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Image Frequency

Image Frequency

In this chapter I'll explain the concept of Image Frequency from a qualitative and intuitive point of view and no mathematical knowledge is required to understand it. Let start having a look at the following picture.

On the left side there is a Black and White. You have to think at this image like if it represents a shoot of series of waves over the water in a given instant of time where lighter areas represent the crests of such waves and darker areas represent the bases. In order to better understand, I cut the image along an horizontal red line and I displayed on the right side the resulting section. As you can see in the image there are several vertical waves.

Let's now have a look at the next image.

If we apply the same concept to the left image you can now see that there is a single big wave exaclty in the middle. As I did before I cut the left image along an horizontal red line and I showed the section on the right picture.

The two images are clearly different. On the first one there is a large number of waves while in the second one there is only one single wave. We explain this concept saying that in the first picture there is a high frequency of waves while in the second one there is a low frequency of waves. The direction of a wave, from left to right, from top to bottom and the shape of a wave is irrelevant. Intuitively we see many waves in the first picture and few (actually just one) in the second picture.

In a single image, like in sea water, there can be many waves at the same time: a lot of small little waves and just few large waves. have a look at the following image.

In the left image we can clearly identify a composition of the two previous picture. We can still identify a lot of little vertical waves and a big wave exactly in the center. This is even more clear if we have a look at the right picture, representing the "depth of the water" along the horizontal red cut.

The above picture are of course a big simplification. What it must be clear from a qualitative point of view is that any Black and White image can be though as a composition of several short and large waves moving in all directions.

If we think about a portrait picture, the vignetting is just a single big wave occupying the center of the picture. A light gradient of shades moving across the picture can still be thought as a wave much much larger. And because these "waves" are few and large, their frequency is very low. On the opposite side we have the texture of the skin, composed by many many little variation of shades. They can be though as a multitude of very short waves and because they are a lot, their frequency is high. Following this approach, the shades that help us to identify the shape of a face, the nose, the lips or the eyes, are waves too, not to large and not to short. Because of this we can say that their frequency is neither high nor low.

To summarize every picture is a composition of high, middle and low frequency waves.