Application of Channel in Photoshop (I)

The term "channel" comes from traditional extensions and is a very important concept in Adobe Photoshop. In the image processing process is mainly used for two aspects, one is to save and adjust the color information of the image; the second is to save and load the selection range.

Type of 1 channel

Adobe Photoshop, there are three types of channels, one is used to describe the color channel of the image color information; the second is used to save and edit the selected range, can also be used as an alpha channel image mask; third is used to record Spot color channel for information.

When you open a new image, the software automatically creates a color information channel. The number of channels depends on the color mode of the image. By default, bitmap mode, grayscale mode, duotone mode, and indexed color mode have only one channel; RGB, Lab mode images have three channels, plus a composite channel for editing images; CMYK mode has four images. The channels, which represent C, M, Y, and K information, respectively, can be considered as the printing plate in the printing, that is, a single printing plate corresponds to each color layer.

Unlike the color channel, the Alpha channel is not used to save the color, but is used to save and edit the selection range. That is, the selection area is stored as an 8-bit grayscale image, which is equivalent to the mask function. You can use the brush or other tools to Editing can also use the channel's calculation function to form a new image.
With spot color channels, users can use a special spot color ink to replace or supplement process color (CMYK) inks. A spot color channel is added to the image, and the channel will be individually designated as an additional printing plate printed with a spot color ink.

2 channel creation

1) The creation of the alpha channel

There are two ways to create an alpha channel, one is to create an alpha channel freely on a blank channel, and the other is to create an alpha channel with a selected range.

If you create a new alpha channel, you can select the New Channel command in the shortcut menu of the Channels control panel, and you can assign a name to the created channel. Note: The color specified when the channel is created is used only for auxiliary display to distinguish between the masked and non-masked areas of the channel and has no effect on the image itself. The opacity setting is for better viewing of the object.

If you need to create an alpha channel using the selected range, you can first create the selected range in the image and then convert the selected range to an alpha channel. If the newly created channel is selected on the channel control panel, you can view the shape of the channel in the workspace.

2) Spot channel creation

To create a spot color channel, select the New Spot Channel command in the shortcut menu of the Channels control panel, or convert an alpha channel to a spot color channel.

Select the "New Spot Channel" command in the shortcut menu of the "Channel" control panel, open the dialog box, enter the name of the spot color channel, click on the color box under "ink characteristics", and select a custom color so that It is easier to provide suitable ink to reproduce the image when printing. The setting of the density parameter (between 0 and 100) is only used to display the simulated print effect on the screen, and has no effect on the actual print output. In addition, if the area is selected before the new spot color channel is created, after the new spot color channel is created, the spot color channel color will be filled in the selection area and the dotted line frame of the selection area will be canceled. This is not the same as making an ordinary Alpha channel.

If you need to convert an alpha channel to a spot color channel, first select the alpha channel you want to convert, then select the “Channel Options” command in the shortcut menu of the “Channel” control panel, and select “Spot Color” in the “Color Indication” area. A single box will complete the conversion.

3-channel application

1) Adjust image levels and correct color cast

In the composite channel, the curve tool or the color grading tool can be used to adjust the level of the image to correct the overall color shift of the image; each primary color channel can also be adjusted separately; the gray of each channel image can also be understood by using the histogram of each channel. Degree information.

2) Conversion of color channels

1 In grayscale, RGB, or CMYK mode, 16-bit channels and 8-bit channels can be converted to each other. The 16-channel image can provide finer details of the color, but its file size is much larger than the 8-bit channel image, and Photoshop limits this image to a lot, such as allowing only one layer, most filters can not be used Can not be printed and so on. In order to reduce the file can convert the file's channel.

2 images are converted to "multi-channel"

You can convert any image synthesized by more than one channel into a multi-channel mode image, but each channel in the converted image will be converted to a spot color channel.

When converting a color image to multiple channels, the new grayscale information is based on the color values ​​of the pixels in each channel. Convert CMYK image to multi-channel to create C, M, Y, K spot color channels, convert RGB image to multi-channel to create C, M, Y spot color channels, convert Lab mode to multi-channel, create three Channels, names are Alpha.

3) Separation of images

Due to the large amount of image data, it is not easy to take away at once. Consider splitting an image by channel. Using the "separate channel" command, you can separate the channels into separate image files. When you save the image file as a file format that does not support channels, you can use this method to retain individual channel information. You can map the separated alpha channels. The image file is saved separately and reloaded when needed. Note: Only stitched images can be separated from the channel.