If a minister preached that “God is in the details,” then the minister of large-format printing would preach that “God is in the resolution.” Today, we work with digital printers and digital cameras with higher-than-ever image quality. The weak link in quality can often be the operator, the choices that he or she makes, and the things that he or she does to the image on its way to reproduction. Resolution is quality.

What’s Resolution?
Resolution is the amount of detail present in the image. The more detail, the more image information. The more information, the more realistic the image. It’s like being there.
Two types of digital file formats are vector and bitmapped graphics. Vector graphics are defined by points, lines, and curves using mathematical equations. They are independent of the output resolution. Vector graphics are appropriate for logos, diagrams, drawings and type. They are a blessing to the sign industry because they can be scaled to any resolution without loss of quality. Also, since their images are defined mathematically (i.e., with text), files are small, even for a large diagram. In fact, the reason that the animated vector-graphic format, Flash, has taken over the Internet is that its graphics, likewise, are mathematically defined. Their small file size enables Web pages to load quickly.
Bitmapped images are digital “mosaics” of microscopic image squares known as picture elements (pixels). The number of pixels must be determined in advance for the intended output, so bitmapped images are said to be purpose-specific. If there are not enough pixels, images will appear “pixelized,” or jagged. If there are too many pixels, images will take too much time to print, with no noticeable increase in quality, as the RIP must process each pixel.
Eyal Friedman, vice president of technical support at SA International, Inc., publisher of the FlexiSIGN and PhotoPRINT software RIPs, said, “If the original image isn’t in a sufficient resolution, the print will come out pixelized jaggy, or pixilated, no matter what the resolution mode that you RIP the image at. The RIP will not compensate for low-quality images.”

Bitmapped Resolution
Two ways to describe image resolution are by area and by linear dimension. Area measurement describes the resolution of a digital camera (megapixels) or of a file (megabytes). Linear measurement describes the number of pixels per linear inch (ppi) or, in metric countries, per centimeter (p/cm).

Digital camera resolution. If you have a digital camera of a certain size in megapixels, you can calculate how big an image you can reproduce from it. Conversely, if you want to buy a digital camera to take images of a certain size, you can calculate the number of megapixels you’ll need (see CALCULATING CAMERA MEGAPIXELS, below).

Scanner resolution. Commercial-quality scanners can reproduce images at up to 10,000 ppi. The actual scan resolution depends upon the size of the original and size of the intended reproduction (Figure 1).

Figure 1: Dialog box from the Kodak Oxygen Scan application for EverSmart scanners shows how image resolution is dependent upon the size of the original and the intended reproduction size. Since output size must be known in advance, bitmapped images are said to be purpose-specific.

Printer resolution. Today’s inkjet printers are capable of extremely high resolution. Printer resolution is measured in inkjet spots per inch (spi). Resolutions of 1440×1440 or 1200×1200 spi are not uncommon. However, not all of this resolution is necessary or even desirable for reproducing most work. The higher the resolution, the slower the printer.
The printer’s resolution defines the number of ink spots per linear inch. However, for high quality, images do not need to be equal to the printer’s resolution. In fact, image resolution depends upon viewing distance.
The normal viewing distance for a document is approximately equal to its diagonal. For example, you would expect to view an 8×10-in. photo at a distance of 13 in., and a 2×3-ft. poster at about 3.5 ft.

Resizing Images
Since bitmapped images are purpose-specific, it is often necessary to change their resolution to meet output requirements and print them efficiently. It is easier to remove pixels from an image than to create pixels that were not there to begin with.

To decrease, downsample, or “res down” the resolution of an image in Photoshop, use the Image > Image Size dialog box (Figure 2). Enter the new image size, or the percentage of the original you want, and check the Resampling box. Resampling adds or removes pixels from the image, while unchecking it only makes the pixels larger or smaller. Images can also be resampled upwards, or “ressed up,” in Photoshop. Usually ressing up greater than 400 percent sacrifices quality. Genuine Fractals, a Photoshop plug-in (Figure 3), uses fractal resampling to res up images and claims to produce higher-quality results.

Figure 2: Photoshop’s Image > Image Size dialog box can be used for resizing images.

Figure 3: To show the difference between ressing up an image in Photoshop vs. Genuine Fractals, a 1” square from a 300-ppi, 6-megapixel digital camera photo was enlarged to 800 percent in both programs.

Resizing in the RIP
Many of today’s software RIPs have the capability of enlarging images directly in the RIP. If in the final output stages, you note that the image needs to be enlarged, you do not need to go back to Photoshop to res-up the image. SAi’s Friedman said, “If the image quality is good and a customer wishes to enlarge the image in the RIP and print it out, then the RIP will be able to achieve a high-quality print. Since the RIP can do this so effectively, there will be no need to resize the image in Photoshop or Illustrator.”

Dennis Dorrity, creative manager at Wasatch Computer Technology, said that the Wasatch SoftRIP “resamples an image using bicubic interpolation, the same method that is used in Photoshop.” He adds that “Photoshop does have variations of the bicubic method that allow for some quality adjustments while resizing.”

Max Derhak, senior software architect at Onyx Graphics, Inc., adds, “The RIP has to provide image data for every pixel sent to the printer. Different interpolation methods can be employed for different image types. Generally, pixel duplication is used for upsampling and pixel subsampling (periodically skipping pixels) is used for downsampling.”

Derhak points out a setting for “Process with interpolation” in Onyx’s Output tab. He points out that this option uses an image resampling algorithm to get the correct number of pixels for output. “This is effective for both upsampling and downsampling, but it is considerably slower than pixel duplication and pixel subsampling.”

Wasatch’s Dorrity points out that the main advantage of resampling in the RIP is speed. “If resampling is done in the application, you have to go through the extra time to resample at the application level.”

The best way to ensure quality of bitmapped image is to remember that they are purpose-specific, and to make sure you have enough resolution from the outset. However, on those occasions when you must do the best you can, it is nice to know that programs like Photoshop, Genuine Fractals, and software RIPs can resample images.

CALCULATING CAMERA MEGAPIXELS

Use this calculation to determine the number of megapixels needed for a certain-sized image, such as an 8.5 × 11-in. magazine photo.

1.Determine sampling ratio (e.g., 2:1, 1.5:1, 1:1)

2. Calculate ppi needed: ppi = screen ruling × sampling ratio

3. Megapixels per sq.in. = ppi^2 (e.g., 300^2 = 90000 pixels = 0.09 MP)

4. Multiply MP/sq.in. by image size (e.g., for an 8.5 × 11-in. photo, 0.09 MP x 8.5 × 11 = 8.5 MP)

Calculating image size

Use this calculation to determine how large an image you can get from your digital camera, for example, a 6-MB digital SLR.

1. Determine ppi needed, e.g. 300 ppi

2. Calculate number of MP per sq.in. (e.g., 300 ppi ^2 = 90,000 pixels/sq.in. = 0.09 MP/sq.in.)

3. Divide camera MP by MP/sq.in. to get available sq.in. (e.g., 6 MP camera / 0.09 = 66.67 sq.in. of image area)

4. Take square root of available image area to get image size, (e.g., square root of 66.67 sq.in. = 8.16 in.)

5. So a 6-MP digital camera will give you an image 8.16 × 8.16 in. at 300 ppi.

6. Convert the image size to the proportion you want, e.g., (6-MP image in the proportion of 8 × 10 would be 7.3 × 9.1 in.)