I have to admit that the title of this article is a little misleading. Bigger is not necessarily better. In fact, it’s usually worse when it comes down to increasing the size of an image for output. Scaling an image by “resampling up” adds pixels that increase its height and width or resolution. The problem is, when pixels are added, image quality can be severely compromised. Edges may appear soft, colors can look dull and contrast may diminish.

INTERPOLATION
When an image is enlarged by resampling in Photoshop, the colors of the new pixels are determined by one of the five interpolation algorithms chosen from the Image Size dialog box (see Figure 1). Nearest Neighbor, the fastest one, simply copies pixels. It doesn’t produce intermediate gradations of color and therefore it’s ideal for scaling line art, but it can devastate an image that contains smooth gradients. Bilinear adds pixels by averaging the color values of surrounding pixels. It too is fast, but it only produces so-so quality results.

Figure 1: The Image Size dialog box shows a list of the five interpolation algorithms.

Figure 2: A detail of an image resampled up. Although the resolution is higher, the areas are softer and the colors have become muddy.)

Bicubic is a more precise method that assesses the values of the eight pixels that surround each pixel in the image. With these calculations, Bicubic produces even tonal variations. It’s recommended for images that have smooth gradients. Bicubic Smoother is recommended for enlarging raster images. The core algorithm is bicubic interpolation, but it smoothes edge pixels and reduces softening by adding pixels of higher contrast in the inner areas. Bicubic Sharper is used for reducing the size of an image, and works by eliminating pixels. It too is based on bicubic interpolation but it enhances the image by increasing the contrast of the edge pixels. Figure 2 shows the affect of each of the algorithms applied to the same image that has been enlarged to 400 percent.

Even if Bicubic Smoother is used, it is still likely that contrast and sharpness will be compromised if the image is enlarged by too much. The question is how much is too much? Most images can be enlarged by 10 or 20 percent using Bicubic Smoother with a minimum of noticeable damage. It’s when images are enlarged by greater values that the effects become more obvious.

HOW MUCH?
Of course, how much also depends on image content. If an image is enlarged that is already soft — like big soft fluffy clouds for example (see Figure 3) — then the damage won’t be as severe as an image whose focus and contrast are more critical, like the image in Figure 4.

Figure 3: Images that are naturally soft, such as clouds, can be resampled to a degree without much harm.

If an image is going to be output to a large-format printer, the first step is to collect enough information at the outset so that resampling up is unnecessary. Be sure that the scanner has the capacity to collect enough information to support the printer’s requirements. If the scanner’s maximum optical resolution can capture at 2400 pixels per inch (ppi) and the printer requires a 300ppi image, then the image can be enlarged by a factor of eight (2400 ÷ 300 = 8) with no loss of quality. Don’t exceed the optical resolution of the device, otherwise the scanner’s software will be resampling using it’s own interpolation methods. In general, Photoshop’s interpolation methods are superior to that of most scanners.

Figure 4: Images whose focus and sharpness are critical will be compromised by too much resampling.

Let’s say the original image is 10 inches wide and eight inches high and is scanned at 2400ppi. It can be safely enlarged to 80"x 64" at 300ppi resolution without loss of data if it is not resampled. To perform this operation go to the Image menu and choose Image Size (see Figure 5). In dialog box uncheck the resample box and enter 80 inches in the width box. The height box will automatically convert the eight inches to 64 inches. Then click OK to enlarge the image without loss of quality.

Figure 5: An image can be enlarged without resampling in the Image Size dialog box.

SHARPER IMAGES
If there is no choice and the image has to be resampled to enlarge it beyond the capabilities of the scanner, then sacn the image at the scanner’s maximum optical capacity first. Then resample up to the desired size using Bicubic Smoother. A sharpening filter like Unsharp Mask (see Figure 6, right) or Smart Sharpen (see Figure 7, below) can be applied in moderation to improve image quality. Remember, these sharpening filters are not cure-alls. They will help in reducing the ravages of resampling but don’t expect them to add detail where there is none in the original image.

Figure 7: The Smart Sharpen dialog box controls the application of sharpening in the Shadows and Highlights of an image. Here I am applying the Gausian Blur feature.

The Unsharp Mask filter is a workhorse for improving the appearance of images that have been enlarged and are destined for large-format printing. It gets its name from an old optical process of masking areas on a film negative and then “bumping” an exposure to increase edge contrast. The digital version of this process essentially performs the same task but is much easier to use and has finer controls. The Amount slider determines the strength of the effect. The Radius slider designates how far the sharpening effect extends beyond the areas of most contrast (the edges). The Threshold slider mitigates the effect in the areas of least contrast so that they appear less grainy.

The Smart Sharpen filter can also help reduce the effects of resampling. It has sharpening controls that are not available with the Unsharp Mask filter. Smart Sharpen presents an extensive interface with a large preview. In Basic mode, the Amount and Radius are adjustable on three different types of blurring; Lens, Motion and Gaussian. Choose Gaussian to sharpen the softening effects of enlargement. When applying this filter Zoom the document window to 100% to get an accurate view of the sharpening. Choose More Accurate to process the file more slowly for a more accurate removal of blurring.

In Advanced mode, sharpening of dark and light areas can be performed. After initial sharpening is applied, click the Shadow and Highlight tabs. If sharpening halos appear, they can be reduced with the Fade Amount slider that adjusts the amount of sharpening in the highlights or shadows. The Tonal Width slider controls the range of tones in the shadows or highlights, and the Radius slider controls the distance of the area around each pixel designating weather it’s a shadow or highlight.

I recommend that all images destined for large-format printers be sharpened to some degree. The images that have been enlarged by resampling may require a bit more surgery. Use either of the filters or a combination of both for best results. Converting the file to Lab Mode prior to sharpening and then applying the filter(s) exclusively to the Lightness channel is a good method of avoiding color shifts. The image can then be reconverted back to RGB before printing with out altering the color relationships.

Ideally, your printing requirements won’t exceed that of your scanner’s capabilities. If you are faced with the necessity of needing to increase the size of your image for output, try these techniques. They can go a long way in improving an image after it has been enlarged for print.