Scancalc 1.0

For help on using help files, press F1.

Background

Using Scancalc

Technical Information

IMPORTANT: Values you enter do not update the result readouts until you click the Compute button or press Enter.

Copyright © 1995-1997, Digital Light & Color
All Rights Reserved

This program may be redistribruted free of charge as long as the entire program and its accompanying help file are copied intact, including this notice.

Written by Jonathan M. Sachs
www.dl-c.com


Background

Scancalc is a program designed to help you decide what image resolution is required to create high quality photographic prints of a given size. This question comes up when you have your own scanner or are using Photo CD files and need to decide what size image to start with. If you choose too low a resolution, the final print may lack detail. If you choose too high a resolution, your image files will be huge and unwieldy.

Scancalc works backwards from the desired print size and printer resolution to compute how detailed an image file is required to get the most out of your printer. The more detailed an image your printer can output, the higher resolution image is required to get the best possible print it can output.

If you have an image of fixed dimensions to work with, you can also use Scancalc to determine the maximum size print you can make from it by entering different output widths until the recommended input image dimensions are approximately equal to those of the input image.

The resolution numbers Scancalc computes are approximate. Images that are a little lower or a little higher in resolution will print about the same. If the image is somewhat lower resolution than recommended, you may be able to get acceptable prints by oversharpening it slightly.

Many factors influence the choice of input image resolution:

Input image resolution

The input image resolution is related to the number of pixels in the input image. Resolution is a little like film grain – the more  pixels in the image, the more information in the digital image. The number of pixels in the input image in turn depends of the size of the original and the scanning resolution. For example, when scanning a 4”x6” print on a flatbed scanner at 300dpi, you get an image consisting of 1200 x 1800 pixels (4x300 = 1200; 6x300 = 1800). The resulting image has 1200*1800 = 2,160,000 pixels. If it is a color image, each pixel requires 3 bytes so the image requires about 6.5MB of memory. The higher the scanning resolution you use (or the high resolution Photo CD image you select), the more pixels you get and the larger the image file. Scancalc computes both the scanning resolution and the input image dimensions in pixels required to produce a high quality print.

Some file formats and image editing programs let you specify an image resolution (usually in dpi) for an image. This number is not really important – what matters is how much detail the image actually contains and this is determined by its dimensions in pixels. The resolution setting merely assigns a nominal size to the image in inches. Typically, when printing, the image is scaled to the desired print size and printed at the printer resolution, regardless of its resolution setting.

Input image quality and subject matter

If the original image is not sharp or lacks contrast or if your scanner does not capture the image data accurately, print quality will suffer. These factors are not within Scancalc’s control – it assumes that the original image and scanner are ideal. In the real world, images are sometimes fuzzy and scanners do not achieve their theoretical resolving power.

Some images are intentionally soft or lacking in contrast. These images can often be reproduced using less than the recommended image resolution. On the other hand, images that have a great deal of fine, high contrast detail may require higher resolution to achieve the same perceived print quality.

Registration and Sharpening

Some loss of image quality invariably occurs during the scanning process.

In color images, there is sometimes a misregistration between the red, green, and blue layers of the image. If this misregistration is consistent, it can be corrected or at least reduced by shifting two of the layers slightly with respect to the third. Picture Window supports this function with its Color Registration transformation.

Ideally, scanners would capture perfectly square pixels with no overlap between neighboring pixels. In reality, the pixels are rounded and overlap quite a bit. This leads to some softening of the image which can be reduced by using sharpening or unsharp masking.

If your image has less resolution that is recommended to create a print of the desired size, oversharpening the image somewhat may yield some improvement. You will need to experiment with this to find the optimum settings.

Cropping

When you crop your digital images you are discarding image data. A cropped image thus contains less information than an uncropped image and does not support as large an enlargement.

Desired output image size

Clearly, the larger the print you want to make, the more data you will need (assuming the viewing distance remains the same).

Type of printer used to produce output image

There are many different technologies used to reproduce photographic images. They can be broken down into three categories:

Continuous Tone

Continuous tone printers are the most “photographic” as they print images using tiny dots, each of which can be any possible color. Although dye sublimation printers are the most common continuous tone printers, there are a number of other continuous tone technologies in use as well such as film recorders and printers that image directly on photographic materials.

Dithered

Dithered printers are now becoming very popular and achieve “near photographic” results. They print images made up of dots of primary color (cyan, magenta, or yellow) and often add black to improve image contrast. To reproduce a full range of brightness levels and colors, dithered printers must place clusters of dots on the page such that their average color approximates that of the input image. Because of this dithering process, these printers must have much higher resolution than a continuous tone printer to get equivalent image quality and the images often appear grainy when viewed up close. Ink jet printers are the most common member of this printer class.

Some advanced inkjet printers use additional inks or support variable ink dot size to provide results closer to those of a continuous tone printer. For printers of this type you may be better off selecting Continuous Tone and then experimenting to find the effective resolution of the printer.

Printing Press

Conventional printing presses are really a special case of dithered printers, but the resolution of the printing press is conventionally specified as a halftone screen frequency in lines per inch (lpi). This number varies from 70-80 for newspapers to 133 for magazines and books, and 150 or higher for fine art reproductions.

Resolution of printer

The resolution of the printer determines the maximum amount of image detail the printer can reproduce per square inch. Scancalc works backward by computing the amount of input data required to fill the output area for the print at the effective resolution of the printer.

Printer quality

Just as scanners never achieve their theoretical maximum resolving power, printers vary considerably in the amount of detail they can reproduce and this actual resolving power depends not only on the advertized printer resolution in dpi but also on the size, shape, and overlap of the dots. You may find that printing at less than the full printer resolution produces results that are barely distinguishable from those printed at full resolution while working with much smaller images.

Certain printers have difficulty reproducing colors accurately. Inkjet printers often have trouble with very pale colors appearing grainy. Some of the new inkjet printers incorporate additional pastel ink colors to address this problem.

Viewing distance

By convention, a standard viewing distance of 10 inches is assumed for prints up to roughly 8”x12”. For larger images, the viewer is normally farther away and cannot see as much fine detail. This means that large images can often be reproduced using a lower resolution and still look sharp as long as you don’t get too close. By contrast, very small images should normally be rendered with higher resolution as the viewer is likely to be closer than 10 inches away when viewing the image.

How much resolution makes an image look photographic?

Assuming a sharp, high quality original image, a continuous tone printer, and a viewing distance of 10 inches, photographic results are obtained at pixel densities of from 150 to 300 dots per inch (dpi). Below 150 dpi, images start to look noticeably soft. Above 300 dpi there is little or no improvement as this approaches the angular resolution of the eye. While there is some improvement between 200 and 300 dpi it is usually minor and most 300 dpi printers cannot really resolve data at this level of detail due to pixel overlap and registration errors. For all but the most demanding applications, 200 dpi is a good rule of thumb.

Using a dithered printer (e.g. an inkjet), you will need roughly 3-4 times the resolution of a continuous tone printer to get comparable results. In other words, inkjet printers need between 600 and 800 dpi to reproduce photographic images well. The paper you print on can make a huge difference as well. High resolution inkjet printing normally requires coated or glossy paper to prevent the ink dots from spreading.

Using a printing press, a 133 lpi screen is pretty much the standard for glossy magazines and this translates into roughly 200 dpi of image data.

If the viewing distance is greater than 10 inches, the resolution can be reduced proportionately. For example, if you double the viewing distance to 20 inches, you halve the resolution requirement to roughly 100 dpi.


Using Scancalc

To use Scancalc, simply enter the necessary information as described below and then click the Compute button or press Enter. The recommended scanning resolution in dpi and input image size in pixels are displayed at the bottom of the window.

Input Width, Input Height

Enter the width and height of the original image here. You can change units if you wish by clicking on the units drop down box. When scanning using a film scanner, enter the dimensions of the film frame; when scanning with a flatbed scanner, enter the dimensions of the original you are scanning. If you plan to crop the image, enter the size of the original after cropping.

Output Width

Enter the width of the print you plan to create from the input image. You can change units if you wish by clicking on the units drop down box.

For example, if you are planning to create an 8”x10” print, enter either 8.0 or 10.0 depending on whether the input image is oriented vertically or horizontally.

Output Height

The output height is computed automatically based on the output width and the proportions of the input image.

Printer Type

The required scan resolution depends on the way the image will be reproduced. Scancalc recognizes three types of printers, and each has its own characteristics:

Continuous Tone

Continuous tone printers create images from dots, each of which can be any color. The most common continuous tone printers use dye sublimation technology. Continuous tone printer resolution is specified in dots per inch.

Dithered

Dithered printers create images from dots, each of which is one of the primary colors (cyan, magenta, yellow). Often black is added as a fourth color to improve image contrast. To simulate a full range of colors, groups of dots must be clustered in the right proportions using a technique called halftoning or dithering. Ink jet printers fall into this category. A continuous tone printer does not need as much resolution as a dithered printer to reproduce images with the same quality.

Printing Press

Four color printing presses are really dithered printers, but their resolution is normally specified by a line screen frequency such as 133 lpi instead of dots per inch (dpi).

Printer Resolution/Line Screen Frequency

Enter the resolution of your printer in dpi or halftone screen frequency in lpi you will be using on a printing press here.

If your printer has different horizontal and vertical resolutions (e.g. 300x600dpi), use the lower number if you want to be conservative or the average of the two numbers. If you are preparing an image for reproduction in print, and you are unsure, check with your printer to see what screen frequency they will be using.

Viewing Distance

Enter the distance from which you will be viewing the output and from which you want the print to appear sharp. You can change units if you wish by clicking on the units drop down box.

For prints up to 8x12, a viewing distance of 10 inches is standard. For larger sizes, the viewing distance is typically larger.

Scan Resolution

The required scanning resolution in dots per inch is displayed here.

Scan Pixels

The required input image size in pixels is displayed here.


Technical Information

This section describes how Scancalc performs its calculations and what assumptions it uses:

The output height is computed as (output width)*(input height)/(input width).

The effective printer resolution is computed based on the printer type. For continuous tone printers the effective resolution is equal to the printer resolution. For dithered printers, the printer resolution is reduced by a factor of 3.5 to obtain an approximate equivalent continuous tone resolution. For printing presses, the effective resolution is computed as 1.5 times the screen frequency. This is generally agreed to be the image resolution needed to make high quality four color images given the screen frequency.

A second resolution is also computed based on the viewing distance as 10.0”/(viewing_distance)*250dpi. This determines an upper limit to the resolution based on an image printed at 250 dpi looking good when viewed from a distance of 10 inches. If this alternate resolution is lower than the effective printer resolution, the effective printer resolution is reduced since the full printer resolution is not needed.

The required scan resolution is then computed as (effective resolution)*(print width)/(image width).

The required scan dimensions are computed as (scan resolution)*(image width) and (scan resolution)*(image height).