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Layers and Layer Masks – Part 1

Non-destructive Image Editing in Photoshop

Layers & Layer Masks – Part 1 


In the days of film – for those who remember that far back! – care and protection of the original negative or slide from loss or damage was of paramount importance, both in storage and during processing. The same is true of digital files and it is vitally important that they too are secured from loss by backing up to an external device such as second hard drive (internal or external) or burnt to CD/DVD. It is equally important that any editing of the files is non-destructive, i.e. the original file as captured remains unchanged. This can be achieved in part by working on a copy of the original file – Image > Duplicate and closing down the original file to avoid mistakes, but it is not the final solution by any means. Image adjustments made through Image > Adjustments are not only destructive in that they are applied directly to the image file and irrevocably change the pixel structure, but they are also global in that they affect the whole image.

Non-destructive image editing involves the use of Adjustment Layers and Layer Masks. The Adjustment Layer applies the effect of the editing (Levels, etc.) on the image and the Layer Mask controls the area of the image over which the editing is applied.

The concept of an Adjustment Layer is not always easy to grasp, but think of it as an overlay placed on top of the image; the adjustment that is made be it Levels, Hue & Saturation, etc. is overlaid onto the image layer below and the effect of the adjustment is seen on the image displayed on the monitor. A good analogy is the effect of placing of a sheet of translucent tracing paper over a print – the image viewed through the paper is diffused, but if the paper overlay is removed the original view of the print is restored. An Adjustment Layer can be adjusted as many times as is necessary or reset to the default values i.e. no adjustment or turned on or off to see the effect on the underlying image and there is no practical limit to the number which can be applied. It is important to re-emphasise that the pixel structure of the image layer is not changed in any way by the use of Adjustment Layers.

Whenever an Adjustment Layer is applied to an image by default a Layer Mask  is also applied. A Layer Mask is a grey scale image which controls the visibility of the layer on which it sits. For each pixel in the underlying image there is a corresponding pixel in the Layer Mask – where the pixels in the Layer Mask are white the effect of the Adjustment Layer is applied to the underlying image, where they are black the effect of the Adjustment Layer is masked from the underlying image and where they are grey the effect of the Adjustment Layer is partially applied to the underlying image. By default the Layer Mask is white, i.e. completely transparent, but by painting on it in black or a shade of grey the locations where the effect of the Adjustment Layer is applied to the underlying image can be precisely controlled.


Apply an Adjustment Layer

With an image open in Photoshop click on the ‘Create new fill or adjustment layer’ icon at the bottom of the Layers Palette (Fig 1) – an alternative and longer route is Layer > New Adjustment Layer – and select the type of Adjustment Layer you want to apply from the menu presented – in this example a Levels layer has been selected. The Adjustment Layer will appear in the Layers Palette (Fig 2) and the Levels dialogue box with the familiar histogram will open (Fig 3). For the purpose of this exercise make a dramatic change to the appearance of the image by drawing the white and black sliders at each end of the histogram in towards the centre to increase the contrast significantly and click on OK (Fig 4).




Fig 1  Layers Palette


Fig 2 Adjustment Layer applied




Fig 3 Levels Dialogue


Fig 4 Levels Adjusted

Clicking on the ‘eye’ icon at the left hand end of the Levels Adjustment Layer toggles the Levels Adjustment Layer on/off. With the Adjustment Layer turned off the image reverts to it’s original appearance when first opened confirming that no permanent changes have been made to the image, i.e. the Levels adjustment is completely non-destructive. When the Levels dialogue is reopened by double clicking on the half filled circle icon the settings are exactly as they were when the dialogue was closed and can be adjusted again if required (Fig 5). This is in direct contrast to the destructive adjustment applied by Image > Adjustments where the dialogue is reset to the default values and the effect of the original adjustment remains confirming that permanent changes to the image were made (Fig 6).




Fig 5  Levels dialogue on re-opening on an Adjustment Layer – non-destructive adjustment.


Fig 6  Levels dialogue on re-opening after a destructive adjustment.


To revert back to the default settings of an Adjustment Layer. i.e. no adjustment, hold down the Alt key – the Cancel button changes to Reset.

Use the Layer Mask

By default the Layer mask is transparent and shown as white in the Layers palette and the effect of the Adjustment Layer is seen over the entire image, i.e. it is global. In many instances we require to adjust only a portion of the image and this may be achieved by painting, preferably with a soft edged brush, on the Layer Mask in black to block out the effect of the Adjustment Layer in specific areas or in a shade of grey (selected by varying the opacity of the brush with black selected) to partially block out or reduce the effect of the Adjustment Layer. If a mistake is made or if you wish to remove the masking from a specific area simply paint over that area in white.

The areas which have been masked out are shown on the Layer Mask icon on the Adjustment Layer and the mask may be seen full size by Alt-clicking on the mask icon (hold down the Alt key and left click on the icon). To revert back to image view Alt-click again on the mask icon. In the example shown in Figs 7 – 10 the Levels adjustment (exaggerated for effect) has been completely masked out on the sky and by 50% in the lower foreground.


Fig 7 Unmasked Image


              Fig 8  Masked Image



Fig 9 Layer Mask icon

Fig 10 Detail of Layer Mask


The Layer Mask can be disabled by Shift-clicking on the Layer Mask icon on the Adjustment Layer and re-enabled by the same action.

So far we have covered the basics of non-destructive image editing in Photoshop. Although Levels has been used as an example the list of available types of Adjustment Layers revealed by clicking on the ‘Create new fill or adjustment layer’ icon at the base of the Layers Palette’ is extensive. In the follow-up articles other ways of using Layer Masks on Adjustment Layers will be outlined together with the use of Layer Masks on image layers and the use of an empty Layer as a form of Adjustment Layer.


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