|OK, everyone out there who really understands rasters, vectors, and RIPs, raise your hands! I thought so, since I’ve asked this question in many seminars I’ve given and found this essential knowledge lacking. Twenty years ago a buyer didn’t have to know this information; in fact, it barely existed. Today, it is at the very heart of what we do and therefore, this subject is a must-know.
In the beginning, it was all about raster, sometimes referred to as bit-mapped. Since what you see onscreen and in print is composed of dots or pixels, all files were once raster files composed of a specific number of dots or pixels. Note that for our purposes you can use the terms pixels or dots interchangeably, but most refer to pixels for onscreen and dots for print. The development of the vector file format and PostScript was a monumental step forward, making desktop publishing possible. But first let’s look at the raster file since it is the file format for photographs and many other types of graphic images. And it is the file format that causes the most grief and confusion.
Part One: Raster Files
In a raster file, each pixel is addressable, which doesn’t mean that you can send your favorite pixels love letters. It means that you can access and alter each dot in a raster graphic and that not only does each individual dot have its own color, but it can be copied and repositioned as well.
Photographs are the most common raster graphic (if you think about it you can understand why). The skin tones look real and natural because each dot or pixel can carry slightly different color information than the one next to it. Since each pixel is addressable, raster files can be manipulated in a variety of ways through pixel-editing programs like Adobe Photoshop. You can make your favorite photo brighter, take out Aunt Millie, give your best girl or guy blue eyes, or update that old product shot all with undetectable reality by using software such as Adobe Photoshop. Altering on a pixel-by-pixel basis would be quite laborious, so image-editing software has features that enable you to do global changes, like color correction or pixel copying, by using automated tools. However, even though you are working on a global basis, the program is actually working on a pixel-by- pixel basis, which is why it looks so real.
Dots or pixels are addressable, but you must address the most critical aspect of a raster file, which is resolution. When a raster file is created by scanning or a digital camera photograph, it is born with a given number of dots or pixels. You can control the “birth” through settings on your scanner or by buying a digital camera with more megapixels. And yes, there are methods of adding pixels like resampling. But the truth is for best results you are stuck with the maximum number of dots or pixels already present in the file. In programs like Adobe Photoshop you can resample down to lower the number of dots or pixels but resampling up, or adding dots or pixels, more often than not produces disappointing results.
How many dots do you need? For the Web or on-screen 72 DPI (dots per inch) will do. However, the rule for print is twice the line screen. Newspapers can be as low as 85 line screen so you can get away with 170 DPI. Many magazines are 133 line screen hence 266 DPI. Quality print ranges from 150 to 200 line screen. The math is easy, but I will give you the rule that most printers follow: regardless of 150 or 200 line screens, use 300 DPI. So there you have it--Web is 72 DPI (or PPI) and most print is 300 DPI.
Because a raster image is created with a specific number of dots or pixels that cover a given area, raster files lose DPI as they are enlarged, and can gain DPI as they are reduced. All this has a very detrimental effect on quality. For example, when you scan an image into your computer, you pick a size and a resolution. Suppose you scan a photo and select 2”x 3” at 300 DPI. Now you want to enlarge it 200% to 4”x 6”. Your resolution will drop to 150 DPI. Since your screen doesn’t go beyond 72 or 96 DPI, you will not notice a difference on your screen. However, a disaster may occur when you go to press because 150 DPI may not be good enough to generate the quality result desired. Conversely if you reduced the original 2”x 3” by 50% to 1”x 1.5” the resolution jumps to 600 DPI as more pixels or dots are crowding into a smaller area.
One would think that in terms of file size 144 DPI is simply double 72 DPI. But DPI is in linear inches and the actual number of dots in a file would be in square inches. Thus a 72 DPI file has 5,184 dots per square inch (72 x 72=5,184). Just imagine a one-inch square box with 72 rows of dots and 72 dots in each row. A 144 DPI file dramatically jumps to 20,736 dots per square inch or four times the 5,184 dots of the 72 DPI file (144 x 144=20,736). And a 300 DPI file leaps to a whopping 90,000 dots per square inch, which is more than seventeen times the file size of a 72 DPI file (300 x 300=90,000). Now add to this the fact that you can get away with certain types of file compression onscreen, which may not work in print. I have seen an onscreen file successfully reduced to 11 kilobytes look great whereas the print version was almost three megabytes.
Now you can start to see the agonizing dilemma of raster files in this world of Web and print. Web needs the smallest file size possible to speed its journey to desktops around the Web, whereas print files are huge. The resolution conflict is even more apparent when you pick a raster image off a PowerPoint presentation or off the Web that looks great onscreen only to have a mess in print. Plus even if you started with 300 dpi and determined the image was too small enlarging it may cause poor results. Large-format printing has had to deal with this problem and has had some success. But that is another story.
The most-common raster file formats are TIF, used primarily for print, and JPG and GIF, which are popular onscreen. But whatever the format, appropriate raster file resolution is essential for good-quality print.
Now how about a “no worries, no need to even think about it” (resolution that is) file format? Tune in to Part 2 next month to learn about the vector file and the PostScript RIP, which gave birth to prepress as we know it today.