In simple terms, a raster image is one formed with pixels, and a vector graphic is one formed of paths. Now what does that really mean? And why does it matter for apparel decoration? Well, here it goes:
What is a Raster Image?
We see raster images every day in modern, 21st century life. All of your handheld devices, monitors and television screens use raster images. Raster images are characterized by its use of pixels. Pixels are little pieces of colour information that when put together very closely, create an image. Ever zoomed in a little too far into a selfie and noticed the little squares that make up the picture of your face? Those are pixels!
Resolution of a raster image is also important, as this is the control we have over the pixels. Resolution is measured in “pixels per inch” (ppi) for screens, or “dots per square inch” (dpi) for print. What this means, is we’re telling the image how many pixels to put in every inch of the image. Higher resolution means more pixels to hold more information and therefore, crisper image, and lower resolution meaning less pixels to hold information. There are advantages and disadvantages to both. Higher resolution will give a crisper image, but will have a much larger file size because of all the extra data. Lower resolution has a more manageable file size, but you will be sacrificing quality. We typically use a high resolution like 300ppi in print to ensure images are high quality, but low resolution images like 72ppi on the web for faster loading times, and where quality isn’t as much of a big deal since screens have a maximum resolution as well.
So know you know about raster images, what makes a vector graphic different?
What is a Vector Graphic?
A vector graphic is composed of paths. Paths are mathematical lines with points directing them where to go and where to terminate. Think of connect-the-dots. Same idea! Vectors don’t use resolution to display their information.
Because they don’t use resolution, they can be scaled up or down without loosing their crisp, sharp edges.
Here’s an an example of two versions of the same illustration, one raster based and one vector based, zoomed in 300%:
See the difference? With the raster version, even at a high resolution, we can see the pixels almost blurring (or “pixelating”!) the edges of each line, whereas the vector version is still sharp!
So why does this matter for apparel decoration?
Glad you asked!
When we take your uploaded art, it is separated so each colour can be burned to a screen for printing. This can be difficult with a raster image, since like the picture above, the edges aren’t always crisp enough to tell where one colour ends and the next begins. With vectors, we always know exactly where colours should stop and start because of the paths.
Also, we have to use printers to get your digital file from the computer to a screen print screen. It’s basically just a beefed up version of your typical desktop printer and works in a similar way by printing tiny dots of colour to make up an image (remember dots per inch? This is where it comes it!). So the printer reads the raster image’s pixel information, and turns it into dots. Modern day printers are pretty great at making sure all those dots line up properly to produce quality images, but they can sometimes slip up for a number of reasons and print one or two dots out of place. Then, when we take that print out and burn it to the screen, sometimes those little dots make it through, giving your print a halftone look, which isn’t always the desired effect. To combat this dot problem, our printer can also print vector paths exactly as they are inputted on the computer! This makes sure that every line is crisp and without dots, so we don’t get the halftone effect. We always prefer to screen print with vector based images for this reason. We want to make sure that the shirt you received is of the highest quality standards!
How do I know if my image is raster or vector?
Simple! Each use specific file types:
Raster images are most commonly .JPEG/.JPG, .PNG, .GIF and .TIFF, but can also be .PSD, .BMP, .HEIF, or .WebP
Vector graphics are .AI, .EPS, .SVG, and sometimes .PDF.
Portable Document Formats (or PDFs) can be tricky sometimes, because many different file types can be converted to a .PDF very effortlessly, meaning that your vector based file may actually have pixels inside. If your .PDF logo file was created by a design professional, chances are you’re good to go. But if Joe down the street offered to change your .JPG into a .PDF, you could have a decoy vector on your hands.