# Image - Gaussian Blur

A Gaussian probability curve is the classic bell-shaped curve with a higher probability in the center of the curve and lower probabilities to either side. A Gaussian blur effect takes each pixel in an RGB or RGBa image and mixes it with adjacent pixels with Gaussian probability so that the pixel has great effect near its original location and less effect (in a bell curve shaped way) farther away from its original location. If a selection is present, the Gaussian blur will ignore any unselected pixels. Gaussian blur will also ignore invisible pixels.

 Amount Number of pixels to blur to either side. Preview Check to see effect in action.

This command is also available on the Transform Toolbar for images as the Gaussian Blur transform operator. However, when used from the transform toolbar the effect is immediate with no preview possible. In the transform toolbar version the parameter specified is used as the amount of pixels to blur.

Gaussian blurs are most often used to create "drop shadow" effects.

Suppose we have an image layer that contains just the monument clipped out of our standard bronze sample image. We would like to add a drop shadow.

We begin by making a copy of the layer, selecting the monument only and then using Hue / Saturation to turn the Lightness all the way down. This makes the monument all black.

We next apply the Gaussian Blur effect, using an Amount setting of 5. The farther the "shadow" is supposed to be from the image, the greater the Amount should be for a credible effect.

We then position the initial layer over the drop shadow layer and then move either the shadow or the upper image so that the shadow is slightly offset. Drop shadows are often used to create the "cardboard cut out" effect seen at the bottom of the monument. If we wish to avoid this effect we can feather the bottom of the monument by applying partial transparency to the bottom with a partial eraser (assuming the image is an RGBa image in which partial pixel transparency is possible).

Gaussian blurs are also used to create "halos" that set off images. There are two ways of doing so. The easiest way is to take the black blurred shadow and to colorize it and to then use the colorized "shadow" as a halo.

Here is a slightly longer version that shows the steps more explicitly.

If we take the black and white image created by reducing lightness, we can Invert it to get a white image (shown here on a black background, since otherwise the white image would be invisible against a white background).

Next we use Colorize to convert the image to bright yellow. We did this by turning the Blue slider all the way down and the Red and Green sliders all the way up in the Colorize dialog.

We can now apply the Gaussian blur to blur the yellow image.

Here is the completed composition. The original image is in a layer above the yellow image, which has been offset slightly upward and to the left. We have also used three other effects:

·      The saturation in the bronze monument has been increased to provide more vivid colors.

·      The bronze monument is an RGBa image. We've used the eraser to partially erase the bottom part of the image to feather it into the background by virtue of increasing transparency.

·      So the yellow halo does not show through the partially transparent monument image, we have erased most of the bottom half of the yellow halo to an even greater degree than the monument.

The illustration above shows four image layers above several drawing layers in a map. The Manifold logo and text are both in separate layers and their shadows underneath are drop shadows in RGBa image layers created using Gaussian Blur. Both drop shadow layers have had transparency increased so their shadows are not so obvious. It is especially important when adding drop shadows to text to use a light touch. Increase transparency so there is barely a hint of shadow for a better effect.

Use drop shadows with logos, with text in image layers and to provide a "drop shadow" for photographs or other images that may be assembled "on top" of the map.

To create the image above, we zoomed into the map, made a screen shot, pasted the screen shot into a new image and then cropped the image down to the size of the detailed view desired. This image went into a layer in the map. We made a copy of the layer, turned the lightness down to zero to create a black square that was exactly the same size as the cropped detail and then used Gaussian Blur on this black square to create a drop shadow. Finally, we offset the drop shadow and added the A and B labels.

See the Cookie Cutter a Large Image with Transfer Selection example topic for a detailed description of how to create a drop shadow in a geographic map.

When a Selection is Present

If a selection is present the Gaussian blur command will operate only on selected pixels.

For example, in the image above a circular region was selected and Gaussian blur applied. Only those pixels in the selected region were blurred.