV	O
,	O
adjacent	O
pixels	O
were	O
in	O
fact	O
blurred	O
out	O
horizontally	O
.	O
</s>
<s>
Some	O
emulators	B-Application
provide	O
some	O
sort	O
of	O
"	O
blur	O
"	O
filter	O
that	O
tries	O
to	O
simulate	O
this	O
effect	O
.	O
</s>
<s>
By	O
using	O
a	O
monochrome	B-Device
monitor	I-Device
or	O
black	O
and	O
white	O
TV	O
(	O
or	O
reducing	O
the	O
colour	O
settings	O
of	O
a	O
colour	O
TV	O
)	O
,	O
it	O
is	O
possible	O
to	O
take	O
advantage	O
of	O
the	O
differences	O
in	O
intensity	O
over	O
the	O
Spectrum	O
's	O
colour	O
range	O
to	O
generate	O
a	O
15-shade	O
grayscale	O
image	O
at	O
256×192	O
resolution	O
.	O
</s>
<s>
Several	O
third-party	O
Spectrum	O
clones	O
,	O
including	O
the	O
Timex	O
Sinclair	O
machines	O
,	O
the	O
Pentagon	B-Device
,	O
the	O
eLeMeNt	O
ZX	O
,	O
and	O
the	O
MB03+	O
Ultimate	O
interface	O
support	O
a	O
screen	O
mode	O
in	O
which	O
attribute	O
blocks	O
are	O
8×1	O
pixels	O
in	O
size	O
rather	O
than	O
the	O
usual	O
8×8	O
.	O
</s>
<s>
In	O
addition	O
,	O
this	O
screen	O
mode	O
can	O
be	O
generated	O
through	O
the	O
use	O
of	O
the	O
MB-02	O
disk	O
system	O
's	O
DMA	B-General_Concept
hardware	O
(	O
where	O
the	O
technique	O
is	O
known	O
as	O
Multitech	O
)	O
,	O
and	O
is	O
also	O
available	O
as	O
Mode	O
2	O
(	O
with	O
a	O
linear	O
byte	O
order	O
)	O
on	O
the	O
SAM	B-Device
Coupé	I-Device
.	O
</s>
<s>
ULAplus	O
is	O
compatible	O
with	O
the	O
standard	O
ZX	B-Operating_System
Spectrum	I-Operating_System
and	O
Timex	O
modes	O
,	O
and	O
adds	O
the	O
ability	O
to	O
redefining	O
of	O
the	O
palette	O
.	O
</s>
<s>
The	O
Timex	O
clones	O
and	O
ZX	B-Operating_System
Spectrum	I-Operating_System
128K	O
implement	O
a	O
'	O
shadow	O
 '	O
screen	O
area	O
which	O
can	O
be	O
switched	O
into	O
place	O
through	O
the	O
use	O
of	O
a	O
single	O
OUT	O
command	O
,	O
and	O
this	O
is	O
often	O
utilised	O
to	O
rapidly	O
switch	O
between	O
two	O
images	O
for	O
this	O
purpose	O
(	O
although	O
this	O
can	O
also	O
be	O
achieved	O
with	O
a	O
standard	O
block	O
copy	O
,	O
albeit	O
not	O
over	O
the	O
entire	O
screen	O
)	O
.	O
</s>
<s>
When	O
viewed	O
on	O
a	O
CRT	O
television	O
screen	O
(	O
the	O
standard	O
at	O
the	O
time	O
)	O
,	O
the	O
flicker	B-General_Concept
is	O
less	O
noticeable	O
than	O
on	O
a	O
modern	O
monitor	O
.	O
</s>
<s>
The	O
image	O
is	O
linked	O
to	O
here	O
(	O
warning	O
:	O
the	O
linked	O
image	O
flickers	B-General_Concept
very	O
rapidly	O
,	O
which	O
could	O
potentially	O
cause	O
seizures	O
in	O
people	O
with	O
photosensitive	O
epilepsy	O
)	O
.	O
</s>
<s>
Rather	O
,	O
the	O
effect	O
is	O
more	O
akin	O
to	O
anti-aliasing	B-Algorithm
,	O
with	O
certain	O
pixels	O
appearing	O
at	O
half	O
intensity	O
.	O
</s>
<s>
The	O
attributes	O
(	O
colours	O
)	O
of	O
an	O
image	O
are	O
alternated	O
at	O
50Hz	O
on	O
the	O
screen	O
,	O
taking	O
advantage	O
of	O
the	O
PAL	B-Language
colour	O
system	O
encoding	O
for	O
mixing	O
.	O
</s>
<s>
For	O
Pentagon	B-Device
machines	O
,	O
a	O
hardware	O
modification	O
is	O
available	O
which	O
directly	O
combines	O
the	O
two	O
alternate	O
screen	O
areas	O
into	O
the	O
video	O
signal	O
,	O
thus	O
eliminating	O
the	O
flicker	B-General_Concept
associated	O
with	O
this	O
method	O
.	O
</s>
<s>
Later	O
ZX	B-Architecture
Spectrum	I-Architecture
compatible	I-Architecture
machines	I-Architecture
offered	O
extra	O
video	O
modes	O
.	O
</s>
<s>
A	O
Sam	B-Device
Coupé	I-Device
mode	O
,	O
where	O
each	O
pixel	O
can	O
one	O
of	O
16	O
colours	O
from	O
a	O
128	O
colour	O
palette	O
.	O
</s>
<s>
A	O
mode	O
supported	O
by	O
the	O
Pentagon	B-Device
computer	O
.	O
</s>
<s>
Two	O
graphics	O
editors	O
(	O
"	O
Draw	O
512	O
"	O
and	O
"	O
Tech-Draw	O
"	O
)	O
support	O
this	O
mode	O
,	O
along	O
with	O
BASIC64	O
and	O
some	O
CP/M	B-Application
implementations	O
.	O
</s>
<s>
The	O
Sam	B-Device
Coupé	I-Device
has	O
this	O
mode	O
,	O
where	O
each	O
pixel	O
can	O
use	O
one	O
of	O
4	O
colours	O
from	O
palette	O
of	O
128	O
.	O
</s>
<s>
A	O
mode	O
supported	O
by	O
the	O
Pentagon	B-Device
computer	O
.	O
</s>
<s>
Attributes	O
are	O
doubled	O
in	O
the	O
vertical	O
direction	O
,	O
the	O
composition	O
of	O
the	O
attribute	O
is	O
identical	O
to	O
the	O
original	O
ZX	B-Operating_System
Spectrum	I-Operating_System
mode	O
.	O
</s>
<s>
The	O
SPEC256	O
,	O
EmuZWin	O
and	O
Es.pectrum	O
emulators	B-Application
have	O
a	O
screen	O
mode	O
256×192	O
pixels	O
where	O
each	O
pixel	O
can	O
be	O
in	O
one	O
of	O
256	O
colours	O
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
extending	O
the	O
word	O
size	O
of	O
the	O
emulated	B-Application
Z80	B-General_Concept
from	O
8	O
bits	O
to	O
64	O
,	O
making	O
eight	O
bits	O
of	O
data	O
available	O
for	O
each	O
pixel	O
;	O
the	O
screen	O
thus	O
takes	O
48KB	O
of	O
memory	O
.	O
</s>
<s>
This	O
mode	O
only	O
exists	O
on	O
the	O
emulator	B-Application
and	O
software	O
graphics	O
must	O
be	O
modified	O
to	O
use	O
it	O
.	O
</s>
<s>
The	O
ZX	O
ULAX	O
emulator	B-Application
provides	O
a	O
15-bit	O
high	O
colour	O
mode	O
.	O
</s>
