Gaussian Noise is Added, Poisson Noise is Applied

Poisson and Gaussian noise, pixel values 0 to 5

There is a fundamental difference between adding Gaussian noise and applying Poisson noise. In practice, people often talk about adding Poisson noise anyway, but this is not accurate. I will be looking at this from the image processing perspective in this article, and I’ll show purely visual examples. An application of this could be… read more

Submitted on 11 January 2014

Why is Deconvolution Difficult?

Lena convolved with the Airy pattern

When taking a picture with a camera, the “true” image is convolved with the point spread function (PSF) of that camera, potentially producing a blurred image. Deconvolution is the process of removing the effect of this PSF again. In this article, I demonstrate that this is not an easy thing to do… read more

Submitted on 24 December 2013

Brandenburger TorPhoto Page

Brandenburger Tor

This is a detail of the Brandenburger Gate (Brandenburger Tor in German) in Berlin, Germany. It is difficult to make an original photo of the thing, since there are so many already, but this is my attempt at one… read more

How is the Size of A4 Paper Determined?

The most common size of ISO A series paper is A4, of which the exact size is 210 × 297 mm. How is that size determined? Amazingly, you can exactly compute all the sizes of the A series sheets from the following two simple rules… read more


Submitted on 4 December 2013

What is a Spectrogram?

Spectrogram of square wave with harmonics 1, 3, 5, …, 19

A spectrogram is a graph that shows the evolution of the spectrum (the frequency contents) of a signal over time. Often, the frequency is on the vertical axis and time is on the horizontal axis. A spectrogram is computed by “chopping up” the signal into chunks and computing… read more

Submitted on 20 November 2013

Naive Square Wave

Naive square wave

After introducing finite-bandwidth square waves in previous articles, I’ll now describe what happens if you follow the naive approach, and just alternate sequences of 1’s and −1’s. As you might expect after reading the previous articles, there are several problems with this. The first problem is that the naive square wave is… read more

Submitted on 27 October 2013

Finite-Bandwidth Square Wave

Square wave with harmonics 1, 3, 5, …, 19

How would you produce a square wave on a digital system? At first sight, this seems completely trivial. You might think that you could just alternate a series of +1 values with a series of −1 values and be done with it. Well, it doesn’t work like that. An ideal square wave needs infinite bandwith, so creating one is… read more

Submitted on 2 October 2013


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