Which image to use from the Image Pac file
Many people discount the smaller units of the Image Pac file
as unusable; they say these are images for position-only or catalog
purposes (the Photo CD disc has carried some resolution myths).
But, each element of the Image Pac File can be used in halftone
reproduction - to the size limitations set by the reproduction
process. For example, a Base/lfi image can be printed in a newspaper
(85 Ipi, uncropped) up to about 1.75 inches wide. That's big
enough for many editorial uses. Using a higher-resolution image
(Base/4 for example) does not yield a better image ~~ just a
bigger file (and longer imagesetting time). The math for scan
Q x Ipi x %
where Q represents the multiplier of oversampling (Nyquist's
2.0:1 or less), lpi is the frequency of the halftone, in lines
per inch, and No is the decimal value of enlargement. Let's do
the math for one example: A 35 mm transparency is needed to print
on a big postcard, 8.75 x !5.75 in. (landscape orientation).
The printing process will be sheet-fed offset at 175 lpi (a high
halftone frequency for smoother tonal rendering).
1.5 x 175 x 5.83 = 1530.75
The Associative Law applies here, so the numbers can be multiplied in any order with the same result. The product of these numbers is 1530.375 - the image resolution we will need to accomplish the task (round to 1530). Now, let's look at the available upper-range resolutions in the Kodak Photo CD Master Discos Image Pac file:
Base*16 2048 x 3072 pixels
Base*4 1024 x 1536 pixels Base 512 x 768 pixels
1.25 x 133 x 1.6 = 266
The product of this math is 266; let's look at the available
Image Pac File components to see if one will satisfy our needs
We need a total "resolution" of 266 in the long
dimension of this image, so we may choose any resolution whose
long dimension provides at least that many pixels. The Base/4
image has 384 pixels in its long dimension, so it will do the
job. Should we use the larger, Base-size image for better quality?
No; the higher resolution of the Base file will not yield a visibly
superior image when taken to halftone printing at this size and
The formula, if the number of pixels is known (but not percentage
of enlargement) for calculating the reproduction capacity of
any image that has already been scanned (i,e,, Photo CD) images)
pixels in one dimension
This formula will tell you, in inches, how large an image
can be in print at the chosen screen frequency (Ipi) and Q factor.
Changing the Q factor between the recommended lower limit ( 1
25) and the recommended higher limit (2 0) will yield larger
or smaller image sizes. Let's do the math:
This is to calculate the maximum dimension of reproducibility
of the short dimension of the Base 16 Image Pac image at 150
Ipi, with a Q factor of 1.25. The result of this calculation
is 10.8936 inches. The other dimension, 3072 pixels, works out
to 16.3404 inches.