Gaussian Blur

This transformation works by computing a weighted average of a neighborhood around each pixel whose size you can specify (see Radius below). Nearby pixels are weighted more heavily than distant pixels. This effect is very similar to the optical blurring that occurs when an image is out of focus, the biggest difference being that the entire image is blurred uniformly, regardless of subject distance. Gaussian Blur is very fast, even for large values of blur radius, but it is only an approximation to true Gaussian Blur. If you need more precise blurring or additional controls for accurate selective blurring, see the Precise Gaussian Blur transformation which has more features but is much slower.

Clicking on the input image display centers the 1:1 preview on the location you clicked on, or you can click and drag to scroll the preview window.

Amount

The amount control lets you control how much of the blur transformation is applied to the input image. You can apply a percentage of the transformation to the entire image, or you can specify an amount mask to restrict the effects of the transformation to only part of the input image.

This control does not vary the blurring radius – it just blends the unblurred image with the blurred image. The overall effect of using a mid-level amount value is like a fog filter. If you need to vary the blur radius selectively, use the Precise Gaussian Blur transformation.

Radius

The larger the radius, the more blurred the result.

Threshold

The threshold is applied as follows: first, the blurred image is computed; next, pixel-by-pixel, the blurred image is compared to the input image. If the difference is less than the threshold, the blurred image replaces the input image; otherwise the original input image pixel is retained.

The effect of the threshold is to restrict blurring to those areas of the input image that are already relatively soft and to leave pixels that differ sharply from others around them alone. This can be very helpful, for example, in smoothing out noise in a clear sky while leaving the adjacent tree line unblurred. A threshold setting of 100% enables full blurring; relatively small values (say less than 10%) are good for selective blurring.

Preview

A 1:1 preview of the blur effect is displayed in the right-hand side of the dialog box. You can resize the preview by resizing the dialog box. Clicking on the input image re-centers the preview at the point you click. Or, you can drag within the preview area to move around the image.

If you enlarge the dialog box, you can make the sliders wider and the preview area narrower or vice versa by dragging the vertical separator line between them.

 Preview Button

Clicking the Preview button causes the full output image to be recalculated, not just the preview area. This can be useful if you want to view the entire image at once and not just a small section, but it can take longer to compute.

 Settings Menu

Blur Chrominance Only

This toggles the Blur Chrominance Only setting. If the menu item is checked, just the color information (chrominance) is blurred, while brightness information (luminance) in the input image is preserved. This can be useful, for example, in reducing color noise in pure blue skies or other areas of solid color.