Before we can talk about converting raster to vector, we need to understand what each is. Starting at the very basic level…each describes a type of drawing or image. So, what exactly is a raster image and how does it differ from a vector image? And why do we need to convert raster to vector?
First, let’s look at the graphic arts. Basically, a photograph is a raster image, while a line drawing is a vector image. A raster image is made of up pixels (dots), each a different color, arranged to display an image, while a vector image is made up of paths (lines), each with magnitude (length) and direction that can be described by a mathematical formula. Rasters cannot be described by a mathematical formula but can be described by a locus, or location (xy).
The really important difference for graphic artists is that raster images do not retain their appearance as size increases. This is why when you blow a photograph up, it becomes blurry. Vector images do retain appearance regardless of size since the mathematical formulas dictate how the image is rendered.
Uses of Rasters and Vectors in Graphics
Raster images are really good for displaying lots of colors in a single image and they allow for the greater definition of color than a vector image. Raster images can also display finer nuances in light and shading at the right resolution. Vector images are scalable, which means that the same image can be designed once and resized infinitely for any size application – from business card to billboard.
Raster images cannot be made larger without sacrificing quality. Vector images cannot display the natural shading and color qualities of photographs. Raster images are often large files, while vector images are relatively small. For the commercial graphic artist, each type of image has its place.
Uses of Rasters and Vectors in Engineering and Mapping
In the industrial, engineering and mapping worlds, both raster and vector images are also used. For instance, in mapping, satellite images and aerial photography are frequently used to capture topography, roads, buildings and other geographic demarcations. These are graphic images, which are raster. In order to create maps from these images, they need to be converted from the raster image to a vector image.
In industrial and engineering applications, sometimes a drawing that was initially created by hand is scanned in order to convert it to digital. Regardless of whether the original image was a photograph or a line drawing, the resulting scanned file will always be a raster file. However, in order to work with a digital file in a CAD (Computer Aided Design) system, the scanned raster file needs to be converted to a vector file. This process is called vectorization. In very simple terms, this means converting the raster pixels, into lines (vectors).
In this instance, the process must start with a high-quality scan. The large format scanner used to scan paper engineering drawings or satellite imagery for raster to vector conversion should be a scanner that is capable of providing a high-quality image. Image quality is everything here. If two lines are muddied and appear as a single line in the scanned image, it will be difficult, if not impossible, to know that they should be separated in the vector image. Once you have the best-scanned raster image possible, rectify the image to make sure it is aligned correctly, scaled correctly, and that it measures correctly on all axes points.
The Process of Conversion
There are many different conversion methods. There is commercially available raster to vector conversion software packages and there are companies which specialize in raster to vector conversion. Probably one of the most important things for a novice to understand is that the raster to vector conversion process is not standardized. There is no single correct method, no “one size fits all” from blueprints to photographs. Even within the same category, blueprints, for instance, there are many different rasters to vector algorithms that can be used and each gives different results. It’s important to recognize that vector representations of graphics can be more abstract than raster representations.
Whether you use an outsource service firm, or whether you attempt the conversion yourself, a clean raster image that has been scanned at the correct resolution will improve the final outcome. Cleaning means de-speckling, getting rid of unwanted artifacts, smoothing and fixing lines and removing any holes. This can be a time-consuming process. One of the advantages of using an outsourced service is that, in addition to the performing the actual conversion, they will also ensure that the drawing is scanned at the correct resolution and that it is clean and ready for raster to vector conversion. In this respect, an outsource service can save you considerable time.
While there are software packages that do provide automatic conversion, it’s important to realize that even the best automatic conversion software, requires some human intervention in order to ensure an accurate final drawing; moreover, it’s safe to say that one raster to vector conversion product does not fit all circumstances. It is not uncommon with 100 drawings, that 30% is converted via one method, the next 40% need to be converted via a different method, and the remaining are hand redrawn. This is when knowing some of the “tricks of the trade” used in conversion can be a huge benefit.