Watkins Printing - Support: Images and Scanning

Watkins Printing
110 West 1200 South
Logan, Utah 84321

Sales | Service | Support | Contact Us

Images and Scanning

Image Types

The first distinction to make between image formats is whether they are vector (object-oriented, mathematical) or raster (bitmap, pixel grid).
Vector images are created using mathematically defined curves and line segments. They can be sized to print or display at any resolution without quality loss because the device is responsible for calculating the necessary units to represent the image.

Vector images should be used whenever scalability is a concern. They are most commonly utilized for logos and multiple spot-color art, such as clipart.

Raster images consist of a rectangular grid of small squares called pixels. Each pixel has a specific number of bits associated with it that describes its value. The larger the number of bits associated to each pixel, the more color information that pixel can describe. At the same time, overall file size increases with increased color depth. They can be scaled, but the effective resolution of the image will change.

A 1-bit raster image has only one bit per pixel, and thus each pixel can only be black or white. In Adobe Photoshop, this is the image mode known as 'bitmap.' Also sometimes referred to as 'line art.'

An 8-bit raster image can assign up to 2⁸ or 256 shades of a color to a pixel, often grayscale values.

24-bit raster images are 'true-color' RGB images, using three 8-bit segments per pixel to describe red, green and blue shades.

32-bit raster images are four 8-bit segments; usually used for cyan, magenta, yellow and black (CMYK) for four-color process printing.

[Formula: Color capacity from bits]
Raster images are used for photographic images and are produced by scanners and digital cameras.

[Back to Top]

Image Formats

Images come in a variety of formats, created for a wide array of media. Proper use of image formats comes from an understanding of their advantages and limitations.
Vector images are most commonly created by Adobe Illustrator, Macromedia Freehand, and CorelDraw in the publishing industry. A great deal of other vector image editing and creating software exists, ranging from CAD/CAM applications to 3D modeling and animation, and for other specialized uses.

EPS - Encapsulated PostScript. This is the preferred format for vector-based images in printing. EPS files also support embedded raster data (such as Photoshop EPS files).

AI - Adobe Illustrator native format.

FH9, FH10... - Macromedia Freehand native format.

CDR - CorelDraw native format.

WMF, WPG, CGM, GEM, PLT, DXF... There are many formats supporting vector-based data, but most of these are not suited for printing and must be converted.

EPS files are not normally rasterized when placed into layout software such as Adobe PageMaker or QuarkXPress, and thus they may appear incorrect, low-resolution, or as a gray box. However, when printed to a PostScript-capable printer, they will print according to the vector data. Most layout software utilizes a header, which is a small raster representation of the vector image; and is created when the vector image is saved. This header can be omitted or included based on the options selected when saving vector-based images.
Raster images are created by Adobe Photoshop, Jasc Paint Shop Pro, Corel PhotoPaint, and a multitude of other raster-based image editing software. They are also created when scanning or taking digital photographs.

TIF - Tagged Image File Format. This is the preferred format for raster-based images in printing. TIF files also support embedded paths and alpha channels, OPI comments, and LZW lossless compression. LZW compression is not recommended in commercial printing because of color problems.

DCS - Desktop Color Separations. Relatively new format used for spot-color based raster images. Adobe Photoshop v5.5 supports this format. It yields varied results in printing, but generally is acceptable. Uses a header.

JPG - Joint Photographic Experts Group. This is a common format used on the internet and in digital photography. JPG is not recommended in commercial printing because of color problems due to lossy compression.

GIF - CompuServe Graphics Interchange Format. Another common format used on the internet. They only support up to 256 levels of color, which makes them ideal for small non-photographic images on the web, but poor for printing.

BMP - Windows/Microsoft Bitmap. Used primarily for Windows operating system.

PCD - Kodak PhotoCD.

PCT, IMG, PCX, TGA, MSP, RAS... Other formats should be converted as necessary.

[Back to Top]

Resolution

Resolution refers to an image's or output device's number of dots available to represent graphic detail in a given area. The units used are spi, dpi, ppi, and lpi.

spi - Samples per inch. The unit used for scanning images. Functionally the same as ppi. All too often, dpi is used to describe scan resolutions, which is technically incorrect.

dpi - Dots per inch. The unit used for a printer to measure the number of machine dots printed in a linear inch. The smallest physical dot an output device can produce.

ppi - Pixels per inch. The unit used for raster images and monitors to measure the number of pixels per inch. A pixel is the smallest distinct unit of a raster image.

lpi - Lines per inch. The unit used for halftone screens to measure the number of rows of halftone lines per inch. (Halftone or screen frequency.) See halftones.

Vector images do not have an inherent resolution of their own. They will print using the resolution that the output device is set to at print time.
Raster images have specific resolutions and must be scanned or rendered at a resolution appropriate for their intended use. If the resolution is too low raster images will look jagged or bitmapped; too high and it will consume disk space and take much longer to print.
To achieve the ideal resolution for an image, first determine 1) what dimensions, and 2) what halftone/screen frequency the image will print at. Then use the following formula:

For example, a 6w x 4h inch image you intend to print at 1.5w x 1h using a 150 lpi screen: 1 / 4 x 150 x 1.5= 56 spi.

A 2w x 2h inch image you want to print at 6w x 6h using a 133 lpi screen would need to be scanned at: 6 / 2 x 133 x 2 = 598 spi. Typically you should scan at resolutions that are evenly divisible by your scanner's optical resolution. With a 600 spi scanner, you would scan the image in our first example at 60 spi and the second at 600.

Some guides indicate to use a ratio of 2 (not 1.5). This is okay, especially for high-definition images, but can yield much larger files and extra scanning time. The acceptable range is between 1.3 and 2.
The correct lpi to use is determined by several factors. The ability of equipment to generate the halftones properly and in register, the ability of plates, inks, and press to transfer the halftones, and the ability of the paper to hold the halftone dots. We use industry standard SWOP specifications, and run between 133 lpi and 150 lpi depending on paper type.
[Formula: Effective resolution]
[Formula: Determine maximum lpi (halftone) for a specific dpi to yield 256 shading levels]
[Formula: Determine values of gray possible at a given lpi-dpi combination]
Resizing is one of the most misunderstood or misused concepts when working with raster images. There are several ways to resize a raster image, and depending on the situation, the resizing method chosen may work well, or may create problems.

Scaling - Raster images placed in layout applications such as InDesign or QuarkXPress can be sized to almost any desired dimension. This is scaling, and essentially overrides the dimensions specified by the image. The quality of the image after scaling depends on its effective resolution, which is the resolution of a placed image after scaling has been applied. Most layout applications will show the image size as a percentage. Thus, a 300 ppi image placed and scaled to 120% is no longer 300 ppi when printed -- it is now 250 ppi.

Resampling - Resizing a raster image in an image editing application such as Photoshop usually offers several choices on how the resizing should be done. Resampling, or changing the pixel dimensions, is a method that actually adds pixels to (or removes pixels from) the image. (In Photoshop, checking the box "Resample Image" during an Image Size change will enable resampling.) This is useful for permanently changing the resolution of an image, but it does not add detail or quality to an image. There seems to be a misconception that images can be resampled to a higher resolution to obtain higher quality. It does help to eliminate unwanted pixelization or reduce "jagginess," but essentially resampling to a higher resolution only produces a blurring effect that masks the otherwise pixelized appearance. Resampling an image to a higher resolution for printing purposes should only be done within a certain amount; generally 200% or less. Beyond that, the result is of dimishing returns: an extraordinarily large file size for very little gain in perceived quality. (This misconception seems to be more common with the popularity of internet web pages and the myriad of low-resolution images that are readily available.)

Alternatively, resampling an image to a lower resolution is acceptable for any instance where an image is simply larger than necessary. A good example is when you have a full page image that only needs to be printed at a very small size. (i.e. An 8 x 10 portrait of someone that is being printed on their business card at a size of 0.8 x 1.) The savings is in time to print, and disk space used; with no quality loss in the printed image.

Resampling an image to a higher resolution is sometimes called "upsampling;" whereas resampling an image to a lower resolution is sometimes called "downsampling."

Remember that downsampling an image is essentially irreversible. Once an image has been downsampled, the loss of detail is permanent. Upsampling only can "guess" as to what detail has been lost. If you intend to use an image for multiple purposes, keep a high resolution copy somewhere.

Resizing / Changing Document Size - You can change the size of a raster image in most image editing applications without affecting the number of pixels in the image. This is changing the Document Size or, in actuality, assigning a dimension to the pixels themselves. An image comprised of 100 pixels by 100 pixels may be printed at 1 inch by 1 inch; or it may be printed at 10 feet by 10 feet. A pixel has no fixed size; it is determined solely by you. By changing the Document Size of a raster image, you are specifying the size that the pixels should adopt. In Photoshop, with the "Resample Image" checkbox cleared, you can observe how the document size and resolution are inherently linked (as indicated by the graphic of a chain link to the right). As you increase the size of the document, the resolution is reduced; and vice versa.
As it applies to graphic design and printing, this relationship is very trivial, because layout applications can override this value through scaling (see above). The only effect Document Size usually has in graphic design, is in determining the size that an image will be when first placed in a layout document.

Canvas Size - Have you ever drawn a picture on paper only to find that you incorrectly guessed where to start, and have run out of room somewhere? Taping an additional sheet of paper to the first to extend your drawing area is very much like the Canvas Size option in Photoshop. This allows you to extend or contract the area of the image without affecting its resolution. Adding "canvas" to the image is similar to cropping in reverse. Subtracting canvas from the image is identical to cropping, except that you are specifying the area to crop in relationship to the edges of the current image (rather than using a selection marquee).

[Back to Top]

Compression

Compression is a means of making file sizes smaller, usually for the purpose of transmission or saving disk space. There is both in-image compression and external compression. The two basic kinds of compression you will find in-image are lossless and lossy.
Lossless compression (such as LZW) compresses an image without sacrificing image quality. It does this by finding repeating groups of pixels and substitutes other information to designate the repitition. LZW compression can dramatically reduce filesize for images with areas filled with solid colors.
Lossy compression like that of JPG images discards image information to an extent based on the compression amount desired. JPG compression exploits known limitations of the human eye, notably that small color changes are perceived less accurately than small changes in brightness. High compression can cause noticable 'artifacting' and blocky-looking areas.
In-image compression as a general rule is not recommended for printing because the compression methods can slow down or even halt the RIP process. They also have a tendency to change the appearance of colors regardless of whether the compression is lossless or not.
If you are transmitting files via email or attempting to save space on a disk, we recommend that you use an external compression program such as Aladdin Stuffit Deluxe for Macintosh or PKZIP for Windows.

[Back to Top]