Research Tips & Stories for Everyone
In the era of digital publishing, scientific figures are prepared and published in digital graphics. Hence, it is useful to know the basics on digital graphics relevant to scientific figure preparations. In this tip post, I wish to introduce the most basic concepts about digital graphics including 1) vector vs. raster images, 2) image resolution, and 3) digital image formats. I also find that this guide document from Nature also provides good information regarding the basics of digital graphics.
Vector vs. Raster Images
Although there are tons of different file formats for digital images, there are two big categories of digital images: vector and raster images.
Vector images are digital images whose geometry is defined by points on a Cartesian plane (2D) connected by lines and curves to form polygons (the name vector comes from the fact that it is made of many vector lines to form polygons). The key benefit/feature of vector images is their capability to being magnified without losing detail/resolution – because vector image is defined by the points on a Cartesian plane that can be easily transformed in the same plane without losing information. It’s linear algebra in action! Because of this capability to keep high resolution regardless of image manipulations (magnification, etc) and easiness in edits without losing details, vector images are the most preferred type for scientific figure preparation for schematic illustrations, plots, and other graphical elements other than raster image-based data.
Raster or bitmap images are digital images based on color information on a predefined grid of dots or pixels (the name raster comes from the fact that it is made of dots in a raster or grid). Raster images are the most common type of digital images because image sensors in measurement devices (camera, microscope, CCD, or photodiode), as well as screen capture, mostly generate raster images due to their straightforwardness (just converting pixel information in sensor/screen into raster image). The key disadvantage of raster images is that their resolution is predefined and degrade by magnifications contrary to vector images (see the above image for example).
Since the major purpose of scientific figures is the delivery of information, high enough resolution in digital images to clearly convey the needed information is a critical requirement for scientific figures. Vector images have an obvious advantage in terms of image resolution as their resolution is independent of magnification. While some journals adopt vector graphics in their PDF version of papers, still the majority of paper figures are published in raster images (in such cases, original vector images are rasterized). Furthermore, many scientific image data are only collected in raster images. Hence, image resolution is an important consideration in scientific figure preparation.
The most common unit of image resolution in publishing is dpi (dots per inch). Dpi is a pretty straightforward unit – higher dots per inch, higher resolution. The above image shows an example of the difference between different dpi for the same image. 300 dpi is the most common resolution for printing (and therefore, for publishing-quality paper figures), giving high resolution where individual dots are not visible in the human eye. 72 dpi is a common choice of resolution for web-based content, but you can see a bit grainy texture due to the low resolution.
Digital Image Formats
There are a lot of digital image formats, but there are a handful of commonly used formats in scientific publishing. For vector images, EPS and PDF formats are commonly used. AI and postscript (SVG) formats are also used widely. For raster images, TIFF format is the most commonly used one for publishing while JPEG, PNG, and GIF are widely used too. Note that vector formats have a lot higher editability as they conserve individual editable elements instead of making the whole figure into a flattened raster image. Therefore, I recommend using vector format until the figure reaches the final version before rasterizing (if needed). Some journals require the submission of vector format figures to allow appropriate copyediting.
Disclaimer. The contents are my personal opinion and do not represent the view of any institution or company I am affiliated/employed. If you find any incorrect information, please feel free to let me know via my email.