The “large” and “small” variations depend mostly on the size of the image (say one from a 16-megapixel camera vs one from a 2-megapixel camera) and the size of the object and its structure. The Structure actions act to enhance the structure within an object: details in the core or spiral arms in a galaxy, dust lanes in galaxies and nebulas, and shock fronts in galaxies, for example. Using the lasso tool to select objects and bright stars SMALL LARGE STRUCTURE Images with a lot of background structure from nebulosity may not work as well, but in general, the images that show the worst gradients are those with fairly empty backgrounds like images of galaxies. No precision selection or feathering needs to be done. The selection is simply made by roughly circling the objects with the lasso tool. It requires the user to select brighter objects to protect such as galaxies or bright stars. It can work well even on complex gradients with fairly rapid spatial variations. This action is designed to remove uneven gradients from an image. Mouse over image for before and after BACKGROUND MAGIC However, in some cases applying a layer mask in conjunction with this action can help preserve fine detail in the regions between the bright and dim areas.
The brighter areas of an image, which have a higher signal to noise ratio, are largely protected by the action. But there is almost always some remaining noise to be processed out. This is the noise inherent in imaging faint objects and is reduced to some extent by stacking multiple exposures. Noise Smoothing reduces the graininess visible in the faint areas of an image. Mouse over image for before and after NOISE SMOOTHING In this case, using a layer mask in conjunction with this action may be beneficial. Brighter parts of the image with higher signal to noise ratio are protected to some extent from the effects of this action, but it is possible that small colored regions like the little pink nebulas in the spiral arms of a galaxy may be adversely affected if they are faint. The Color Noise Smoothing action is usually best run before the Noise Smoothing action. It does not generally affect the overall noise or graininess the Noise Smoothing action is intended for that application. The Color Noise Smoothing action is designed to reduce the blotchy colors that can appear in the background and faint regions of an image. Mouse over image for before and after (image by Josh Knutson) COLOR NOISE SMOOTHING Objects with a large dynamic range in brightness will benefit most from this action: the Orion Nebula, Lagoon Nebula, and Andromeda Galaxy are all good examples. Compressing the overall brightness range allows more detail to be seen, and generally, the colors in the bright areas are boosted as well. The Compressor action acts to reduce the brightness of washed out areas of an object while boosting the faint areas of an image. Mouse over image for before and after COMPRESSOR The Veil Nebula is a prime example, but even objects like the Orion Nebula or Dumbbell Nebula would be well enhanced. Faint objects in dense star fields will benefit greatly from this effect. The Star Killer action is designed to significantly reduce the size of brighter stars in an image to allow more details from faint nebulosity to show through. There are ten unique actions (plus two variations for 12 total) and each is described in more detail below. The actions tend to be on the aggressive side because it is easy to lower the effect using the opacity slider in the Layers palette.
This allows you to fine-tune the intensity of the action or to apply a layer mask to make the action's effect more selective. The action runs on the current image and the result is always left as a separate layer. Once installed (see instructions at bottom), the actions appear in the Actions palette in Photoshop and a selected action may be “played” by simply pressing a button.
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