Gaussian Blur Filter

Unsharp Mask Sharpening applies the Gaussian Blur filter and creates a "file" with the blurred result.

Unsharp Mask Sharpening then compares the blurred file from the Gaussian Blur filter to the original file of the photograph.

The Gaussian Blur filter doesn't blur image information with little detail.

So, the two files are similar where there isn't much detail in the photograph.

They're very different in areas where there's detail.

The Gaussian Blur filter blurs detailed areas.

So the Unsharp Mask Sharpening detects edges because the Gaussian Blur filter blurs areas with detail, and not areas with little detail.

File	Detail	Little Detail
Gaussian Blur Filter File	Lots of Blur	Little Blur
Original File of the Photograph	No Blur	No Blur

Go to Sharpening Methods Compared with Step Wedges.

Note

Pixels don't have spaces between them.

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

Arach, the Spider

You could imagine a spider-like robot moving from pixel to pixel.

Let's call this robot Arach.

Arach, our sharpening filter, detects no edges, in the sky below.

Then, Arach finds an edge in the sky.

Let's say it's the edge between deep-blue sky and a cloud.

The pixel values show the edge.

Arach does two things.

First, he lightens some of the pixels on the bright side of the edge.

The "80" column of pixels below, to the left, is changed to a brightness level of 60.

Second, Arach then darkens some of the pixels on the dark side of the edge.

The "100" column of pixels below, to the left, is changed to a brightness level of 60.

We see this increase in contrast as increased sharpness.

No Sharpening	Unsharp Mask Sharpening Amount: 400% Radius: 1 Pixels Threshold: 2 Levels

Here's the edge enlarged greatly, so you can see the sharpening.

No Sharpening	Unsharp Mask Sharpening Amount: 400% Radius: 1 Pixels Threshold: 2 Levels

How does the math work with sharpening?

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