Note: Photoshop tools and menus are used here as examples which may be different from other image editing programs.  Layer is called "Lens" in Corel PhotoPaint. Other than that, all operations and effects are similar to layers.

Layers

When you open an image, the image consists of one raster layer called the background. Think of this as a piece of paper which you have drawn a picture on. Layers allow you to work on part of your image without effecting other areas. You can create several new layers to make different adjustments such as color correction, exposure, contrast, or any other changes you like to make. Until you combine the layers to create one final image, each layer remains independent. You do not need to create new layers for simple image corrections such as sharpening, cropping, or color corrections.  However, if you need to make several major changes to the image, layers will make your task a lot easier.

There are three types of layers: raster layers, vector layers, and adjustment layers. Raster layers contain pixel-based information. Vector layers contain instruction data for drawing vector lines, shapes, and text. Unlike raster layers, vector layers can be added to images of any color depth. Adjustment layers contain color correction information.

The layer you are editing is called the active layer and include several tools for making corrections and adjustments. These are the Layer palette, the Layer Properties dialog box, and various commands in the Layers menu. The Layer palette shows all  layers, and order in which they are stacked, its current properties, and for a vector layer, a button for each vector object you draw. The Layer Properties dialog box shows the settings for an individual layer. The Layers menu contains the general commands for creating, managing, and merging layers.


The layer palette

When you create or open an existing image, this image is called the Background. You cannot make changes to the background as you would with other layers unless you convert a background to a regular layer. Changes to an image affect only the active layer. You select a layer to make it active, and only one layer can be active at a time, unless you group several layers together.

To select a layer:

With the Layers palette, you can control whether a layer is visible.

To show or hide a layer:

The stacking order determines whether a layer appears in front of or behind other layers.

To change the order of a layer:

1. In the Layers palette, select the layer that you want to move.
2. Choose Layer > Arrange, and choose an option from the submenu to arrange the layer
3. Bring to Front to make the layer the topmost layer.
4. Bring Forward to move the layer one level up 
5. Send Backward to move the layer one level down
6. Send to Back to make the layer the bottommost layer

You can also change the order of layers by dragging.  

1. In the Layers palette, select the layer that you want to move.
2. Drag the layer up or down in the Layers palette. When the highlighted line appears in the desired position, release the mouse button

By linking two or more layers, you can move their contents together.

To link layers:

1. Select a layer in the Layers palette.
2. Click in the column to the left of any layers you want to link to the active layer.

To unlink layers:

In the Layers palette, click the link icons to remove them.

Creating layers

Newly added layers appear above the selected layer in the Layers palette. You can add layers to an image in several different ways:

1. By creating new layers or turning selections into layers.
2. By converting a background to a regular layer or adding a background to an image.
3. By pasting selections into the image.
4. By using the type tool or by using a shape tool.

To create a new layer using default options:

Click the New Layer button at the bottom of the Layers palette. The layer uses Normal mode with 100% opacity and is named according to its order of creation.

To add a new layer and specify options:

1. Choose Layer > New > Layer.
2. Alt-click (Windows) or Option-click (Mac OS) the New Layer button at the bottom of the Layers palette to name the layer, and set other layer options.
3. Click OK.

To turn a selection into a new layer:

1. Make a selection.
2. Choose Layer > New, and choose a command from the submenu
3. To copy the selection into a new layer, choose Layer Via Copy.
4. To cut the selection and paste it into a new layer, choose Layer Via Cut.
5. The selection contents appear in the same position relative to the image boundaries.

To convert a background into a layer:

1. Double-click Background in the Layers palette or choose Layer > New > Layer from Background.
2. Rename the layer, and click OK.

To convert a layer into a background:

• Select a layer in the Layers palette.
• Choose Layer > New > Background from Layer.

Adjustment Layers

An adjustment layer is a kind of raster layer that you can use to make color corrections to a multi-layered image without affecting the other layers. To make color corrections to one layer or to an image containing a single layer, you may prefer to use the Brightness/Contrast, Posterize, Hue/Saturation/Lightness, and other Color menu commands. The adjustment layer contains the information on the changes made to the image. You can edit an adjustment layer without altering the pixel values. To make the adjustment permanent, flatten the image. You can create several adjustment layers to an image.

1- Adjustment layer

1. Use the Levels adjustment tab to map the brightness values in the image to new values. The original values are the input values; the new values are the output values.
   In the Channel drop-down box, select the color channel to edit. You can edit all the R/G/B values simultaneously or select a single channel.
2. Input Levels
   Modify the Input levels to darken the darkest values and lighten the lightest values. This increases the contrast in the image. The Input levels boxes correspond to the diamonds on the slider underneath them. The box and diamond on the left represent a lightness value of 0. The middle box and diamond control the gamma curve. The box and diamond on the right represent a lightness value of 255.Type values in the boxes or drag the diamonds to set new levels. As you increase the value in the left box, all lightness levels in the image lower than the new value are set to 0 (black). As you decrease the value in the right box, all lightness values in the image higher than this value are set to 255 (white).
   Change the gamma value to reset the lightness of medium grey and alter the center of light balance.
3. Output Levels
   The Output levels modify the edited Input values to another set of brightness values. Modify the levels to lighten the darkest pixels and darken the lightest pixels. You set the lowest range and highest range for brightness levels. The Output levels boxes correspond to the diamonds on the slider underneath it. The box and diamond on the left represent the minimum (darkest) value in the image. As you increase this value, all levels in the image below it are set to the new level (lightened). The box and diamond on the right represent the maximum (lightest) value. As you decrease this value, all levels in the image above it are set to the new level (darkened). You can also use the Output levels to invert an image by dragging the black diamond to the right (or typing a high number in the box), and then dragging the clear diamond to the left of the black one (or typing a low number in the box).

2- Curve adjustment layer

The Curves adjustment layer is similar to the Levels adjustment in that you use it to adjust lightness values. However, with Curves, you can the change any lightness value on the 0-255 scale. Use the Curves Adjustment tab to map any brightness value to a new value. This tab gives you more flexibility than the Levels tab because you can edit any value on the 0 to 255 (black to white) scale.

The graph charts the relationship between the input and output levels. At the lower left of the chart, both values are 0 (black). At the upper right, both values are 255 (white).
Select a channel to edit from Channel drop-down box. You can edit all the R/G/B values simultaneously or select a single channel.
You can change lightness values by entering numbers in the numeric edit control or dragging the graph line and adding a point to it. To use the numeric edit control, enter an input value and the new output value.

To add a point to the graph line, move the cursor over line. When the cursor changes to an arrowhead and displays "+add,’" click to add the point. You can then drag the point to a new location or use the numeric edit control to move it to a new brightness value. You can add up to 16 points.
You can also drag a point along the line to substitute values. To remove a point, drag it to the top or bottom of the graph and release the mouse.

3- Brightness/Contrast Adjustment Layer

Use a Brightness/Contrast adjustment layer to lighten or darken the underlying layers and change the amount of shading, or contrast, between areas.

Brightness Setting: A value of 0 displays the image at its original Brightness level. Drag the slider to the right or type a positive number to lighten the image. Drag it to the left or type a negative number to darken it.

Contrast Setting: A value of 0 displays the image at its original Contrast level. Drag the slider to the right or type a positive number to increase the contrast and make the image sharper. Drag it to the left or type a negative number to decrease it and make the image more blurry.

4- Color Balance Adjustment

Use the Color Balance adjustment tab settings to shift the balance of a color and its opposite on the color wheel. There are three pairs: cyan and red, magenta and green, and yellow and blue. Increasing the amount of one decreases the amount of its opposite. This tab contains two panels, one for tone balance, the other for color balance.

5- Hue/Saturation/Lightness Adjustment

Use the settings on the Hue/Saturation/Lightness adjustment tab to change the hue, saturation, and lightness values for any color or range of colors in the layers underneath. When the 3 sliders and boxes are set to 0, the image has its original values.
The tab contains three concentric hue rings:

1. The outer hue ring, displays a color wheel. It represents the original hues. When the tab opens, this ring and the inner ring are displayed fully saturated. Increased saturation levels are not visible.
2. The inner ring displays the cumulative effects of all the adjustments. It rotates and changes as you adjust the settings, but does not reflect increases you make to the saturation level.
3. The middle ring is the control ring. When the tab opens, this ring is half saturated. It reflects all adjustments you make, including those to the saturation level. If you edit all the colors in the image by choosing Master in the Edit drop-down box, the entire ring appears. It shows all the colors at their current values. If you restrict the editing to a color range, only the segment of the ring at the color range appears. Drag the controls in the color segment to set where in the range the adjustment starts, where it is at full effect, where it begins to taper off, and where it ends.

Opacity

A layer's opacity determines to what degree it obscures or reveals the layer beneath it. A layer with 1% opacity appears nearly transparent, while one with 100% opacity appears completely opaque.

To specify opacity for a layer:

1. Select the layer in the Layers palette.
2. In the Layers palette, enter a value for Opacity or drag the Opacity pop-up slider.

Blending modes

You use layer blending modes to determine how the pixels in a layer or set of layers are blended with underlying pixels in the image. By applying modes to layers, you can create a variety of special effects.

To specify a blending mode for a layer:

1. Select the layer in the Layers palette.
2. Choose an option from the Blending Mode pop-up menu.

The blending mode specified in the options bar controls how pixels in the image are affected by a painting or editing tool. To select a blending mode for a tool:

Choose from the Mode menu in the options bar.

• Normal: Edits or paints each pixel to make it the result color. This is the default mode.
• Dissolve: Same as Normal, except the result color is a random replacement of the pixels with the base color or the blend color, depending on the opacity at any pixel location. This mode works best with the paintbrush or airbrush tool and a large brush.
• Behind: Edits or paints only on the transparent part of a layer. This mode works only in layers with Lock Transparency deselected and is analogous to painting on the back of transparent areas in a sheet of acetate.
• Multiply: Looks at the color information in each channel and multiplies the base color by the blend color. The result color is always a darker color. Multiplying any color with black produces black. Multiplying any color with white leaves the color unchanged. When you're painting with a color other than black or white, successive strokes with a painting tool produce progressively darker colors. The effect is similar to drawing on the image with multiple felt-tipped pens.
• Screen: Looks at each channel's color information and multiplies the inverse of the blend and base colors. The result color is always a lighter color. Screening with black leaves the color unchanged. Screening with white produces white. The effect is similar to projecting multiple photographic slides on top of each other.
• Overlay: Multiplies or screens the colors, depending on the base color. Patterns or colors overlay the existing pixels while preserving the highlights and shadows of the base color. The base color is not replaced but is mixed with the blend color to reflect the lightness or darkness of the original color.
• Soft Light: Darkens or lightens the colors, depending on the blend color. The effect is similar to shining a diffused spotlight on the image. If the blend color (light source) is lighter than 50% gray, the image is lightened, as if it were dodged. If the blend color is darker than 50% gray, the image is darkened, as if it were burned in. Painting with pure black or white produces a distinctly darker or lighter area but does not result in pure black or white.
• Hard Light: Multiplies or screens the colors, depending on the blend color. The effect is similar to shining a harsh spotlight on the image. If the blend color (light source) is lighter than 50% gray, the image is lightened, as if it were screened. This is useful for adding highlights to an image. If the blend color is darker than 50% gray, the image is darkened, as if it were multiplied. This is useful for adding shadows to an image. Painting with pure black or white results in pure black or white.
• Color Dodge: Looks at the color information in each channel and brightens the base color to reflect the blend color. Blending with black produces no change.
• Color Burn: Looks at the color information in each channel and darkens the base color to reflect the blend color. Blending with white produces no change.
• Darken: Looks at the color information in each channel and selects the base or blend color--whichever is darker--as the result color. Pixels lighter than the blend color are replaced, and pixels darker than the blend color do not change.
• Lighten: Looks at the color information in each channel and selects the base or blend color--whichever is lighter--as the result color. Pixels darker than the blend color are replaced, and pixels lighter than the blend color do not change.
• Difference: Looks at the color information in each channel and subtracts either the blend color from the base color or the base color from the blend color, depending on which has the greater brightness value. Blending with white inverts the base color values; blending with black produces no change.
• Exclusion: Creates an effect similar to but lower in contrast than the Difference mode. Blending with white inverts the base color values. Blending with black produces no change.
• Hue: Creates a result color with the luminance and saturation of the base color and the hue of the blend color.
• Saturation: Creates a result color with the luminance and hue of the base color and the saturation of the blend color. Painting with this mode in an area with no (0) saturation (gray) causes no change.
• Color: Creates a result color with the luminance of the base color and the hue and saturation of the blend color. This preserves the gray levels in the image and is useful for coloring monochrome images and for tinting color images.
• Luminosity: Creates a result color with the hue and saturation of the base color and the luminance of the blend color. This mode creates an inverse effect from that of the Color mode.