Demystifying image file types

April 21, 2015

The three little letters after a file name can be a source of great confusion to the uninitiated. As an Account Manager you’ll be required to request and deliver many different types of files for different purposes; and, in the self-interest of sounding intelligent, you’ll want to know you are using the right lingo at the right time.

Filename extensions

A “filename extension”, or “suffix”, is the “dot xxx” extension after a file’s name. It indicates the “encoding” (file format) of the file, plus it gives you the biggest clue as to the file’s intended use. Examples of filename extensions are .jpg, .png, and .gif.

A full list of industry filename extensions can run into the hundreds - literally. Don’t worry, the number of extensions that an Account Manager needs to be familiar with is considerably smaller!

Vector vs Bitmap

The first thing you’ll need to understand is that there are two main file types for image files: “bitmap” and “vector”. Once you understand these two concepts, you’ll be able to better grasp why we have so many different file types and how you can apply them.

Bitmap images

“Bitmap” images (also known as “raster” images) are exactly what their name suggests: a collection of “bits” that form an image. A “bit” is the smallest possible unit of information for a computer. Bitmap images are made up of pixels in a grid (when you see them on-screen), which are then reproduced as dots (when you view a printed page with a magnifying glass).

Bitmap images are resolution dependent. “Resolution” refers to the number of pixels in an image and is usually stated as dpi (dots per inch - print terminology), or ppi (pixels per inch - digital terminology). It refers to the number of dots/pixels that will fit into one inch of printed page or screen. The more dots/pixels in that inch, the clearer, crisper and better quality the image becomes.

Standard computer displays will show images comfortably at 70 to 100 pixels per inch, which is why the image resolution of photos for web has been 72 dpi until fairly recently. With the introduction of “Retina” displays (from Apple), the resolution of images needs to be higher at 220 ppi+.

In order for a bitmap image to render well in print, the resolution needs to be around 300 dpi/ppi. If you print an image that has been saved at 72 dpi it will appear “pixelated” or blurry. There are not enough dots per inch in a 72 dpi/ppi image to see the image clearly enough on the printed page.

Types of bitmap images:

  • Line-art: images containing only two colours - usually black and white. These images are sometimes referred to as “bitmaps” because a computer has to use only 1 bit (on = black, off = white) to define each pixel.
  • Greyscale: an image containing various shades of grey, as well as pure black and white.
  • Multi-tones: images containing shades or two or more colours. The most popular multi-tones are duotone (usually black and a second spot colour) and tritone (usually black and two other spot colours).
  • Full colour images: these colour images can be described using a number of different colour spaces, such as RGB, CMYK or Lab.

Key points to note:

  • The pixels/dots are in a grid.
  • Bitmaps are resolution-dependent.
  • Resizing affects the quality. You can make a high resolution bitmap smaller without losing quality. If you make a low resolution bitmap larger the image will be come pixelated/blurry and unusable.
  • All scanned images and images from digital cameras are bitmap.
  • Individual elements cannot be grouped.
  • You can convert from one bitmap format to another by using image editing software (such as Adobe Photoshop) and using the “save as” command.
  • Bitmap images (in general) do not support transparency. The notable exceptions are .png and .gif.

Common bitmap formats include: JPG/JPEG, BMP, GIF, PNG, PCX, TIF/TIFF, PSD

Vector images

“Vector” images are made up of objects, rather than pixels or dots. Each of these objects can be defined by mathematical statements and have properties assigned to them (such as fill, colour, outline).

Vector graphics are resolution-independent because they can be output to the highest quality at any size or scale. You can take a tiny vector graphic and enlarge it to billboard size without losing quality.

Vector format is the preferred file format for creating logos, maps and other graphic images that have to be re-sized frequently.

Key points to note:

  • Vector images are scaleable and high resolution at any size.
  • Individual elements can be grouped.
  • Uses more processing power than bitmaps.
  • Tend to take up less memory, and have much smaller file sizes than raster-based bitmaps.
  • Vector images must be converted to bitmap images to display on the web.
  • A vector image can easily be converted to bitmap format, but cannot be converted back again to vector.

Common vector formats include: AI, EPS

What about PDF files?

PDF files can trip up the unwary. They are neither bitmap nor vector, per se. PDF stands for “Portable Document Format”. It is a file format for capturing and sending electronic documents in exactly the format generated by the original programme (e.g. Microsoft Word, InDesign, etc). With the exception of pdf files created by Adobe Illustrator (see below), it is best to consider pdfs as being for viewing only, rather than as a file format you use for image creation.

The Illustrator exception:

An Adobe Illustrator .ai or .eps vector file can be saved as a .pdf. It will look exactly as per the layout of the page in Illustrator, and it will retain all the vector properties of the .ai file. This particular type of pdf file can then be re-opened and worked on in Illustrator - in vector format. This is an anomaly in the pdf world.

Note: Any pdf document can be opened up in Photoshop, but the opened image will be in bitmap format, and you will not be able to edit the individual elements of the image.

File Formats

The following image file formats are the most common that an Account Manager will need to deal with on a day-to-day basis.

  • .ai  Adobe Illustrator document; the native vector file format of Adobe Illustrator.
  • .bmp  bitmap image file. An outdated and limited file format that is not suitable for use in print.
  • .eps  Encapsulated Postscript; a flexible file format that can contain both bitmap and vector data.
  • .gif  Graphics Interchange Format; mainly used for internet graphics. Multiple Photoshop images can be saved as an “animated gif” to give the graphic image or message a sense of motion.
  • .jpg  Joint Photography Experts Group; compressed format for internet and print use. Depending on the degree of compression selected, image quality can be lost in this format.
  • .png  Portable Network Graphic; depending on the resolution selected a png is suitable for either web or print. Along with .gif, this is other file format that retains transparency.
  • .psd  Photoshop Document; the native file format of Adobe Photoshop. Layout programmes, such as InDesign and Illustrator, can accept psd files. Designers like psd files as they retain their original layers and the quality is uncompressed.
  • .tif  Tagged Image File; an uncompressed bitmap file format. Retains data quality well. Not widely used these days.

 

Mac vs PC

All image file formats can be used on either a Mac or PC if the user is running graphics software, such as Adobe Creative Suite. As a vast proportion of graphics professionals are Mac users, we can pretty much assume they will be able to work with vector files. Typically, our clients will be PC-based and running basic software such as Microsoft Office. In this instance, our “PC users” will be unable to open/place vector files, therefore we would give them bitmap images only. When you supply image files to your client, it pays to ask who will be using the images and the purpose of use.

One way you can supply images and logos is in two folders: “Mac” and “PC”. Mac format files will be vector-based (.ai and .eps) and PC format files will be bitmap (.jpeg, .png, .gif). That way you can be assured that PC users will know which folder to access and that any image they select will be usable. Mac users will want the highest quality images possible, and they will thank you for supplying vector formats. Mac users can also use the bitmap file versions if they need to (e.g. for web).

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