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 resolutions is: 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). The enlargement rate is 583% (8.75 /1.5 inch). The frequency (previously noted) is 175 Ipi. The Q factor we'll use for this example is 1.5. Here's the formula: 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 We're seeking a resolution that accommodates the long dimension of the image at 1530 pixels of data. Curiously, the Base*4 Image Pac element has slightly more, so we can use the Base/4 image to accomplish our goal. Using the higher resolution Base*16 file would not yield a better image in the final halftone. Many people familiar with the reproduction requirements of halftones would scoff at the above calculation, citing "conventional wisdom" or "we've always done it that way" logic for using a larger image. But using a large image doesn't generate a better reproduction; to the contrary, it only impedes the system, taking more storage space, more imagesetter time, and more network time. Another example of the use of Photo CD discs in editorial work will demonstrate the choice of smaller Image Pac file components, those with less resolution than the Base file. We need a photo for reproduction in color to print in a magazine that uses 5-column layout for its editorial content. The column width is 1.2 inches. The image comes from a 35mm transparency (though it has been scanned to Photo CD). Screen frequency for the magazine is 133 Ipi, and this time we'll use a multiplier (Q factor) of 1.25. The section of the original transparency to be used measures .75 in. (thus the enlargement of 160% - 1.6 in our formula). Here's the formula for the calculation: 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 for resolution: Base.........512x768 Base/4......256 x 384 Base/16....128 x 192 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 screen frequency. 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) is: pixels in one dimension (Qxlpi) 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: 2048 (1.25 x 150) 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. Be sure to test both dimensions of an image whenever working with the calculations to be sure the proportions of the original image fit the window in the printed page. Square images from Pro Photo CD Master discs and images cropped to different proportions must be checked carefully to ensure that they will not cause trouble later in the process. Next Colorite Home Page OPTIMIZING PHOTO CD -- MENU