Using channel calculations to blend layers and channels

Using channel calculations to blend layers and channels

(Photoshop)

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 from 0 (off or black) to 255 (on or white). 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 (See "Changing image size and resolution on section 65 for information on adjusting an image s pixel dimensions.)

Using the Apply Image command

The Apply Image command lets you blend one image s layer and channel (the source) with a layer and channel of the active image (the destination).

To use the Apply Image command:

Open the source and destination images and select the desired layer and channel in the destination image. The pixel dimensions of the images must match for image names to appear in the Apply Image dialog box.

Note: If the color modes of the two images differ (for example one image is RGB and the other is CMYK) you can copy a single channel to another channel between images but you cannot copy a composite channel to a composite channel in another image.

Choose Image > Apply Image.

Choose the source image layer and channel you want to combine with the destination. To use all layers in the source image select Merged for Layer.

Select Preview to preview the results in the image window.

Select Invert to use the negative of the channel contents in the calculation.

For Blending choose a blending option.

For information on the Add and Subtract options see About the Add and Subtract blending modes on section 277 For information on other blending options see "Selecting a blending mode on section 241

Enter an opacity to specify the effect s strength.

Select Preserve Transparency to apply the results only to opaque areas in the result layer.

Select Mask if you want to apply the blending through a mask. Then choose the image and layer containing the mask For Channel you can choose any color or alpha channel to use as the mask. You can also use a mask based on the active selection or the boundaries of the chosen layer ( Transparency) Select Invert to reverse the masked and unmasked areas of the channel (See "Using channel calculations to blend layers and channels

(Photoshop) on section 275

Using the Calculations command

The Calculations command lets you blend two individual channels from one or more source images. You can then apply the results to a new image or to a new channel or selection in the active image. You cannot apply the Calculations command to composite channels.

To use the Calculations command:

Open the source image or images.

Note: If you are using more than one source image the images must have the same pixel dimensions.

Choose Image > Calculations.

Select Preview to preview the results in the image window.

Choose the first source image layer and channel. To use all the layers in the source image choose Merged for Layer.

Select Invert to use the negative of the channel contents in the calculation For Channel choose Gray to get the same effect as would be obtained by converting the image to a grayscale image.

Choose the second source image layer and channel specifying further options as described in step 5.

For Blending choose a blending mode.

For information on the Add and Subtract modes see About the Add and Subtract blending modes on section 277 For information on other blending modes see "Selecting a blending mode on section 241

Enter an opacity to specify the effect s strength.

Select Mask if you want to apply the blending through a mask. Then choose the image and layer containing the mask For Channel you can choose any color or alpha channel to use as the mask. You can also use a mask based on the active selection or the boundaries of the chosen layer ( Transparency) Select Invert to reverse the masked and unmasked areas of the channel.

For Result specify whether to place the blending results in a new document or in a new channel or selection in the active image.

About the Add and Subtract blending modes

The Add and Subtract blending modes are available only for the Apply Image and Calcula- tions commands.

Add   Adds the pixel values in two channels. This is a good way to combine nonover- lapping images in two channels.

Because higher pixel values represent lighter colors adding channels with overlapping pixels lightens the image Black areas in both channels remain black (0 + 0 = 0). White in either channel results in white (255 + any value = 255 or greater).

Add mode divides the sum of the pixel values by the Scale amount and then adds the Offset value to the sum For example if you wanted to find the average of the pixels in two channels you would add them divide by 2 and enter no Offset value.

The Scale factor may be any number between 1.000 and 2.000 Entering a higher Scale value darkens the image.

The Offset value lets you lighten or darken the pixels in the destination channel by any brightness value between +255 and -255 Negative values darken the image positive values lighten the image.

Subtract Subtracts the pixel values in the source channel from the corresponding pixels in the target channel As with Add mode the result is then divided by the Scale factor and added to the Offset value.

The Scale factor may be any number between 1.000 and 2.000. The Offset value lets you lighten or darken the pixels in the destination channel by any brightness value between

+255 and -255.