Image Resolution for Color EPS Page Layout Document with RGB or Lab: FlightCheck

In the last “Preflighting 101″ post, we covered text issues in the entry entitled, “Preflighting 101 Part 2 Common Issues- text”. Today, we are delving into the ever important (both technically and communication wise) area of layout graphics images. Markzware FlightCheck preflighting software checks with precision. After all, an image speaks a thousand words. (I wonder how many words that is with today’s exchange rate and image filled, Internet world.) Anyway, back to the potential printing problems and printing solutions with images in your print documents:

Preflighting 101-
II Common Preflight Problems

II b. Bitmap (Tiff) Image Problems
Problems with bitmap images are often discovered when preflighting, as Markzware FlightCheck preflighting software can report. Some of the more common difficulties have to do with improper scaling and image resolution, incorrect color models (one reason you should preflight color), complex clipping paths and poor image quality.

Image Formats
When creating or placing images for print, attention needs to be paid to the file format of the images. The most common formats for image types are JPG (*High Quality), EPS and TIFF. Formats that are likely to give you problems include PICT, GIF, PCX, BMP and WMF. These formats, and others like them, are low-resolution or compressed images (mostly 72 dpi). Low-resolution images are only usable for display on monitors or televisions. They do not have enough resolution to reproduce in high-quality print unless scaled to reach the preferred 300 dpi (i.e. a 72 dpi image scaled to 24% results in 300 dpi output resolution). Compressed images may not print at all if the output device cannot handle the decompression algorithm. Markzware FlightCheck preflighting software will check your document for these types of images and warn you if they exist in the document. If you are preflighting manually, you will need to check the formats of each image to make sure they are usable in print.

Images are often scaled in page layout applications. This increases RIP time and may size them beyond their acceptable image resolution, so that they print incorrectly.

Enlarging or reducing an image beyond these thresholds will cause it to reproduce poorly. It is best to use an image-editing program, such as Adobe Photoshop, to change the size of an image and place it in the document at 100% scale. Be careful with images that may be used several times in a document.

Markzware FlightCheck Ground Controls Part 2

Proper Resolution
Proper image resolution for printed material is between 1.5 and 2.0 times the linescreen at which the job will be printed. We measure linescreen in lines per inch (lpi). Low-resolution images will not render correctly, and excessively high-resolution images will cause excessive RIP time. High resolution image errors can easily be corrected in an image editing application. To solve Low resolution image errors you might want to look for a substitute image that does have the required resolution or scale it.

When working with excessive image resolution in Adobe Photoshop, you can adjust the image resolution in the Image Size dialog box. A high-resolution image can be safely reduced to 40% of 750 ppi. When reducing an image’s resolution, be sure to maintain its height and width while reducing image resolution and file size. The same rules apply to decreasing image resolution. However, increasing image resolution has more restriction. A 300 dpi image can be safely increased to 120% of its original resolution. Beyond this, even color correction will not maintain the quality of the image and the image will need to be re-scanned. Increasing image resolution reduces image sharpness, so always remember to re-sharpen. (Note- you cannot artificially res-up the dpi of an image in Photoshop. It may say it, but it will not work!)

In addition to image resolution, you should also be aware of problems that may occur when rotating bitmap images. Rotating these images may cause increased RIP time or difficulties getting the image to output. Bitmap image rotation should be done in an image-editing program and placed upright in the document with zero rotation applied.

Color Model
In this digital age, more and more images will be in RGB or LAB rather than CMYK format. To preserve the original colors and to be able to decide whether the image will be used for print, monitor or television, the images are converted at the last moment. Converting from RGB or LAB to CMYK though requires color correction. An image in RGB or LAB format will not reproduce well in four-color printing. The RGB or LAB color models should only be used for display on a monitor or television. Every RGB or LAB image must be converted to CMYK before it will print correctly. Preflighting software will warn you of any RGB or LAB images it finds and will check the color model. You will also need an image-editing program like Photoshop to change the color model. Markzware FlightCheck will find an application that will open the image. Then you can launch the application and have the image open automatically. Once the image is converted, you may need to make slight color corrections. For training on color correction, contact Digital Media and ask about the Scanning & Color Correction in Adobe Photoshop training CD.

Issues with duotones, tritones and even quadtones also come up when dealing with color model problems. These images are made from one or more spot colors and (most of the time) black. The problem that can arise when using these images is with the linescreen. The screen angle of spot colors can default to the same screen angle as black when an image is RIPped. If the spot color and the black both print on the same screen angle, the image will not reproduce properly. To eliminate this problem, it is necessary to alter the screen angle of the spot color in the image software that was used to create the image or in the RIP. If you are preflighting manually, you will need to open each duotone and check the screen angle settings individually.

Clipping Paths
Clipping paths are used to mask or hide image backgrounds. A path is created with the Pen tool. At each transition in the path, a point is used to indicate the change in line direction. Points add PostScript instructions, complications the PostScript file and slowing the RIP process down. Proper clipping paths should not be created with and excessive number of points, but will use Bézier curves to reduce the amount of points. Overly complex paths will increase the amount of time it takes to Rip the Image and may cause problems once the graphic is imaged.

DCS Files
DCS is an EPS image file format that is composed of five parts. The first is a master file that is a low-resolution composite file. This file is meant for use in the page layout document. The other four files are print separation files, one for each CMYK plate. DCS images require careful handling. All DCS components must be in the same folder and follow a standard naming convention. A file named “Doorway” saved in DCS format will become five files: “Doorway,” the master file; “Doorway.C,” the cyan image; “Doorway.M,” the magenta image; “Doorway.Y,” the yellow image; and “Doorway.K,” the black image. Individual separation files cannot be reconstructed if they are lost. For this reason, it is critical to check all of the files when sending DCS images with your print job. It is also important to keep the original names of the files so they will link properly when output.

II c. EPS Graphic Problems
EPS, or EPSF, stands for Encapsulated PostScript File (or Format). It is used to describe files that contain PostScript commands, which are essentially printer instruction or recommendations on how to draw objects. To avoid confusion, it is important to make a distinction between a bitmap EPS and a vector EPS image. A bitmap EPS is saved with and EPS file format in a program like Adobe Photoshop. The information contained in the file is strictly the binary pixel data of the image. On the other hand, a vector EPS is an image created in Adobe Illustrator or Macromedia FreeHand that will consist of specific drawing commands your printer interprets in order to render elements like curves or patterns in your art drawings.

For our purposes, we will use the terms EPS to describe images comprised of drawing commands that essentially mathematical formulas. These EPS images are files created in illustration or draw programs such as Adobe Illustrator of Macromedia FreeHand. Unlike binary image data where the image resolution depends on the number of pixels per inch, the resolution of vector images is determined by the output device and may be scaled without noticeable loss of quality. Illustration programs create these images with vector rather than pixel maps. Vectors are the lines (also called paths or Bézier curves) that are drawn and then stroked or filled with color. Because these images use a mathematical process to create the shapes and colors of an image, they tend to be much smaller in file size than bitmap images. Instead of the image holding information about each pixel the entire image, it only holds information about each object in the drawing. Each object has a description such as, “ Fill a circle that is 12 points wide and 12 points tall with a 30% fill of Pantone 233.” This takes up less disk space that images that containing all the pixels data to print an object.

Color Issues with EPS Graphics
You should also take care when an EPS image contains custom colors that do not match the color names used in the page layout document. If the EPS and the page layout document are both supposed to use Pantone 201, but the colors are not assigned the exact same name, tow different colors will be created in the page layout document. For instance, if the EPS image contains a color called “Pantone 201 CV” and the document contains a color called “Pantone 201,” then when the EPS image is imported into the document, the page layout program will not recognize “Pantone 201 CV” and “Pantone 201” as the same color. This would cause the “Pantone CV” in the EPS to image on a different film separation than the “Pantone 201” plate. To avoid this problem, make sure the color name in the illustration program exactly matched the color name in the page layout program. If you cannot edit the EPS image, change the color name in the page layout document before importing the image.

Font Issues with EPS Graphics
If text is used in an EPS graphic, then the font for that text will be required when the graphic is output. The same problems that arise when a font used in document text is not provided can come up here (see Text Problems in the Common Problems section). Preflighting software will examine each EPS graphic and check for fonts used in the graphic. IF the font is not installed on you computer, you will receive a warning.

To prevent font problems with EPS graphics, it is sometimes a good idea to convert text to outline paths in the illustration program. This causes all characters to be drawn using PostScript commands so that the actual font is no longer required to output the job. Otherwise, if you are outputting a document with EPS graphics in it, you will need to check the font usage to ensure that the designer can provide the fonts.

Thanks to Arnold Roosch of Markzware for touching this up! Use FlightCheck, software from Markzware for preflighting during prepress for print quality assurance, available via the FlightCheck page.