Wondering What File to Deliver?
If you’re not a graphic designer or printer, you may often have some confusion about the many different types of graphic files we work with. It would be nice if there were a simple answer to the questions you’re likely to have, but there are many different variables at play. Different projects have different requirements, with different file types that meet those specific needs best. Even one file type might have many possible different specifications. To try to help you out, in this blog post I’ll give a brief overview of the basics of graphics files, with a cheat sheet at the end with some common file types I often come across and use.
Color
One of the most basic things to understand about graphic files is color, and how it is used in printing and reproduction. First I’ll explain how color is reproduced on computer monitors, then I’ll dig into the main types of color printing, roughly sorted by complexity.
Color on computers is represented and stored in RGB values. RGB is an abbreviation you’ve probably heard, and what it stands for is fairly obvious. “RGB” refers to the primary colors, red, green, and blue. Each pixel on a computer screen can create red, green, and blue light in various amounts. Pretty much any possible color is able to be reproduced by combining red, green, and blue light. If you combine any two of them at full intensity you end up with a secondary color, cyan, magenta, or yellow. All three together at full intensity create white, if all three are at minimum intensity you end up with nothing, black. This is called additive color mixing. Keep all of that in mind for later.
First up in the hierarchy of printing; spot color. This is when the colors used for printing are mixed before they are printed, so the end result you see and the inks used to print it correlate directly. If there’s an area of orange in a spot color print, the ink used to print it is likely that exact color. Types of prints that fall under the spot color umbrella that you might recognize are as follows: A black and white photocopy, by which I mean true black and white, no gray tones. A greyscale photograph, with gray tones. Monotone is a word used to describe something created or printed in a single color; essentially, one spot color. A black and white or grayscale image is technically monotone, as well as theoretically being a single spot color. Screen prints are a great example of spot color printing that people can recognize easily. While colors may sometimes be layered in a screen print to create other colors, the actual ink colors are likely obvious. Another print process which uses spot colors, but which is more niche and less analog than screen printing, is risograph printing. The average person probably can’t tell the
difference between a screen print and a risograph print; both processes look fairly similar because they are both spot color processes.
The next, and most common type of color printing these days, is CMYK printing. CMYK is to color printing what RGB is to computer screens. CMYK refers to the secondary colors, cyan, magenta, and yellow, with the addition of black, called “key”. While RGB screens are based on producing colored light, CMYK printing is based on absorbing light, since paper cannot produce light. When white light hits yellow ink, the ink absorbs all the other colors contained in that light and reflects the yellow component back at the viewer. Where in RGB adding all three colors together creates white, in CMYK adding all three colors together results in black, because there are no colors of light that the inks aren’t absorbing. Mixing various amounts of each color ink is able to effectively create any color - though in reality there are some limitations. This is called subtractive color mixing. Because ink formulations aren’t perfect, adding all three together usually doesn’t reach a sufficiently dark black, which is where key - or black - ink comes in to play. A CMY black will often have K printed over it as well to make it look even darker. In spot color printing you rarely see colors that aren’t the ones used for printing, CMYK is the opposite though. You will almost never see one of the ink colors printed pure in CMYK printing, instead they are almost always combined on the paper to create other colors. This is the process most often used to reproduce color photos for that reason. The inks aren’t technically mixed on the paper, but are instead printed in dots so small that your eye visually mixes them together. The density of these dots is measured in dots per inch, “dpi”. 300 dpi is the standard for an average sized print, though large posters are often 150 dpi, and banners might be 75 dpi or less. If the viewer is likely to be further from the printed asset, the dots can be larger.
The files
From here we move on to digital files used to store graphics. There are two main types of graphics files, raster files and vector files. The easiest way to think of raster files is as a sheet of gridded paper being used as a paint-by-numbers. If you painted a picture of a flower on that paper, you could break the image down into what specific color is on each specific square. Each square is equivalent to a pixel, and the color information for each pixel can be stored as either RGB or CMYK data. Raster files are best suited for storing photographs and other texture rich images, because it is easy to store the information of every visible pixel even when they’re all different. Raster files have a density that corresponds to the dpi used in CMYK printing, in this situation called pixels per inch, or “ppi”. A 600x600 pixel image that is 300ppi would be 2x2 inches. People often use the word “size” for several different aspects of raster files, but it’s best to use the word “size” to refer to how much storage space the files takes up (usually in megabytes), “resolution” to refer to its dimensions in pixels (for example, 2000x3000px), “print size” to refer to how
large it would print, and “pixel per inch” to make sure it is actually high enough resolution to be printed (a 70ppi file would print at 70dpi, which would look terrible for something viewed up close). If a raster file has been saved at 100x100 pixels, it can never be made larger again, so with raster files it is important to keep a backup of the original file if you ever scale one down.
The other main type of graphic file is known as vector. Vector files are unintuitive at first if you haven’t worked with them before. Vector files mathematically draw every shape contained in the image. A vector file is essentially like using the graphing equations you learned in high school to create a whole image. If a square is drawn in a vector file, the file doesn’t store every pixel of the image, it just stores the information “a square of this size and color is located here”. This makes vector files very precise, and much smaller for simple images. The biggest benefit of vector files comes from the fact that they don’t store any pixel information, they don’t have an inherent limitation for how large they can be printed, unlike raster files. A mathematically drawn circle is still a mathematically drawn circle no matter how large you print it.
Both raster and vector files can sometimes have invisible layers that are important to the person printing them. Layers also play a big role for the person making the original file, but that aspect doesn’t matter as much after the file is finished. If you reference the chart below you’ll see that jpgs don’t have layers, which is part of the reason why they shouldn’t be sent out to printers.
Raster files can sometimes have compression, where the computer discards information that is similar. If two pixels next to each other are almost the same color, compression will just change them to be the same color so the file is more efficient. This is helpful for storing files because their storage size becomes smaller, but it makes images look worse. Any image that is important should not be compressed. Compression is often also called “quality”, especially when saving jpgs.