<chapter id="layers">
<title>Layers</title>

<para>
This chapter gives an overview of how layers work in &krita;, with special
notice of adjustment layers.
</para>

<sect1 id="layers-background">
<title>Background information on layers</title>

<para>
Extensive use of &krita; will almost require you to have some knowledge of
layers. Using layers, you can work on one part of the image without touching
the rest of it, and most effects are best applied on a layer, instead of on
the whole image. Of course, if you do want to apply an effect to an entire
image, &krita; does offer you that possibility, and there is nothing against
it.
</para><para>
The idea behind layers is quite simple. As the name suggests, layers lie on
top of each other, and together form the layer stack. The final resulting
image is that what you see when looking through the stack from top to bottom.
This means that usually the upper layers of your image will have more or less
transparency, since you cannot look through a layer which has no transparency
(or as &krita; denotes it, is 100 percent opaque). A layer higher in the stack
gets applied later than one lower in the stack. For example, the effect that
layer number 4 adds to the image, is applied to the result from applying
layers 1 through 3.
</para><para>
Every image you edit in &krita; contains layers. When you create a new image,
the layer box (usually shown at the bottom right of your screen, see <link
linkend="commands-palettes-layers-layers">this section</link>) will contain
one layer. The painting and editing you do is then applied to that layer. Once
you add more layers, you can choose on which part of the image you want to
work, by selecting the respective layer. All further painting is then applied
to that layer.
</para><para>
Layers are also an excellent way to check whether adding certain effects (or
applying certain image modifications) come out right. Add a layer which
contains what you want to try out, and show or hide it with the eye icon in
the layer box. You can especially profit from this method if you have multiple
effects: show and hide them in any combination, and decide which you like
best. And since you can move the layers around, you can also experiment with
the order in which the effects are applied.
</para><para>
See the <link linkend="tutorial-select-layer">Selections and layers
tutorial</link> for a small hands-on introduction.
</para>
</sect1>

<sect1 id="layers-adjustment">
<title>Adjustment Layers</title>

<para>Adjustment Layers are layers that consist of a filter and an optional
selection. The filter effect is applied to the composite image of all
layers under the adjustmentlayer in the current layergroup. The big
thing is, adjustment layers apply these effects non-destructively. The
original image data is not modified.
</para><para>
Almost all &krita; filters are suitable for use in adjustment
layers -- even filters that would downgrade the image quality. For instance,
the raindrops filter converts to 8-bit RGB before working its magic. If you
would try to use this filter directly on a 16-bit L*a*b* layer, &krita; would
warn you about the conversion to RGB and back again this filter would cause.
Not so with adjustment layers: the original data isn't touched, so applying
the filter is safe.
</para><para>
What about the colorspace of an adjustment layer then? In order to examine
this issue, you need to know what happens when &krita; renders an adjustment
layer.
</para>

<sect2 id="layers-adjustment-selections">
<title>Adjustment Layers and selections</title>

<para>If the currently active layer has an active selection, then that selection
will be copied and used as a mask for the adjustment layer. If there is no
active selection, then there will be no mask and the adjustment will apply to
the entire extent of the layers under the adjustment layer in the current
group. There is <emphasis>no</emphasis> way of adding a mask to an existing
adjustment layer.
</para><para>
If there is a mask in the adjustment layer, you can edit the mask using the
ordinary painting tools and painting operations.
</para>

</sect2>

<sect2 id="layers-adjustment-projection">
<title>A note on projections</title>

<para>
&krita; composites the layers bottom to top, within each layer group. The
aggregate -- or the projection as we hackers call it -- is then filtered by
the adjustment layer. If there are layers on top of the adjustment layer,
those are composited onto the projection. &krita; converts all layer data before
compositing, so if the bottom-most layer in an image is grayscale, all layers
are converted to grayscale before compositing -- and that means that the
adjustment layer projection will be grayscale, too.
</para><para>
With this knowledge you'll understand why &krita; can often offer better
performance working with layers on top of an adjustmentlayer which is on top
of a complex layer structure: &krita; uses the projection and doesn't even look
anymore at the layers under the adjustment layer. Unless, of course, you
change one of them.
</para>
</sect2>

</sect1>

</chapter>
