4) Layers and selection tools

[MartinB]

Layers and Selections

Everyone knows how powerful Photoshop is to the extent that some people are intimidated by it and assume you need a degree in graphic design to access it’s magical powers. For astroimaging you will only need to access a small number of its features but with a bit of understanding and practice you will soon be whizzing your way around. Along with levels and curves layers and the selection tools are the vital features to understand. Subsequent primers will outline processing methods which make regular use of these tools.

Making a Selection

After performing a histogram stretch any change made to improve one part of the image is likely to have a detrimental effect on another. For example, the highlights within an image have good signal to noise ratio and can usually be improved with some sharpening however, apply the same sharpening process to the faint, dark areas and you will introduce more graininess. Therefore it is very helpful if we can select the portions of the image to which we wish to apply a change.

At the top left of the tool bar panel you will see the 2 selection tools, one will appear as a hashed square ( or possibly an elipse) – the marquee tool, the other, immediately below will probably look like a lasso or possibly an irregular shape – the lasso tool.

To see the range of “sub tools” under any one tool bar button just press and hold the left mouse button.

With an image active select the rectangle marquee tool. Place the cursor over the image, press the cursor and drag. This will pull out a rectangle with a corner anchored at the point where you started the drag. If you hold the shift key whilst you drag the selection will remain square. If you hold the alt key whilst you drag when you release the left mouse button the rectangle will centre on the point where you started the drag. Hold down both the shift and alt keys and you will produce a square centred over the start point for the drag.

The same rules apply to the elliptical marquee tool except in this case holding down the shift key will produce a circle. Holding down the Alt and Shift keys is very useful for manually selecting stars.

Now have a look at the lasso tool. There is a free hand lasso tool which allows you to draw whatever shape of selection you wish. The only problem with this is that it is actually quite difficult to be precise, my hand always wanders off track when I use it. For more accurate work I find it easier to use the polygonal lasso. Click to start your selection then each click sets an anchor point. When you get close to your initial start point a tiny circle appears near the cursor and the next click will complete the selection. To complete the selection at any other time just double click. There is also a magnetic lasso tool which I don’t use since astro selection choice is usually a bit too subtle for it to do a good job.

If you have made one selection and wish to make another just hold down the shift key at the same time. If you already have multiple selections and wish to remove one hold down the alt key whilst dragging.

Feathering a Selection

Nearly always when you make a selection you will need to set an amount of feather. If you make any changes to your selection there will be a very sharp cut off between the selected and non selected parts of the image. To reduce this apply a feather, this reduces the weight of the change both in and out from the edge by the specified number of pixels. As soon as you click on your selection tool a small feather box appears in the tool bar at the top. The size of the feather depends on the size of the selection and the sort of change you are making. Just experiment. If you make a selection and want to change the feather click on select on the menu bar and pick feather from the sub menu.

When the feather is chosen the selection line will become smoother, in some cases it may become very small. If the feather range is larger than the selection size PS will not make the selection. The new selection boundary is feathered both outside and inside.

Colour Range Selection

Quite commonly when processing we want to select on the basis of brightness, for instance, sharpening works well on bright portions of the image but not on the darker areas. These bright areas may be spread over different parts of the image. Manual selection in this situation is tedious and probably not very accurate. This is when the colour range selection tool becomes very useful. Click on select on the menu bar and choose “colour range”.

In the dialogue box you will see the following: -

Select – the sampled colours option will be fine (we are looking for brightness not colour details. If you are selecting from a colour image it is best to initially make a duplicate copy to a new layer and desaturate it to make it black and white. More about this in the layers section)

Fuzziness – 40 is a good value. You will be clicking on a brightness level within the image. This sets how much either side of this brightness level will be selected. Increase the fuzziness and the area selected increases.

Selection/image – select image – the unaltered image will appear in the dialogue box

Selection preview – choose quick mask – the non selected parts of the main image will appear with a transparent pink mask over them ie the whole of the image when you start

You will see 3 dropper buttons, the one on the left is a simple select tool. Click on either the main image or the image in the dialogue box and all parts of the image with a similar brightness level will be selected. You can see what has been selected since the pink mask will be removed from these areas. The middle dropper has a + by it. Select this and click on other parts of the image to add to your selection. Obviously the – dropper on the right removes selections. Some people select a brightness level with the left dropper and then slide the fuzziness slider to achieve the change they want. I prefer to use the + dropper tool since I think it gives more control.

I often reverse the way the images are displayed by changing the selection/image radio buttons to choose selection and choosing “none” for selection preview. By zooming in the main image onto the critical areas you can get very precise control of the final selection.

Once you have made your selection click ok. The image will now appear with the selection crawling ant hash lines. To apply a feather return to the select menu and choose feather.

Magic Wand tool

Back to the tool bar palette, 2nd from top on the right is the magic wand tool. This does a similar job to the colour select tool in a slightly different way. I rarely use it except with Russell Cromans Gradient Exterminator software. The tolerance setting has a similar role to the fuzziness slider.

Modify/expand

If you feel your selection is a bit tight you can expand it by a defined amount by going to Select and choosing modify then expand. Choose the number of pixels by which you wish to expand your selection.

So that’s about it for the selection tools. Seems a bit complicated written down but the idea is very simple and soon comes together with a bit of practice.

Layers

The ability to work in layers is one of photoshop’s most useful features.

Open an image in photoshop.

Click on Window in the menu bar and select layers.

The dialogue box should appear on the right. Your image will be the only one in the box and will be called Background and highlighted in blue. Right click on this layer and select duplicate layer. A box will pop up asking for a name – background copy will do. It also gives you the option of sending it to another image you have open or creating a new image. You can also duplicate via the layers menu or by right clicking the main image and choosing duplicate image.

Have a look at the layers panel. There are tabs for layers, channels and path. We are only interested in layers at this stage.

Blend mode – top left with “normal” selected as default. Clicking on the arrow to the right of this box reveals all the different blend modes. Don’t panic, you won’t need to use or understand most of these. The most important are normal, colour, luminosity, darken difference and overlay. I will explain what these blend modes do in later primers. Normal is the simplest and simply treats the layers as paper images placed one on top of the other. In other words the top layer blocks the view of the other layers.

Opacity – this is set to 100% by default. Moving the slider over to the left reduces the opacity allowing the lower layer to show through. At zero opacity the top layer becomes completely invisible.

Locking tools – You can lock an number of features of the layer to prevent unwanted adjustments including transparency, image pixels and position, or you can lock everything. I don’t use this feature.

Fill – This controls the opacity of an fills made to the layer. Since we don’t go painting on top of astroimages normally it can be ignored. If you have a play you will see that it acts like the opacity slider.

At the bottom of the Layers window are a number of icons which replicate the functions found in the layers menu.

In CS2 the layers you create can be moved around by dragging with the mouse. Layers can be grouped together by shift and left clicking on the desired layers. This group of layers can be merged using the layers menu merge layers command. The whole set of layers can be merged into one image using the flatten image command in the layers menu. Other versions of photoshop move and link layers slightly differently, check the help file.

Make some adjustments to the duplicate layer and play around with the opacity settings to see the effect it has.

Eyeball – to the left of each layer is a small box containing an eyeball. The eyeball indicates that the layer is operating (it may not be visible if it is covered over by another layer). To make the layer inoperative click on the eyeball to remove it. This is a very useful tool to review and adjust changes made to the layer.

Always make sure that you are editing the intended layer, it is very easy to spend a lot of time making adjustments to the wrong layer without realising it. The active layer is highlighted.in CS2, in other versions it is indicated by a small brush symbol to the left of the layer.

Creating a new layer by pasting

Whenever something is pasted on top of an image a new layer is created, called layer 1 by default. You can use a selection tool to select part of an image then copy and paste it into the image. You can move this selection around using the Move tool at the top right of the tool palette, you can resize it or rotate it using the scale and rotate tools in the edit menu – transform. You can edit this layer in any way you wish.

So that is the briefest of overviews. A quick glance at the PS help file will quickly tell you that there is far more to layers than mentioned about, I haven’t touched on the difference between fill and adjustment layers for instance. This primer has been written on a need to know basis!