In traditional art, a technique known as glazing is commonly used by many artists to build up layers of paint on a paper or canvass.  The result can give a brilliance to a painting that cannot be achieved by mixing paint all at once in a single application.  This is because paints that are over mixed tend to become muddy.

For example, this apple image was painted originally in a grayscale form, then color was added.

In traditional painting, the artist would let the first layer dry completely, then and only then would he start on the new layer, otherwise, his colors might begin to lift and blend with the new paint, ruining his painting.

You can see here the two layers of paint used to make this image.

In our case we don't have to wait for paint to dry.

The layer panel on the sidebar helps us manage our separate layers of paint, or images, or photographs, or text for that matter.

Use the button to add a new layer.  This new layer will become the active layer.  Only one layer is active at a time.

Use the button to remove the current layer.

Merging layers.  When you have created an image with a number of layers, you may find it helpful to reduce the number of layers by merging some of them together.  When you have finished with an image, you might even merge all the layers before saving the file.  When you save a file with layers, only the current layer will be saved unless you use the special Layered format.

Use the button (merge) to merge all of the layers.  They will be merged using all of the layer settings as they are, so the results will be the same as what you saw before merging.

Use the button (merge with next layer) merges your current layer with the one underneath.  Unlike merging all of the layers, this function may alter the appearance of your image.  Since you are merging two layers together, it is not possible to keep layer settings such as transparency from both layers.  The transparency from the top layer will be used when blending the two images, but the transparency of the second layer will remain a setting associated with the layer, and not become part of the image until it is merged with another layer.  This is usually not a problem, but it is something to be aware of.

Use these two controls to change the positioning of layers in the stack.  The up arrow moves your current layer up by one, and the down arrow moves it down by one.

Swap Mixing.

As covered in the section on the image menu, swap mixing if very similar to layer mixing, and it can be done right from the layer panel.  If you find yourself using the swap image often, you may be comfortable using swap mixing by itself, and not involve yourself with a lot of layers.

The button swaps the current image with the swap image.

  You can see the association of the main and swap images on the layer panel.

Right clicking on the layers panel will give you a context menu with several swap mixing options.  You can activate swap mixing, swap images, and merge the current layer with the swap image.

If you use swap mixing, it will bypass the regular relationship of layers.  The swap image will become the bottom most layer.  It otherwise functions just like a regular layer.

This drop-down control lets you select the size of the thumbnails shown on the layer panel.

Examples of small and jumbo thumbnails

Clicking on the layer modes button lets you select the mode for the current layer.

The slider to the side lets you select the opacity for the current layer.  (The bottom layer can not be transparent)

Default mode is currently the Multiply mode.

Additive Adds the RGB values of the two layers.

Subtractive subtracts the current layer from the layer below.

Multiply mode multiplies the the pixels in the two layers (and divides them by 255). The result is similar to painting with ink on ink. The result is always darker.

Divide is the complement of multiply. Use it as you see fit.

Screen is similar to additive but keeps an image from ‘blowing out’ or becoming overly bright. It’s often useful for effects like stars and fire.

Around gray combines two colors around their middle brightness level.

Tracing paper makes the layer transparent like, a lot like tracing paper.

The little eyeball icon indicates the visibility of a layer.  Clicking on it will toggle the visibility of this layer.  If the layer is the bottom layer, a checkerboard will be displayed in its place.

Right clicking on the layer panel will give you a context menu with some additional visibility options.

Exclusive visibility will cause all but the current layer to become invisible.

Make all visible will turn all the layer's visibility back on.