Raster images are made of pixels. A pixel is a single point or the smallest single element in a display device.
Vector images are mathematical calculations from one point to another that form geometrical shapes.
vector vs raster
I've magnified a raster and vector image above to better see the differences between the two, but you cannot always see the difference at a glance. Vector graphics also display an outline or wireframe view which is important for processes that require vector art. More details below.
When a raster image is scaled up, it usually loses quality. A raster image can be enlarged by either adding more pixels or enlarging the size of the pixel. Either way you are spreading the original data over a larger area at the risk of losing clarity.
A vector program will use a mathematical formula to build an image that can be scaled to any size without losing quality.
Raster / Bitmap Images
Raster images' dimensions are measured in pixels. Because raster images cannot be enlarged without losing quality, different suppliers have specific size requirements for their processes; they require a specific pixel resolution: a specific amount of pixels within each inch. The amount of pixels within each inch in the image represents the image pixel resolution or ppi (pixels per inch).
How large a raster image can be printed - and maintain quality - depends on 2 things:
the pixel dimension of the image (e.g. 6824 pixels wide by 2345 pixels high)
the pixel resolution: pixels-per-inch (ppi) required by the particular printer
Some offset printers (paper printing) require a minimum of 300 ppi
Some screen printers (cloth printing) require a minimum of 240 ppi
Large format printers (banners, billboards) vary a lot because it also depends on the distance from which the sign is going to be viewed - could be as low as 20 or more than 200
How to determine what size your raster image must be, for good quality printing:
Multiply the resolution required by the area to be printed. Examples:
If a printer requires a minimum of 300 ppi and you want to print an image in an area that is 5 inches wide, multiply 300 pixels x 5 inches (300 x 5 = 1500). Your image must be at least 1500 pixels wide.
If a printer requires a minimum of 240 ppi and you want to print an image in an area that is 12 inches wide, multiply 240 pixels x 12 inches (240 x 12 = 2880). Your image must be at least 2880 pixels wide.
Can we enlarge the pixel dimension and resolution of a raster image?
Raster images have a certain amount of pixels within each inch. A 72 ppi image has 72 pixels in every inch. A 300 ppi image has 300 pixels per inch. Usually the higher the ppi, the higher the quality. When you are required to provide a high resolution image file, the file must have been created or scanned at both the dimension and the resolution required. E.G. if you need to print an image at 2 inches wide and 300 ppi is required, your image must be created/scanned at a minimum of 600 pixels (2in x 300dpi).
Once the image is created at a certain dimension, you may not be able to use this image at a larger size without losing quality. When you manually increase the resolution with a program like Photoshop, Photoshop randomly adds pixels and the result will most likely be a high resolution image of poor quality.
Sample of a raster image below:
Raster image made of pixels
How to determine what dimension your existing image can be printed at:
Divide the pixel dimension of your image by the resolution required by your printer.
To view and edit an image pixel dimension you must use a photo-editing program like Adobe Photoshop or Paint Shop Pro to open the raster file.
If your image is 1993 pixels wide & printer requires 300 ppi (1993 χ 300) can be printed at 6.643 inches
If your image is 1993 pixels wide & printer requires 240 ppi (1993 χ 240) can be printed at 8.304 inches
Common raster image files: jpg, jpeg, gif, png, tif, tiff, bmp, psd and pdfs originating from raster files
Common raster programs: paint programs like Photoshop & Paint Shop
Common raster images: photographs, illustrations with soft blends of color gradients
Vector graphics are made of mathematical calculations that form objects or lines - they do not use pixels therefore they are resolution-independent.
Vector graphics can be enlarged and printed at ANY SIZE!
Instead of pixels, vector graphics use objects and lines (shapes) to represent images. Vector graphics can be scaled to any size without losing quality.
Some Vector programs have two different views; preview/normal view which displays the image as we normally would see it and an outline/wireframe view which displays the outline of every object in the file. This vector outline/wireframe is important to some companies like engraving & vinyl-cut signs because it guides the equipment they use to create their products.
For categorizing purposes I have named the above: photo-realistic vector, vector illustration, and vector lineart.
1. Photo-realistic vector: a photograph was traced as vector using a multitude of colors to allow the image to have a closer resemblance to original. This vector image can be used in many different processes like CMYK or digital printing, but as you can see from the outline/wireframe view, there are too many objects for it to work for engraving or vinyl-cut signs, or any other process that is guided by the vector outline. Photographs are best printed as they are: raster images... Learn more.
2. Vector illustration. This is a hand-drawn, simplified illustration using a limited amount of colors and color blends. Color blends can be created in vector programs but they are actually raster effects so this image is NOT 100% vector. The raster effects show up as boxes on the outline/wireframe view. This vector image is suitable for many processes except those that are guided by the vector outline such as vinyl-cut signs or engraving.
There are other ways to achieve color blends,
please see traced vectorization of images with color blends
3. Vector lineart. This is 100% vector art; NO COLOR BLENDS. The outline/wireframe is acceptable for all processes including those that are guided by the vector outline.
Common vector graphic file format: eps* ai, cdr, ps, pdfs originating from vector files
Common vector graphic programs: drawing programs such as Illustrator, CorelDraw, FreeHand
Common vector graphics: logos, illustrations, drawings
* eps and pdf files can originate from either raster or vector programs, and/or can include raster & vector elements. Is EPS Vector?
If I am creating a new design which software should I use; Raster or Vector Graphic Program?
It depends on the design itself. If it's going to have photographic elements with continuous tones and blends of color, you are probably better off using a paint program like Photoshop which is a raster program that specializes in photo editing, or use any other PAINT program.
If you want your final design to look like an illustration instead with clear contrasts between objects & shapes, then use a vector program.
You MUST use a vector program when creating art for plotters, vinyl-cut signs, engraving and other specialty items.
Ideally a company that has a logo design with photographic elements, also has a secondary version of their logo in vector format that can be used for those specialty items that require vector art. Sample below:
Raster and Vector logo
DPI vs PPI
DPI - Dots per Inch
This is the amount of ink dots the printer will put on each pixel of your image. The DPI is set by the actual printer device.
PPI - Pixels per Inch
Digital raster images are measured in pixels, or picture elements. How many pixels per inch is determined by the device you create the digital image with: camera, scanner, or graphics software and can be modified with a photo editing software like Photoshop.
Made of pixels
Represents and edits photo and photo-like elements better than vector programs with the use of continuous tones. The use of different color pixels allows for smooth blends of colors.
Is bound by the number of pixels in the image. It cannot be scaled up without losing quality.
Large dimensions & detailed images equal large file size.
Some service providers like engravers, stencil-cut signs, etc, must have vector art.
It is more difficult to print raster images using a limited amount of spot colors.
Depending on the complexity of the image, conversion to vector may be time consuming.
Made of mathematical calculations that form objects and lines.
Can be scaled to any size without losing quality.
Resolution-independent: Can be printed at any size/resolution.
Number of colors can be easily increased or reduced to adjust printing budget.
A large dimension vector graphic can maintain a small file size.
Vector art is required by many service providers.
Can be easily converted to raster
It is not the best format for photographs or photo-like elements with blends of color.
Letter Visibility Chart
Letter Height (Inches) Distance For Best Impact Maximum Readable Distance