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CHAPTER 4 Graphics
COLOR SPACE Decode ARRAY
ICCBased Same as the value of Range in the ICC profile of the image’s color
space
Indexed [ 0 N ], where N = 2n − 1
Pattern (Not permitted with images)
Separation [ 0.0 1.0 ]
DeviceN [ 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 Dmin value greater than Dmax . 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 2n − 1 is mapped to 0.0 (black).
The Dmin and Dmax 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.
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