Previously, I wrote about the different ways to use scientific illustration to communicate research. In this article, I will introduce the technical aspects of digital images and the most common image file types that scientists will encounter when preparing images for publication.
Raster Versus Vector
Digital images are classified based on how information is stored. Images can either be in raster or vector format (Figure 1). Images in raster format are made of a specific number of pixels – tiny dots coming together to form a picture. Conversely, vector images consist of mathematical formulae, known as paths, linking one point to another to form lines and shapes. Vector images are made of thousands of paths linked together like a wire-frame. Each path encodes information such as colour and length.
Since the details of vector images are encoded in mathematical paths which remain the same regardless of image size, they can be scaled limitlessly without becoming pixelated. However, as information is represented in a fixed number of pixels in raster images, there is a limit to which raster images can be enlarged before visible loss of image quality is observed. When working with raster images, an important factor to consider is image resolution. Simply put, resolution is the amount of detail that can be represented in an image. Resolution is determined by the number of pixels in the image. The common units of resolution are dots per square inch (dpi) and pixels per square inch (ppi). The more dots or pixels there are in the image per area, the higher the resolution. Imagine an image made of 300 dots compared to the same one made of 72 dots. More details will be represented in the image comprising of 300 dots as compared to that of 72 dots. As a result, an image with higher resolution will appear sharper and with greater clarity than one with lower resolution. An image with higher resolution can also be scaled to a greater extent than one with lower resolution before discernible loss of image quality occurs. If there are few pixels in an image, extensive increases in image size will cause each pixel to become large and noticeable, resulting in pixelated images (Figure 2).
The pixels of the image on the left are so small that they are not visible. The intricate details of the stained cells are clearly visible. On the other hand, the pixels of the image on the right are larger and visible. The details of the stained cells are lost. Image modified from Carl Zeiss, Flickr.
At what resolution should your images be set? That will depend on the complexity and size of the image as well as the distance from which the image is viewed. Resolution as low as 30 dpi is sufficient for the printing of images viewed from a greater distance (e.g. billboards) while images viewed from a closer distance (e.g. on a page) should be printed at higher resolution for details to be discernible (at least 300 dpi according to most journal publication guidelines). A picture of a uniformly blue sky can be appreciated at a lower resolution while a colourful microscopy photo showing multiple fluorescent stains will require higher resolution before every detail can be clearly distinguished.
|Type||How information is stored||Resolution dependent?||Scalability|
|Raster||Pixels or dots||Yes||Can only be resized to a certain extent before noticeable image quality loss is observed|
|Vector||Mathematical formulae||No||Can be resized infinitely without image quality loss|
Common Image File Types You Will Encounter When Preparing Images for Publication
Raster and vector files are further classified into different formats, depending on the type and extent of data compression. As a scientist, you will prepare images for publishing on a variety of platforms, such as research manuscripts, posters, and the Internet. These are some of the most common image file types which you may be working with.
Note: The files listed here are compatible with most popular graphics design programs. These files (and more) can also be readily converted from one to another, using either online converter tools or the export functions in graphic design software.
JPEG (also known as JPG)
JPEG is named after the Joint Photographic Experts (JPEG) Group, which created this file format. JPEG files are the most common type of compressed file encountered. JPEG files can store a lot of information in a small file size. As JPEG images undergo “lossy” compression (i.e. compression which degrades image data), some of the information and image details are lost. JPEG files are popular for applications in which space maximization is important. For example, digital cameras often store images in JPEG format to maximise the number of photos which can be stored in a SD card. Due to their small size, JPEG files are also commonly used in Internet applications as they are easy to upload to web pages.
JPEG is usually not recommended for figures used in publications such as manuscripts and posters. This is because loss of information during compression may cause the image to be pixelated and blurry when resized. Repeated saving of JPEG files will also degrade image quality.
Advantages: Small file size
Drawbacks: Loss of image information
Good for: Photos for Internet applications
TIFF (also known as TIF)
Tagged Image File Format (TIFF) is a common uncompressed image file type. As the files are not compressed, TIFF files retain detailed image data. However, this means that they can take up a lot of data storage space! TIFF files are extremely versatile when it comes to colour. They can be in grayscale, cyan-magenta-yellow-key (CMYK), or red-green-blue (RGB) colour schemes, making them easily editable to suit the requirements of the publication. TIFF files are also compatible with Windows and MacIntosh operating systems. The flexibility and high quality of TIFF images makes it the gold standard for journal publications.
Advantages: No loss of image information
Drawbacks: Large file size
Good for: Images, photos and figures for publications where high detail and resolution need to be retained
Portable Network Graphics (PNG) are vector image files. Unlike JPG files, PNG files undergo lossless compression (i.e. compression which does not degrade image data) and, as a result, are larger than JPG files. Like TIFF images, there is no degradation in image quality when PNG files are saved and re-saved. PNG supports greyscale and RGB colour schemes, but not CMYK. PNG is especially suitable for saving line drawings such as graphs at a small file size. Some journals accept figures submitted in PNG format.
Advantages: Relatively small file size
Drawbacks: Does not support all colour schemes
Good for: Line drawings
EPS stands for Encapsulated PostScript. It is a graphics file in vector format. EPS is a versatile file type supported by several different drawing and vector editing programs such as Adobe Illustrator and CorelDraw. Another advantage of EPS is that it is compatible across different operating systems. As such, EPS is recommended for master image files as they are easily editable on several different platforms without worrying about compatibility issues. Being a vector file, it can also be scaled infinitely without loss of image resolution. Although EPS files are still used today, they are increasingly being replaced by newer file formats such as Adobe Illustrator (AI) and Portable Document Format (PDF). In addition to TIFF, journals may also accept image files submitted in EPS format.
Advantages: Versatile file type which can be scaled infinitely without loss of image resolution
Drawbacks: Older file type which may be replaced by newer file formats
Good for: Master copies of image files which require editing
Before preparing any image for publication, always check the author guidelines of the journal and the requirements of the graphic designer and web master. By understanding the mechanics behind digital images, I hope that the number of instances in which an image is rejected because of low quality will be reduced!