CHAPTER 4
346
Graphics
COLOR SPACE
Decode
ARRAY
ICCBased
Same as the value of
Range
in the ICC profile of the image’s color
space
[ 0
N
]
, where
N
=
2
n
−
1
Indexed
Pattern
Separation
DeviceN
(Not permitted with images)
[ 0.0 1.0 ]
[ 0.0 1.0 0.0 1.0 … 0.0 1.0 ]
(one pair of elements for each color
component)
It is possible to specify a mapping that
inverts
sample color intensities by specify-
ing a
D
min
value greater than
D
max
. For example, if the image’s color space is
DeviceGray
and the
Decode
array is
[ 1.0 0.0 ]
, an input value of 0 is mapped to 1.0
(white); an input value of 2
n
−
1 is mapped to 0.0 (black).
The
D
min
and
D
max
parameters for a color component are not required to fall
within the range of values allowed for that component. For instance, if an applica-
tion uses 6-bit numbers as its native image sample format, it can represent those
samples in PDF in 8-bit form, setting the two unused high-order bits of each
sample to 0. The image dictionary should then specify a
Decode
array of
[ 0.00000 4.04762 ]
, which maps input values from 0 to 63 into the range 0.0 to 1.0
(4.04762 being approximately equal to 255
÷
63). If an output value falls outside
the range allowed for a component, it is automatically adjusted to the nearest al-
lowed value.
Image Interpolation
When the resolution of a source image is significantly lower than that of the out-
put device, each source sample covers many device pixels. As a result, images can
appear jaggy or blocky. These visual artifacts can be reduced by applying an
im-
age interpolation
algorithm during rendering. Instead of painting all pixels cov-
ered by a source sample with the same color, image interpolation attempts to
produce a smooth transition between adjacent sample values. Image interpola-
tion is enabled by setting the
Interpolate
entry in the image dictionary to
true
. It
is disabled by default because it may increase the time required to render the im-
age.