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Technique

Le présent article n'est pas disponible en Français
June 2009

Digital images sometimes require to be rescaled in order to provide the quality that is required for reproduction at varying sizes. In this article we talk about fractals, software and the factors to take into account when rescaling images but it is also important to place this subject in context and go back to dot screens, halftones and four colour rosettes. It is important to understand the print technology because much of it is still used today.

Resolution and scaling

In the latter part of the 20th century I used to shoot all of my work in black and white. After processing and printing the pictures were re-shot on a process camera with an 80-line screen. Does this all sound like gibberish or do you remember the days before scanners when everything had to be re-photographed to make separations for printing?

Archaic as it may seem much of the terminology from that time still has meaning and is in daily use for printers and designers. This article will go through an explanation of image resolutions for typical printing and then move on to show the different methods of effectively upsizing images beyond their native size.

Diffraction

Another means of bending light is known as diffraction, the tendency for light to spread around the edge of an obstruction, or after passing through an opening. Figure 1 shows what happens to light when it passes through a small slit.

 

Shooting an image through an 80-line screen gives an effect similar to this image.

Line screens

So what are an 80-line screen and the more common 150lpi screen?
In black and white photography images blend smoothly, displaying continuous tones from light to dark. Printing presses only have the ability to use black ink to reproduce photographic images. To accomplish that patterns of small black dots are printed close together. The human eye blurs the dots and interprets them as shades of grey. This process of using individual, solid-coloured dots to simulate shades of grey is called 'halftoning'.
To achieve a halftone using traditional processes the original image is 'screened' or re-photographed onto a negative through a screen or a sheet of plastic or metal with an ultra-fine pattern of holes, much like photographing through a kitchen sieve. The film is then used to make the printing plate but this process has now been superseded in most workflows by outputting from page layout software to film or direct to printing plate.

 

Close-up detail of printing rosettes on an image for offset printing.

Four colour

Colour printing works in exactly the same way as described for black and white printing except there are four plates (Cyan, Magenta, Yellow and Black - usually known as CMYK) and they have to be printed one after the other in perfect alignment.

The angle of the halftone screen is changed for each colour to prevent interference patterns and the result is a perfect rosette of dots that the human eye merges into a continuous tone. Misaligned screens create a 'moiré' pattern - the halftone pattern that makes it difficult to scan a previously printed image.

Lpi (or L/cm in Europe) is a measure of how many halftone lines are printed per linear inch (or cm).

Typical screen values are as follows:

  • Local newspaper (black and white) - 80lpi (31.5 L/cm)
  • Low-end colour magazine - 133lpi (52 L/cm)
  • Glossy magazine - 150lpi (59 L/cm)
  • Art book - 200lpi (79 L/cm)
 

Detail of printing rosettes on an image for offset printing.

Quality factor

So, how does the line screen in use relate to image size?

Traditionally the screen ruling is multiplied by a 'Quality factor'. This is normally quoted as 2 x, so...

A 133lpi screen value requires images at 266dpi (133lpi x2 = 266dpi)
A 150lpi screen value requires images at 300dpi (150lpi x2 = 300dpi)

Digital images are created from dots or pixels (pixels per inch is expressed as a ppi value).

The Canon EOS-1Ds Mark III generates 5616x3744 pixels at its largest file size 'L'.

Typically a designer will ask for an image at "300dpi" (118 d/cm) when they actually mean pixels per inch (ppi) or pixels per cm (ppcm).

The maths is easy: 5616 pixels divided by 300ppi (118ppcm) = 18.72 inches or 47.59cm. These figures represent the maximum size that an image from the EOS-1Ds Mark III can be printed in a glossy magazine. Or do they?

The quality factor relates to the number of pixels that are averaged into printing dots. The concept was developed long ago when all images were scanned or screened with a process camera. Quality factor 2 means that four camera pixels are averaged into one print dot (on each plate). Quality factor 1.5 means that two pixels become one dot.

Digital quality factors

Times have moved on and, unlike film, digital images have no grain. The quality factor can safely be set at 1.5 or even lower. There has long been a belief among people in the know that the traditional quality factor of 2x was always too much, certainly for digitally originated files. A 1.5x (225ppi or 85.5ppcm) actually gives a sharper and more pleasing image on paper than one at 300ppi/118ppcm.

 

The EOS-1Ds Mark III can generate a file that can be printed at 18.72 inches in a glossy magazine at 150lpi.

This all means that the EOS-1Ds Mark III can realistically generate a file that can be printed at 24.96 inches or 63.40cm wide in a glossy magazine using a quality factor of 1.5 - effectively bigger than A2 in size without re-sizing the image in any way. There you go - the word 're-size' has crept out.

Because digital images have no grain, and contemporary cameras generate a file that is of better quality than a scan from film, images can be re-sized or stretched by a substantial margin with no visible loss of quality. It is safe to say that 150% is possible with no other tool than a copy of Photoshop software - beyond that depends on the final use and the quality of the original file.

The final use of the image determines how the image should be treated when contemplating re-sizing. Getty Images specifies that its photographers supply all files at 48-50Mb RGB TIFF files, regardless of how they were captured in the first place. Market experience has shown Getty Images that with a quality file of that size they can meet all of the demands that clients makes of them, including the use of images for billboards and advertising hoardings at one extreme, and the web at the other.

To Getty Images and the photographers who work for the company the Canon EOS-1Ds Mark II was an important step. No longer did images need to be upsized before supply. Prior to the arrival of the EOS-1Ds Mark II the original EOS-1Ds was the only DSLR that Getty Images would accept images from. To do that necessitated interpolating (re-sizing an image by creating extra information) the images to meet the 48Mb size guideline.

The way in which interpolation is achieved plays a major role in the quality of the final file. When the chip size on DSLRs was smaller I had to do this on a regular basis to meet client expectations. At that time an EOS D30 with a 3Mb chip would produce a good quality A4 image if the upsizing process had been handled correctly

 

To get images to A4 size (3508x2480 pixels) with the EOS D30 (2160x1440) required upsizing by 163% because 3508 divided by 2160 = 1.63.

What software is necessary?

There are several pieces of software on the market specifically for re-sizing images and as many ways to use them as there are users. Photoshop has always been a contender from smaller scale upsizing and using the proven method of 'stair interpolating' in 10% increments, it could achieve excellent results up to around 200%. With the advent of Photoshop CS the latter method was surpassed in quality and speed by the smooth bi-cubic method built into the software.

For interpolation upwards from 150% I have, in the past, used a variety of software - the favoured two being Genuine Fractals by onOne software and pxlSmartScale by Extensis - but PhotoZoom Pro 2 by BenVista and others can also be used. The comparison images below show the results of upsizing a portrait to 1m high at 300dpi - not a normal way to use the software but a basis for comparison. An image of 1m high would normally be of lower resolution even when outputting through a large format inkjet printer.

 

Comparison crops from an image enlarged to 1m high, 300dpi by three different pieces of software. Left - pxlSmartScale; centre - Photoshop CS; right - Genuine Fractals 3.5.

It may not be clear from the screen image but, viewing the high res versions of the files, Genuine Fractals and pxlSmartScale have a clear advantage over Photoshop CS even at this small increase in size. The images are crisper and have more accurate contrast.

Getty Images has a specific department for generating large file sizes and, whilst most things are possible, the bulk of the output is to a maximum of 300Mb, about a 250% scale up from a 50Mb file. Because billboards and other large outputs typically use low screen resolutions of around 50 lpi a 300Mb file is usually sufficient.

During the course of testing software for this article I found it possible to scale up a 50Mb file, slowly but reliably, to around 1200% using a combination of Genuine Fractals (maximum of 600%) and then Photoshop CS to double that. This sort of scaling would normally be handled by RIP software on specialist printers, but it is useful to know how to achieve such a result - the resulting file was about 5Gb.

 

Move your mouse over the image to see a 250% upsize in Genuine Fractals yielding a 300Mb image.

The three pieces of software all provided the ability to re-size images very competently. For sizing up to the 300Mb benchmark it is very similar in quality to Genuine Fractals and has controls for sharpness, detail and contrast if you are not so confident in Photoshop.

Genuine Fractals was the easiest 'specialist' software to use and provided excellent results up to its maximum 600% limit. Genuine Fractals works differently to pxlSmartScale in that it converts the files to resolution independent fractal images. Once the file has been saved in the proprietary .stn format from within Photoshop (it is a plug-in), the same image can be output to multiple sizes, maintaining both sharpness and quality.

Photoshop, well this software is the 'Swiss Army knife' of image software. It may not offer the very best result when doing large scale re-sizing, but it is close if you are not looking at the result at 400%.