Chapter 4 -- Preparing Raster Images
Prior to the digital technology of today, image capture and scanning worked entirely with film. Scanners then progressed technologically and began outputting digital data. At this point, pixels were created as physical images were scanned into a digital format. Today, with digital cameras, images originate in a digital form, cutting out the middleman.
Most of the current personal "point-and-shoot" cameras output JPEGs, a compressed format of images that exhibits loss of data along with said compression. Higher end and DSLR (Digital Single-Lens Reflex) cameras now have the ability to output Camera RAW images as well as JPEGs. The RAW format shows all of the information captured by the sensor(s) of the camera and exhibit minimal to no compression. RAW files are far larger than other formats and cannot be placed into most software programs. However, this can be dealt with by opening the files in Photoshop (or other compatible software) and saved in a different format.
The amount of pixels in an image is directly related to the resolution of the image. Resolution is vitally important when it comes to raster images, and is usually measured in ppi (pixels per inch). As you might expect, high resolution images have a higher ppi, while low resolution images have a lower ppi. What this boils down to is the more pixels there are in am image, the smaller they are, and the finer the details appear to be. Conversely, less pixels mean that the pixels will be larger, and the image will be fuzzy and have little detail. Though opinions differ on what ppi/resolution is best for print, one solid rule to follow is to never scale a raster image up or down, as it will affect the clarity of the image.
When it comes to cropping images, it is always best to play it safe. Avoid cropping it early on if there is any question as to whether or not it will need to be cropped further in the future. At the very least, keep the uncropped, original version of the image as a back up, just in case. Similarly, when considering rotating an image, it is safest to either to keep it at its original rotation or make sure it is scanned at the proper angle, or to rotate it at only 90-degree increments. This is due to the fact that rotating an image requires the software to interpolate (redrawing the image and placing the pixels in the appropriate location for the newly rotated image) the pixels within the image. Rotations of 90-degree increments are easily done by the software and do not effect the detail of the image. Any other angle of rotation will result in some loss of detail, and multiple rotations will increasingly deteriorate image quality.
One common type of raster image is a bitmap, or line art, image. This format uses only black and white pixels and should not be confused with the grayscale format which uses black and white pixels along with various shades of gray. Bitmap images can be very useful in situations that require handwritten type or signatures, especially considering the very high resolution of their scans.
Appropriate raster image file formats for print include TIFF (tagged image file format), Photoshop EPS (encapsulated PostScript), Photoshop Native (Photoshop document), and Photoshop PDF (Portable Document Format). Each of these has its pros and cons, but each should produce acceptable prints. One thing to keep in mind with Photoshop Native files deals with how it creates transparencies. The issue here is that other software like Illustrator, InDesign, etc. do not properly handle the blending used in Photoshop. The easiest way to avoid issues is to create elements which would normally pose problems in InDesign or Illustrator. File formats inappropriate for print are PNG (Portable Network Graphics), BMP (bitmap), GIF (Graphics Interchange Format), and JPEG (Joint Photographic Experts Group). These all have different issues, whether it be with color or resolution, which can make for poor quality printed images.
Chapter 5 -- Vector Graphics
Unlike raster images, vector graphics do not contain pixels and are generated mathematically. As a result, these images are completely resolution independent and can be scaled freely without affecting quality.
File formats for vector graphics include EPS (Encapsulated PostScript) and Native files (Adobe Illustrator). Vectors can also be exported into PDFs. This can often be the ideal choice when printing because it doesn't require fonts to be supplied along with the file. There are several vector formats that are not appropriate for print like WMF (Microsoft Windows Metafile Format), EMF (Enhanced Metafile Format), and any of the raster formats like TIFF, JPEG, PNG, and BMP that are offered for exporting. Each of these formats could cause all or part of the file to be rasterized.
Many fonts are available to be embedded within Illustrator for EPS or AI files so that they may be placed into other programs. Embedding fonts into a file includes the proper font information to allow for display and printing of the text, but not editing. Embedding a font does not always guarantee that the information will survive through the printing process. Opening the file without the proper fonts installed, using fonts that forbid embedding, or issues during imposition or processing by the RIP (Raster Image Processor) could cause corruption of information.
Outlining text (or converting fonts to outlines) can eliminate issues with fonts included in vector graphics. This turns the text into an image rather than copy. However, fonts contain information (called hinting) that refines the display and printing, and this information is destroyed during outlining. Without hinting, some fonts could have a "bloated" appearance on smaller printers. Still, outlining can be a good substitution to embedding fonts.
One final thing to remember about vector images is to simplify your paths. This means that you should make less clicks, even if you might think that more points makes for more detail and a better image. Years ago, drawings with too many points would overload the RIP, causing printing to slow dramatically or even stop altogether. Though that is not an issue today, it is still best to minimize the number of points in your drawings as it will lead to smoother images.
Sources:
McCue, Claudia. "Preparing Raster Images." Real World Print Production with Adobe Creative Suite Applications. Berkeley, CA: Peachpit, 2009. 77-96. Print.
McCue, Claudia. "Vector Graphics." Real World Print Production with Adobe Creative Suite Applications. Berkeley, CA: Peachpit, 2009. 97-105. Print.
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