BBC Sky at Night Magazine
Discover how masks can help you apply enhancements to specific areas of your astro images
Masks can be found in many image processing and editing software apps. Here we take a look at how they can be used in Adobe Photoshop to improve an image of the Heart Nebula, IC 1805. Photoshop masks allow us to cover an adjustment we make to an image with a screen and control the areas in our final image where the adjustment shows through. By using masks, we can be specific with our image enhancements. Two popular masks for astro-processing are ‘Hide All’ and ‘Reveal All’ masks. These are found in the ‘Layer’ menu (‘Layer > Layer Mask’) or accessed via shortcuts, which is covered later in the article.
‘Hide All’ and ‘Reveal All’ masks perform opposite functions. After carrying out an adjustment on a new layer, ‘Hide All’ masks create a screen from the ▼ Before: with its ‘Levels’ adjusted, our nebula image is ready for demonstrating Photoshop’s mask techniques previous, unaltered layer, and place it on top of the adjustment. This means that you can select areas where you want the change to appear by ‘rubbing’ through the screen. Meanwhile, ‘Reveal All’ masks use the adjusted layer as the screen, placing it on top of a copy of the preceding layer. This allows you to select the areas you don’t want the change to appear. Out of the two, we use ‘Hide All’ masks more often.
Applying ‘Hide All’ and ‘Reveal All’ masks in Photoshop consists of several stages. The first involves creating a new layer on which you perform adjustments (press ‘Ctrl, Shift, Alt, N and E’ simultaneously for this). In this example we are using an image of the Heart Nebula, IC 1805, to demonstrate a ‘Hide All’ mask. We’ve used the ‘Levels’ function (‘Image > Adjustments > Levels’) to stretch
the image (see opposite, top right) and we are now ready to get more precise with our adjustments.
The second stage is to perform an amendment via Photoshop’s ‘Adjustments’ menu (‘Image > Adjustments’) and adding the adjustment into the new layer (right click and select ‘Merge Down’). We want to boost the brightness of our image and increase the contrast of some of the nebulosity. Screenshot 1 shows our Heart Nebula image after the ‘Brightness/Contrast’ function is applied and merged. However, we only wish to make this adjustment to some outer parts of the nebula – not the core, which is now overexposed.
This is when we can apply our mask. In this instance, we will use a ‘Hide All’ mask because it will allow us to ‘brush in’ the ‘Brightness/Contrast’ change to the areas we want. A shortcut to the ‘Hide All’ mask is to press ‘Alt’ while clicking the mask button (indicated in Screenshot 1). This is in the toolbar directly under your layers. (To get a ‘Reveal All’ mask, click on this icon without pressing the ‘Alt’ key).
For the ‘Hide All’ mask you’ll notice this action makes the main image revert back to its appearance before we made our adjustment. This is because our adjusted image is now sitting beneath the layer. Next to the layer, a black screen appears (see Screenshot 2). If a ‘Reveal All’ mask is used, the screen is white. We then click the brush icon (see Screenshot 2). We can alter the size of our brush by using the square bracket keys – ‘[‘ makes our brush size smaller, and ‘]’ larger. Ensure for a ‘Hide All’ mask your brush is white – Screenshot 3 indicates how to identify this. For ‘Reveal All’ masks the brush needs to be black.
We now brush in the areas we want to see the increased ‘Brightness/Contrast’ adjustment. As we move over these areas, we can see the change we saved down to the layer below appearing through without affecting the rest of the image (see Screenshot 3). The black screen next to our layer also shows the areas we are amending.
The last stage is to apply a ‘Gaussian Blur’ (‘Filter > Blur > Gaussian Blur’), which enables us to blend our brush strokes. This means our changes are subtly integrated into our image. It is an essential follow-on step after using either mask. The ‘Radius’ slider allows us to control our blur, so that changes appear natural. Each time we make a change that we are happy with, we start a new layer that carries these changes forward by pressing ‘Shift, Alt, Ctrl, N and E’ simultaneously. We repeat these steps with other functions in Photoshop’s ‘Adjustments’ menu.
Our final image (see opposite, top, left) shows vibrance, selective colour and ‘Hue/Saturation’ amendments applied with our ‘Hide All’ mask.