Bitmap (also known as raster) images are composed of many small dots, or pixels, which combine to create an image. Most photos that you are likely to see online, or in catalogs, are bitmap images. It is likely you will recognize the files types: JPEG, GIF, and PNG. These are the most common bitmap image types (though PDFs are typically vectors, they can also be bitmaps). The problem with bitmap images is that because they are composed of small dots called pixels they cannot be resized without compromising the image quality. When you increase the size of a bitmap image you are just enlarging the pixels, and once they become a certain size the individual pixels become distinguishable- thus the term “pixelated.”
To avoid these issues, you must save bitmap images at the exact size necessary for your print. Otherwise you will run the risk of a low-resolution print. There are 72 dots per inch, or DPI (most internet images are 72 DPI), in a low-resolution image. To be digitally printed (sublimation, direct-to-garment, wide format), an image should be at least 300 DPI. Having a higher DPI allows for sharper images with greater detail that will jump off of whatever surface they are printed on.
Vector images, however, are more flexible. Vector images are created through the use of mathematical formulas rather than pixels, using lines and curves to create images. This allows vector images to be resized without issue, and makes them the choice for images that will need to be resized often. For example, your business logo ought to be a vector image so it can be used to print on a wide variety of media to promote your brand. EPS, AI, and PDF are common vector image types. Vector images can be saved as bitmap images, but bitmap images cannot be saved as vectors.
A good way to think of the difference between the two is the vector should be your “source file”- the original from which you can save and create bitmap images in the exact size necessary for a specific print. Also, it is difficult to vectorize photographs or full-color images, so vector images are great for logos and similar images. However, photographs and images with a large number of colors should be saved as bitmap files.
Screen printers typically prefer to work with vector images for several reasons. First and foremost is the aforementioned scalability of vectors. It takes far less time and effort on the printer’s behalf to adjust vector images. Also, vector lines are much smoother since they are actual lines rather than a collection of pixels, and it creates a smoother print. If the screen printer is performing the color separation, then vectors are much easier to color separate. The colors in a vector image are constrained, whereas in a bitmap image the colors are spread among thousands of tiny pixels. Using vector artwork for screen printing ensures the highest quality print.
