Learning about LAYERS

Expert astrophotographer Will Gater provides a beginner's guide to the power of layers-based image editing

- Will Gater

As any astrophotographer will tell you, the cold nights that are spent out capturing data - whether for a simple DSLR camera nightscape or a multihour deep-sky masterpiece - are only the opening stages of a long journey to produce a final image. An enormous amount of work goes into the processing of even quite a simple data set. We may think of stacking and calibration software programs – like DeepSkyStacker or Nebulosity - when we hear the phrase 'astro-image processing, but often much of this work is done in Photoshop or GIMP too - certainly more than a few final tweaks.

These programs allow you adjust different aspects of an image (like the brightness, contrast and colour balance), but they're also important in another respect, as they are what's known as “layers-based' editors. This means that multiple images, or sets of image data, can be brought into these programs as separate layers.

You can think of regular image-editing programs as being similar to the process of painting, where each daub of new paint is added to the canvas, changing the overall image. But a layers-based editor allows multiple, individual layers to simultaneously combine to make the image on screen. Each of these also can be tweaked, adjusted, masked, aligned or blended in myriad ways while processing.

Because each image layer can be switched on or off, and has different treatments or processing techniques applied to it in isolation, it's easy to vary its contribution to the overall picture, or even delete its effect completely. In the painting analogy this is akin to having the ability to remove, tweak or even change the hue of paint strokes that have already been applied to the canvas.

In a non layers-based editor, to go back and tweak an adjustment to part of the image, like a brightness boost or a curves enhancement, you will often need to restart the editing process, if such alterations are possible in the first place.