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2 - Unsharp Mask Sharpening Math

In the last page, we saw how the Unsharp Mask Sharpening travels from pixel to pixel.

At each pixel, it compares that pixel's brightness level with those of the surrounding pixels.

How does the Unsharp Mask Sharpening decide whether to change the pixel or to skip it?

Note

Pixels don't have spaces between them.

But for readability, the representations below have spaces between the pixels.

Sky

Here's a representation of an area of sky.

The Unsharp Mask Sharpening was likened to be like a robot, that was named Arach earlier.

Arach performs the calculations below.

The center pixel is multiplied by 9, and the surrounding pixels by -1.

Here are the calculations all together.

The center pixel has a brightness value of 100.

The calculation result is 100.

Arach says, "If the calculation value is the same as the center pixel value, then don't do any sharpening."

So, Arach doesn't perform sharpening.

Now let's look at an example where the pixels are not the same.

The calculations are as follows.

Here are the calculations all together.

The center pixel has a brightness value of 100.

The calculation result was 200.

Arach says, "If the calculation value is different from the center pixel value, then do sharpening."

Arach performs sharpening.

Sharpening changes the color, as well as the brightness, to increase contrast.

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