The Supernova filter is a filter in GIMP that applies a starburst type effect to a layer. The Supernova filter is found in the Filters menu, under the Light and Shadow sub-menu and it has a few settings that can be adjusted to adapt how it operates. It could be used in many ways, but one possible option is to use it to add a fake sun to a photo.
You should note that the filter responds to existing pixels in a layer which means its appearance will vary from use to use. In fact if you apply it to a transparent layer, the result will be a soft edged circle with no spokes emanating from it.
The dialog has the following features and settings that affect how the Supernova filter functions.
Preview
Firstly, like many other filters in GIMP, there is a thumbnail preview of the image you are working on with the effect applied. You can use the two magnifying glass buttons to zoom in and out of the preview. You can also turn the preview off by unticking the Preview box. The Show position tick box shows and hides cross hairs in the preview thumbnail and these can make it easier to place the center of the Supernova filter more accurately.
Center of Nova
The X and Y inputs make it easy to quickly place the center of the Supernova pixel perfectly, as long as you know the co-ordinates for the center. You can get these by looking at the rulers before opening the Supernova filter dialog, but this not as intuitive as placing the center manually in the preview thumbnail.
Color
If the color is set to white, the spokes or rays emanating from the Supernova will be colorless. If you set a color, the center of the Supernova will still appear white, but the spokes will be colored and this can help to make them more defined.
Radius
The Radius slider adjusts the size of the center of the resulting Supernova and makes it appropriate for use in different types of images. You should note that if the Color is not set to white, the Supernova will appear smaller.
Spokes
This changes the number of spokes or rays of light that emanate from the Supernova. This may need some experimentation to achieve the best results, as their appearance can be affected by the pixels of a layer that the filter is being applied to.
Random Hue
The Random hue setting has no effect if the Color is set to white. However if a color has been set, this slider will introduce other colors to the final result. A low setting can produce quite subtle effects, while higher numbers can produce brightly colored and surreal effects that can be well suited to images that are not intended to appear realistic.
The Supernova filter in GIMP is a fairly flexible tool that can be used to enhance photos with naturalistic effects or to apply highly stylized effects to fantasy images or designs. While there are a few settings that can be adjusted, they are quite easy to experiment with and you will very quickly be able to produce the exact result that you want with just one or two attempts.