Cris’ Image Analysis Blog

theory, methods, algorithms, applications

Why do we keep using the word "pixel"?

According to Wikipedia, the ultimate source of knowledge, the word pixel comes from “picture element.” This means that a pixel is a part of a picture, and yet everybody I know keeps using that term to refer to a part of an image. Because it is “image analysis” that we do, right? Not “picture analysis”? Of course, this is just an insignificant detail that I’m blowing way out of proportion. No, my real beef with the word pixel is more complex.

The word pixel seems to be used only in the context of 2D images. For 3D images we have a different word: voxel. So if I have a 3D image, and take one 2D slice out of it by selecting a set of voxels, these voxels all of a sudden, magically, become pixels! And what happens when you record a multi-spectral volumetric image? Or a volumetric time series? What do you call the elements of a 4D, 5D or 10D image? Dean et al. use the word imel (for image element) in their ICS file format specification (P. Dean et al., “Proposed standard for image cytometry data files”, Cytometry 11(5):561-569, 1990, DOI:10.1002/cyto.990110502). Yes, it’s more general. Yes, it’s more awkward. And yes, it still hides the fact that the images that we analyse are sampled and discretised representations of some continuous reality. When we sample a function we obtain samples, not pixels. The data sets that we analyse are collections of samples. A digital image is composed of samples, whether it be a 2D image, a 3D image or a 10D image. Only after you paint a little rectangle on the screen with the color of your sample does it become a pixel.

PS: I’m suggesting the word lixel for samples in a 1D signal. You heard it here first!