{"id":31065,"date":"2022-04-21T00:00:00","date_gmt":"2022-04-21T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=31065"},"modified":"2022-04-29T14:12:06","modified_gmt":"2022-04-29T14:12:06","slug":"learning-about-layers","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/04\/21\/learning-about-layers\/","title":{"rendered":"Learning about layers"},"content":{"rendered":"\n

Expert astrophotographer Will Gater <\/strong>provides a beginner\u2019s guide to layers-based image editing<\/p>\n\n

\"\"
For this nightscape of the Milky Way over the Cornish coast a layers-based editor was used to blend frames that focused separately on the foreground and sky<\/figcaption><\/figure><\/div>\n\n

As any astrophotographer will tell you, the cold nights that are spent out capturing data \u2013 whether for a simple DSLR camera nightscape or a multihour deep-sky masterpiece \u2013 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 \u2013 like DeepSkyStacker or Nebulosity \u2013 when we hear the phrase \u2018astro-image processing\u2019, but often much of this work is done in Photoshop or GIMP too \u2013 certainly more than a few final tweaks.<\/p>\n\n

These programs allow you adjust different aspects of an image (like the brightness, contrast and colour balance), but they\u2019re also important in another respect, as they are what\u2019s known as \u2018layers-based\u2019 editors. This means that multiple images, or sets of image data, can be brought into these programs as separate layers.<\/p>\n\n

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.<\/p>\n\n

Because each image layer can be switched on or off, and has different treatments or processing techniques applied to it in isolation, it\u2019s 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.<\/p>\n\n

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.<\/p>\n\n

Abundant applications<\/strong><\/h5>\n\n

For astrophotographers, the practical applications of layers-based image editors are numerous. A basic example is the creation of manually stitched lunar mosaics. In such projects tens, if not hundreds, of frames \u2013 with different \u2018footprints\u2019 of the Moon\u2019s surface \u2013 can be loaded as individual layers into a large digital \u2018canvas\u2019. Layers-based editing then allows them to be manually manipulated, aligned and blended to create a composite of the lunar disc, or a highlighted section.<\/p>\n\n

Another common use of layers-based image editing in astrophotography is to blend two or more exposures, or images with different processing treatments or filters applied. This is often done with nightscapes and long-exposure, deep-sky imaging of bright targets.<\/p>\n\n

In this article we will look at three introductory examples of how to use a layers-based editing programme in the scenarios above. We will only scratch the surface of what\u2019s possible with these techniques; there are many more applications. These include high-pass filtering, creating animated GIFs, image stacking and blending, and even adding annotations to your images. Once you start using layers-based software your astrophoto processing workflow won\u2019t be the same again! <\/p>\n\n

Case study 1<\/span><\/strong><\/h3>\n\n

Creating a manually stitched lunar mosaic<\/h4>\n\n
\"\"
A wide-field mosaic of the region around the crater Gassendi (above), stitched together manually in a layers-based editor<\/figcaption><\/figure>\n\n

Making a lunar mosaic from scratch is a great way to introduce yourself to the basic mechanics of layers-based editing. Such mosaics are created by combining multiple, overlapping images of the Moon\u2019s surface. While there are plug-ins and software that can do this automatically, manual composition in a layers-based program gives you full control over the process. In this example we will assume the individual frames begin life as video captures taken through a high frame rate camera and telescope.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 1<\/span> <\/strong>CAPTURING THE FRAMES <\/h5>\n\n

Don\u2019t just think about focusing and exposure, but also about creating the best possible data for manual composition. Give each video capture an overlap between neighbouring frames, as this makes it easier to match and align. Monitor how well your mount is tracking: if it\u2019s not keeping the framing stable, thin gaps can creep into the final mosaic.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 2<\/span> <\/strong>PREPARING THE DATA FOR MOSAICING <\/h5>\n\n

You can now process the videos from Step 1. First, open a video in alignment and stacking software such as the free AutoStakkert! Under \u2018Image Stabilization\u2019 (IS) be sure to select \u2018Expand\u2019 as this will ensure there\u2019s space around the edge that we can make use of. Once AutoStakkert! has done its work, open the images in the free software RegiStax and apply gentle wavelet sharpening (left) before saving each file as a PNG.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 3 <\/span><\/strong>START ALIGNING FRAMES IN THE LAYERS-BASED EDITOR<\/h5>\n\n

Now import all of your frames into the editor. In Photoshop this can be done by selecting \u2018File > Scripts > Load Files into Stack\u2019, while in GIMP it\u2019s \u2018File > Open as Layers\u2019. Select the first two frames of your imaging sequence and start aligning them. In Photoshop you can move and rotate a layer using the \u2018Move Tool\u2019, while in GIMP you\u2019ll have to use a combination of the \u2018Move Tool\u2019 and \u2018Rotate Tool\u2019.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 4<\/span> <\/strong>EXAMINE THE QUALITY OF THE ALIGNMENT <\/h5>\n\n

Check the rotational and positional accuracy of the alignment. You can do this by examining the \u2018Difference\u2019 image of the two frames by setting the upper layer\u2019s blending mode to \u2018Difference\u2019 in the \u2018Layers\u2019 menu. The closer the pixels in the \u2018Difference\u2019 layer match with those below, the nearer to solid black they\u2019ll appear, making slight offsets noticeable. Use the arrow keys to nudge the frames, then switch back to \u2018Normal\u2019 blending when you\u2019re done.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 5<\/span> <\/strong>BRING IN ANOTHER LAYER AND REPEAT THE PROCESS <\/h5>\n\n

Now repeat some of the earlier steps by selecting another layer and aligning it to the co-aligned first and second frames. This will, of course, have to be done in sequence for all frames in their separate layers. If you notice a hard edge between frames \u2013 or frames that show poorer quality data than the one(s) underlying them \u2013 you can use a soft-edged eraser tool to remove it.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 6<\/span> <\/strong>BALANCE LIGHTING BETWEEN FRAMES IF NECESSARY <\/h5>\n\n

The last task \u2013 before blending the image into a final, single composite that can be sharpened and tweaked further \u2013 is to even out any differences in brightness between frames that sit next to each other. Do this by making very small tweaks to the brightness and contrast of an individual layer, for example using the \u2018Levels\u2019 tool via \u2018Colours > Levels\u2019 in GIMP and \u2018Image > Adjustments > Levels\u2019 in Photoshop. <\/p>\n\n

Case study 2<\/span><\/strong><\/h3>\n\n

Blending data in nightscapes using a layer mask<\/strong><\/h4>\n\n

There are many ways in which layer-based editors are useful when you are processing \u2018nightscapes\u2019. For example, they can be used to blend two or more images held in separate layers. A common example of this in nightscape imaging is when you are bringing two different exposures together, or perhaps two treatments of the same data. Here, we are going to look at how you would do that using what\u2019s called a layer \u2018mask\u2019.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 1 <\/span><\/strong>CAPTURE AND PROCESS THE TWO FRAMES YOU WANT TO BLEND<\/h5>\n\n

First capture and process the two images you want to blend. This could be one long exposure (or multiple stacked ones) at a low ISO for the foreground and a shorter one for the sky. It could also be two images made from just one exposure: one processed to bring out details in the starry sky and another, using the same data, where you\u2019ve focused on brightening the foreground, eg with a curves enhancement. You can apply noise-reduction filters or vignetting calibration now, but don\u2019t make levels or curve stretches too harsh at this stage.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 2<\/span> <\/strong>BRING THE IMAGES TOGETHER INTO TWO LAYERS<\/h5>\n\n

Take one image and place it into a new layer in the other. In Photoshop click on \u2018Select > All\u2019, then \u2018Edit > Copy Merged\u2019, before pasting into the other image file. In GIMP, click on \u2018Select > All\u2019, \u2018Edit > Copy Visible\u2019, then \u2018Paste as > New Layer\u2019. Now, align the layers using the techniques discussed in Case Study 1. In some instances, especially if one exposure was taken minutes after the other, there may not be perfect alignment and there will be a small, insurmountable offset of a few tens of pixels due to the sky\u2019s motion. Make an alignment and crop areas that don\u2019t overlap.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 3<\/span> <\/strong>APPLY A LAYER MASK TO BLEND THE LAYERS <\/h5>\n\n

Now apply a layer mask to make one layer visible \u2018through\u2019 the other. In our example, the image processed for the stars is the upper layer in the layers window. To add a mask in Photoshop click the black-circle-in-a-white-rectangle icon at the bottom of the \u2018Layers\u2019 window. In GIMP, right-click on the upper layer and click \u2018Add Layer Mask\u2019 and choose the \u2018White\u2019 option. In GIMP you now use a large, diffuse paint brush to \u2018paint\u2019 a mask and reveal the desired area of the underlying layer. In Photoshop you do the same, but with the eraser tool instead.<\/p>\n\n

\"\"<\/figure>\n\n
STEP 4 <\/span><\/strong>FINAL ADJUSTMENTS <\/h5>\n\n

You should be close to having a final image, with the desired parts of one layer showing through the other. You can now make tweaks to the brightness and contrast of the two layers. Now that you have isolated the different areas with a mask, you can make those adjustments to one layer or the other and can see \u2018live\u2019 how they affect the composition. When you\u2019re happy, flatten the image \u2013 \u2018Layer > Flatten Image\u2019 in Photoshop, and \u2018Image > Flatten Image\u2019 in GIMP \u2013 to merge all the layers and make final global adjustments, eg colour balancing.<\/p>\n\n

Case study 3<\/span><\/strong><\/h3>\n\n

Using layers to deal with bright areas in deep-sky imaging<\/h4>\n\n

In this final case study we\u2019re going to bring together many of the techniques we\u2019ve explored in the previous two step-by-step projects. In deep-sky imaging, having the ability to blend together two data sets, or two different images, in separate layers can be very useful when tackling some of the brighter targets.<\/p>\n\n

\"\"
M31 lends itself to layer masking due to difference in the brightness of its central regions and outer areas\u2019<\/figcaption><\/figure>\n\n

With objects like the Orion Nebula, M42, and the Andromeda Galaxy, M31, the core regions are quite bright compared to their surroundings. What this means is that if you set your exposure levels on your camera to pick up the faint detail in the outer regions of these targets, chances are the centre is going to be overexposed. Typically the way around this is to capture another set of shorter exposures of the central regions, then stack them separately; this data can then be blended with the other stack made from data exposed for the faint outer areas.<\/p>\n\n

Let\u2019s imagine we\u2019re trying to combine the two images of the inner regions of M31 below. We\u2019ve already seen how to bring two images into a layers-based editor and align them, and we could use a layer mask to then mask the overexposed core, allowing the data showing the central region to come through more clearly.<\/p>\n\n

But there\u2019s another way to achieve a similar effect. That is to change the blending mode of the two layers. With the data showing the overexposed core in the upper layer, we could set its \u2018Blend Mode\u2019 (at the top of the layers window in both Photoshop and GIMP) from \u2018Normal\u2019 to \u2018Soft Light\u2019. The result: a much better looking image, without the substantial blown-out core area, that can then be flattened down and processed to taste in numerous other ways.<\/p>\n\n

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These images of M31 were taken with a monochrome camera and illustrate how a tweak to the \u2018blend mode\u2019 can bring together two layers<\/figcaption><\/figure><\/div>\n\n
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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
\n

Will Gater <\/strong>is an astronomy journalist and science presenter. His latest book, The Mysteries of the Universe, <\/em>is published by DK<\/p>\n<\/div>\n<\/div>\n\n

ALL PHOTOS: WILL GATER<\/p>\n\n

<\/p>\n","protected":false},"excerpt":{"rendered":"

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