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How to use Channels in Photoshop

About channels

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    Channels are grayscale images that store different types of information:

    • Color information channels are created automatically when you open a new image. The image's color mode determines the number of color channels created. For example, an RGB image has four default channels: one for each of the red, green, and blue colors plus a composite channel used for editing the image.
    • You can create alpha channels to store selections as grayscale images. You use alpha channels to create and store masks, which let you manipulate, isolate, and protect specific parts of an image. In addition to supporting alpha channels from Photoshop, you can save, load, and delete selections as alpha channels in ImageReady.
    • You can create spot color channels to specify additional plates for printing with spot color inks.

    An image can have up to 56 channels. The file size required for a channel depends on the pixel information in the channel. Certain file formats, including TIFF and Photoshop formats, compress channel information and can save space. The uncompressed size of a file, including alpha channels and layers, appears as the rightmost value in the status bar at the bottom of the window when Document Sizes is chosen from the pop-up menu.

    Note: As long as you save a file in a format supporting the image's color mode, the color channels are preserved. Alpha channels are preserved only when you save a file in Adobe Photoshop, PDF, PICT, Pixar, TIFF, or Raw formats. DCS 2.0 format preserves only spot channels. Saving in other formats may cause channel information to be discarded.

Using the Channels palette (Photoshop)

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    The Channels palette lets you create and manage channels and monitor the effects of editing. The palette lists all channels in the image--composite channel first (for RGB, CMYK, and Lab images), then individual color channels, spot color channels, and finally alpha channels. A thumbnail of the channel's contents appears to the left of the channel name; the thumbnail automatically updates as you edit the channel.

    Illustration of Channels types with these callouts: A. Color channels B. Spot channels C. Alpha channels
    Channels types A. Color channels B. Spot channels C. Alpha channels
Managing channels (Photoshop)

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    You can rearrange channels, duplicate a channel within or between images, split a channel into separate images, merge channels from separate images into one new image, and delete alpha and spot channels when you're finished with them.

Adding spot colors (Photoshop)

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    Spot colors are special premixed inks used instead of, or in addition to, the process color (CMYK) inks. Each spot color requires its own plate on the press. (Because a varnish requires a separate plate, it is considered a spot color, too.) For information on printing spot color plates

    If you are planning to print an image with spot colors, you need to create spot channels to store the colors. To export spot channels, save the file in DCS 2.0 format or PDF.

    Note: ImageReady supports Photoshop spot color channels as alpha channels.

Using channel calculations to blend layers and channels (Photoshop)

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    You can use the blending effects associated with layers to combine channels within and between images into new images using the Apply Image command (on single and composite channels) and the Calculations command (on single channels). These commands offer two additional blending modes not available in the Layers palette--Add and Subtract. Although it's possible to create new combinations of channels by copying channels into layers in the Layers palette, you may find it quicker to use the calculation commands to blend channel information.

    The calculation commands perform mathematical operations on the corresponding pixels of two channels (the pixels with identical locations on the image) and then combine the results in a single channel. Two concepts are fundamental to understanding how the calculation commands work.

    • Each pixel in a channel has a brightness value. The Calculations and Apply Image commands manipulate these values to produce the resulting composite pixels.
    • These commands overlay the pixels in two or more channels. Thus, the images used for calculations must have the same pixel dimensions.

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