What is a digital image?


We all have tens of thousands of images stored in our computer in this day and age.

Here’re some images from my own personal collection, the ones that I took on a holiday to the northern part of Australia a few years ago. There are pictures of water lilies, birds, trees, sunsets, Aboriginal rock art, and crocodiles.

Every image in this collection is stored in an individual file in the file system of my computer. Let’s select this particular crocodile image here and we can see which file it is in. This is a Mac, so I come up here and I will look at the file in the Mac’s file browser utility called Finder. Under Linux, Windows, there are equivalent applications. And here we can see the particular file that holds that crocodile image. We can see that it has a .JPG extension, that means it is a JPEG image and we will talk about what that means shortly, and we can see that it is 3.7 megabytes long.

Now we can open that image in the regular image browser on the Mac, and in this case it is the application called Preview. There we see the crocodile looming large on the screen. And we can zoom into this image and as we zoom in we see that the image becomes more and more block-like. And as we zoom right in we can see that the image is made up of these tiny little blocks. These are the smallest elements within the picture. We call them pixels, that’s short for picture elements. So what you are seeing here are individual pixels that comprise this particular image.
Let’s go back to where we were.

For each particular image, as well as the data that comprises the image itself, the pixels that make up the picture, there is a bunch of additional information which is referred to as metadata. And we can look at the metadata for this image by clicking this button here. Now, the metadata is displayed up in the top corner, and there is quite a bit of it. First of all, we can see the model number of the camera that took these pictures. We can see the resolution of the image and that is that the image is 3248 pixels wide by 2160 pixels tall. We can see that the image is 3.7 megabytes long on the disk. We can see the focal length or the zoom setting that I used when I took this particular picture. I can see the F-stop or aperture setting. And I can also see the exposure time or shutter time when I took this particular picture.

More modern cameras would have additional metadata, for things like the GPS coordinate of the place where the image was captured.

There are many, many different types of image format commonly used today. If you go and look on Wikipedia, you can find that there are hundreds of different types of image file format. I have listed here just a few of the most common ones. JPEG are format files which typically have a file extension of .jpg or .jpeg. TIFF files, GIF files, Portable network graphics or PNG files, and many others. When we actually look inside an image file, it has really three separate components, and they live within what we call a container. So here we have an empty container. And one of the first elements in the container is what is called the header, and the header contains information about how big the image is, how wide it is, how tall it is and the type of compression, and the type of pixel format. Is it eight bits per pixel, ten bits per pixel or whatever.

The next object in the container is what is called the metadata, and that has information about the actual picture that was taken. It might have the GPS coordinate of the camera when the picture was taken. It might have the F number and the focal length, and whether the flash fired or not. So it is information about that particular picture that was taken.

And then, finally we have the actual pixel data itself, the information which represents the image and it may be compressed or not compressed.

So when we talk about image file format, we need to actually talk about two things. We need to talk about the particular container that holds this information and there are different types of containers. And we also need to talk about the different compression methods which are applied to the image pixel data.

So when we commonly talk about a JPEG file, JPEG format file, correctly we should be saying that this is an EXIF container that holds JPEG compressed image data.

Most of us have lots of digital images captured using cameras or phones. Each image comprises millions of picture elements or pixels. The images are stored in files, typically in JPEG format, and we’ll see what’s inside one of these files.

Professor Peter Corke

Professor of Robotic Vision at QUT and Director of the Australian Centre for Robotic Vision (ACRV). Peter is also a Fellow of the IEEE, a senior Fellow of the Higher Education Academy, and on the editorial board of several robotics research journals.

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