Calculations

Calculations

Learn how to use the Calculations method to make very precise selections in Photoshop.

When to use

Using Calculations is most useful when selecting things like hair, fur, or feathers in Photoshop.

Instructions

Getting Started with Calculations

• Begin by opening one of the photos of the pillow from the resources folder.
• Go to the Image Menu and choose 'Calculations' from the dropdown.
• Right away you will see your image converted into black and white.
• In the Calculations Panel that appears, we need to determine which channel has the highest amount of contrast. 
• Under 'Layer' you will see 'Channel' and if we click the dropdown menu we can choose from either 'Red', 'Green', or 'Blue'.
• In this case, the green channel has the best contrast for our image, so we will go with that.

Working with Channels

• Go to the Window Menu and choose 'Channels' from the list.
• Select the 'Alpha 1' channel that has been created at the bottom of the panel.
• The goal is to make all of the areas we want to keep solid white, and all of the areas we want to remove should be filled with black.
• Press Command/Ctrl+L on the keyboard to apply a Levels Adjustment.
• Move the right-hand slider towards the center to push the highlights, and move the left slider towards the center to increase the shadows.

Dodge and Burn

• After applying a Levels Adjustment, we will continue to push the highlights and shadows using the Dodge and Burn Tools.
• Select the Dodge Tool (O) from the toolbar.
• Check along the top toolbar of the interface to make sure that 'Range' is set to 'Highlights' and the 'Exposure' is set all the way at '100%'.
• Begin brushing over the edges of the pillow and the tassels, using the left and right bracket keys on the keyboard to make the tool smaller or larger.
• You should notice the areas that we have gray or light gray are turning into more of a solid white color.
• Once you have gone over the edges, you can use the regular Brush Tool (B) to fill the inner part of the shape with solid white. 
• To make the background solid black, you can do the same with the Burn Tool (O) or by painting with a solid black brush.

Making Selections from Channels

• Once your subject is filled with white, and the background area is filled with black, hold the Command/Ctrl Key and click on the layer thumbnail icon of the 'Alpha 1' channel to activate a selection around the object. 
• Next, click on the 'RGB' channel to see your full-color pillow.
• Return to the Layers Palette and click on the 'Add Layer Mask' button at the bottom of the panel. 
• To check your selection, add a Solid Color Adjustment Layer filled with a contrasting color, and move it to the bottom of the Layers Palette. 

Hints & tips

• When using the Calculations method for isolating subjects from a background, the goal is to use a combination of Levels Adjustments, Dodge and Burn (O), and the Brush Tool (B) to get as much contrast as possible leaving you with a solid black and white channel to work from. 
• Remember that once you activate a selection using the alpha channel, you will need to click the 'RGB' channel to see the full-color photo again.
• Calculations can be an excellent way to isolate hair, feathers, or fur that would otherwise make for some very difficult selections.