The Moire Effect

I have written about this effect, and its most common cause in CCTV, many times but I think it’s necessary to take a closer look.

To start with, this is a great little video showing the effect in action….

The two pieces of mesh have the same grid pattern. As the top piece is moved, a pattern appears. This is the Moire.

I see this daily in CCTV Images that have been obtained by taking either a single image, or a video, directly from a television or monitor screen.

OK, sometimes this is required. The immediate publication of some form of imagery is often an ideal way to obtain recognition intelligence at an early stage. Especially  if the native, evidential acquisition of the visual material would delay this process.

The evidence can be recovered correctly later, and further images publicized at a later time.

Let’s now look at this image loaded into Amped FIVEImage 4

The image was obtained by taking a picture of the original image on a monitor. This replicates a CCTV Image being played through and displayed on a monitor, and then an image or a video taken of that monitor by another imaging device, such as a cell phone camera or bodycam.

You can clearly see the moire effect pattern, (and all the guys learning FIVE in a recent training class!!)

The pattern is produced in exactly the same way as the patterns seen earlier with the two pieces of mesh.

The original image is made up of pixels. It is displayed on a monitor, also using pixels. The capture device, which was a cellphone in this case, again uses pixels. When all those grids collide… the pattern is created and the resulting image is damaged.

Luckily, using FIVE makes it easy to considerably reduce the damage caused by obtaining an image in this manner.

The pattern is made up of regular frequencies in the image. By removing those frequencies whilst analyzing the image in the Fourier domain, the majority can be removed within seconds…

Here, I have removed the areas of constant frequency..

Image 6

Which results in a much cleaner image for publication…

Image 5So, if you have to deal with images that have been snapped from a monitor using another imaging device, remember that there is a quick solution in Amped FIVE to the Moire effect problem.