3.3. Median Blur
| 3. ぼかしフィルター |

### 3.3. Median Blur

#### 3.3.1. 概観

While the 「Gaussian」 blur filter calculates the mean of the neighboring pixels, the 「Median」 blur filter calculates the median:

図17.12 Calculating Median

A 3x3 neighborhood. Values in ascending order. Median surrounded in red.

This calculation does not create a new value, and an unrepresentative pixel in the neighborhood will not affect the result. So, the filter preserves edges and rounds corners. It is used to reduce noise, especially salt and pepper noise, and delete scratches on photographs.

#### 3.3.2. フィルターの呼び出し方

You can find this filter in the image menu under Filters → Blur → Median Blur…

#### 3.3.3. オプション

図17.13 「Median」 filter parameters

Presets, Preview, Split view

  These options are common to GEGL-based filters. Please refer to 「Common Features」.

Neighborhood

  The shape of the neighborhood. Three options: Square, Circle(default), Diamond. Differences are subtle and not predictable: experiment, on-canvas editing makes this easy.

  図17.14 「Median」 Neighborhood

  Radius=3..........Left: Square

  Middle: Circle..........Right: Diamond

半径

  The radius of the neighborhood. Increasing radius increases blur. Contrary to the 「Gaussian」 filter, edges are not blurred. Corners are rounded and convex surfaces are eroded.

  図17.15 「Median」 blur vs 「Gaussian」 blur

  Left: Origin

  Middle: Median

  Right: Gaussian

  Increasing radius too much can create unwanted effects:

  Left: Origin

  Right: Radius=100

Percentile

  By default, the Median Blur filter finds the median value at the neighborhood of each pixel. In spite of its name, the filter can actually find *any* arbitrary percentile, not just the median (i.e., the 50th percentile). The "Percentile" parameter controls the percentile used for the color components. Lower values shift the image toward darker tones and higher values toward brighter ones.

  Left: Origin

  Middle: Percentile=0. Image is darker. Stems are enlarged but sharp.

  Right: Percentile=100. Image is brighter and much blurred.

Alpha Percentile

  To see the effect of this parameter, you need an image with transparency. If the alpha channel is opaque everywhere, the result will also be fully opaque, regardless of the percentile.

  Lower values for the 「Alpha percentile」 parameter shift the image toward more transparency, and higher values shift the image toward more opacity, where a value of 50% is balanced. Roughly speaking, values less than 50% make the opaque regions of the image smaller, while values greater than 50% make the opaque regions of the image larger.

  Left: Origin. The image has an alpha channel. A circle is transparent.

  Middle: Percentile=0%. Transparent circle is enlarged.

  Right: Alpha percentile=100%. Transparent circle is reduced.

#### 3.3.4. Using Median Blur

Reducing salt and pepper noise

  Left: origin (from Wikipedia)

  Middle: radius = 1 applied twice

  Right: radius = 1 applied three times

Reducing scratches

  Left: origin

  Middle: radius = 2

  Right: radius = 1 applied twice. The image is less blurred.

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