Lesson 12: Layers

The implementation of a layering system makes an essential feature of every decent image editing application. Every image consists of one or more layers, with everyone of them having a varying degree of opacity, size and other properties. Layers enhance image editing very much, offering the ability to save various image data inside your project and deploy them at will and in various ways.

The concept of layers is simple, yet very powerful in its simplicity. You can imagine layers as a stack of transparent slides on top of each other. You can create as many as you want and you can manipulate them in every logical way:

• you can activate any one of them to work on and independently of the others,
• you can handle them as groups,
• you can enable or disable them,
• you can resize them,
• you can move them,
• you can manage their order in the stack,
• you can control their transparency,
• you can make them interact with each other in creative ways.

The digital means -that is to say the computer- puts in the hands of the photographer tremendous power to conduct effective manipulations that once took much time and effort and considerable technical means to implement inside the traditional good old darkroom.

Introducing layers

Let me show you how easy and effective it is to create layers.

Duplicate a layer

You can duplicate a layer by

• right-clicking on it and from the context menu Duplicate Layer, or
• Layer > Duplicate Layer [SHIFT+CTRL+D], or
• drag-and-dropping the layer on the image window.

Grouping layers

Working independently on layer data

Sometimes in a big project it is practical to work separately with a layer on its own: copy the layer [CTRL+C], create new from clipboard [CTRL+SHIFT+V], work on your data, copy, go back on the layer and paste. But there is a faster way to open the data in a separate image: just drag the layer on the toolbox. Try it!

Ways to create layers with image data

• Copy your image data (from anywhere), select the existing layer on top of which you want the new layer, create a new layer and then paste on it.
• Edit > Paste as > New Layer: GIMP creates a new layer, putting the image data from the clipboard on the canvas on the top left.
• Select the existing layer on top of which you want the new layer and drag&drop an image file from anywhere on your computer on your image directly.
• File > Open as Layers...: open multiple files with one shot, arranged as layers. If any of these files have multiple layers (for example they are XCF files), all the layers of all the images will be present in the Layers dialog!

Moving layers and the transform lock

The transform lock enables you to perform transforming manipulations on multiple items connected together.

Repeat: the transform lock applies not only to layers - it applies to any channel and any path that currently has the transform lock.

The size of layers

Every layer may be of a size different from other layers or even the canvas.

Basic layer transformations

Under the submenu Layer > Transform there are some commands to conduct manipulations which remind those under the submenu Image > Transform described in Lesson 3. These commands do not act on the image, but on the layer level.

Scaling layers

Interpolation

Interpolation is well explained in the GIMP's help file. To make the story short, when transforming digital image data, some process of recalculation of pixel values is inevitably engaged. There are varying methods for this and GIMP offers four methods, your safest bet in terms of quality being the cubic. In many cases in GIMP you will be offered to select the interpolation method, through a drop-down-list containing the four methods. You can set the default for this with Edit > Preferences > Tool Options > Scaling > Default Interpolation, so that you save two mouse clicks for one more choice.

When scaling, there is a general rule of thumb that you must always be aware of: fearlessly scale down but avoid to scale up. Of course GIMP cannot generate data where there is no data! When scaling down, the new data come out of the interpolation - with some loss of quality theoretically, since any intepolation is inherently connected with the recalculation method. When scaling up, it is simply impossible to create pixels where they do not exist, so you always have a loss of optical quality depending on the degree of the magnification that you apply.

Rotating layers

In "Basic layer transformations" above, those commands work on the whole layer. To rotate a layer at a specific angle, or even to rotate a selection, the Rotate Tool is used.

Merging layers

The commands Merge Down and Merge Visible Layers have to do with layer merging. It would not be wise to repeat here the well-written GIMP's documentation with all the nuances of the thing and i strongly suggest that you give it a good read. These tools as well made, offering you all possible logical options to do your job nice and neatly. Please excuse me for being sincere once more: this school offers the basics, and you should already know that if you intend to be proficient, you will find yourself delving into documentation files quite often.

New From Visible

Another smart and sophisticated command that merges all visible layers into a new one. As stated in the documentation, the aim is to further manipulate the result, but keep the steps that created this situation. Example: You want to selectively blur some areas of your multilayer image. You create a new layer from what you see, blur it and then apply a layer mask to erase the parts you want your original work to show.

Another use of this command might be this: you want to check the histogram of your work which consists of more than one layers. But the histogram works only on one layer. You enable only the layers that you want, and create a layer out of them.

Rejecting layer data

Note: if you reject layer data, they are permanently lost from your project. In the next lesson you will learn about using layer data partially but without losing any data - i.e. about non-destructive editing.

Opacity, transparency, translucency... name it as you like

Again let us try to escape words and make things clear. We want to get the job done, right?

The two layer locks

In the Layers dialog under the Opacity slider in the dialog area labeled Lock, there are two buttons: Lock pixels and Lock alpha channel:

• Lock pixels: if checked, layer pixels cannot be modified. Layer protected.
• Lock alpha channel: the transparent pixels on the layer cannot be modified. Whatever you do on the layer will affect only non-transparent pixels. For example, if you paint on a transparent layer, the paint acts only on the layer's content, not on the transparent areas.

Layer modes

It is possible for a layer to interact with the layer below. The normal mode represents the simplest rule possible: every pixel of the layer below is simply replaced with the one of the layer up. Some creative minds have thought of sophisticated ways of how a layer might blend with the layer below, hence the various layer modes. We have seen about the Overlay mode in Lesson 7.

Manipulations engaging layer modes are a challenge even for advanced users, requiring experience, experimentation and imagination. A certain action may well destroy an image or produce unexpected results. I just mention Layer Modes here only to remind that if you really want to advance you must be ready for work: read the documentation and experiment quite much.

You change layer mode with the first drop-down-list in the Layers Dialog:

And of course using the layer's opacity slider, you can control the intensity of the effect in reference.

Exercise: Creating a drop shadow

In the end of this Lesson, a simple example of creating a "drop shadow".