An image can be treated as a set of channels as follows:
- 32 bit RGB image has 4 channels: Alpha, Red, Green, and Blue. Each channel is 8 bits deep, containing intensity levels from 0 to 255 for the particular component of the image which that channel represents.
- 24 bit RGB image has 3 channels: Red, Green, and Blue. It is the same as a 32 bit image without the Alpha channel.
- 16 bit RGB image has 4 channels: Alpha, Red, Green, and Blue. The Alpha channel is 1 bit deep, containing 2 intensity levels. The Red, Green and Blue channels are 5 bits deep, containing 32 intensity levels each.
- palette image (<= 8 bits) has a palette which can be treated in some respects as having Red, Green, and Blue channels, each 8 bits deep.
An image is formed on the monitor by combining the Red, Green, and Blue channel values for each pixel.
The Alpha channel is non-standard and may be used for masking or any other purpose.
Alpha, Red, Green, Blue:
These functions extract the selected channel from the current image and set the palette to a 256 level grayscale.
Swap Red & Green, Swap Red & Blue, Swap Green & Blue
These functions swap the specified channels in the current image. For a palette image, the channels in the palette are swapped. For an RGB image, the channels in each pixel are swapped.
R to G to B to A to R
This function shifts all the channels over one to the channel on the right. The Red channel goes to the Green channel; the Green channel goes to the Blue channel; the Blue Channel goes to the Alpha channel (or the Red channel if there is no Alpha channel); the Alpha channel (if there is one) goes to the Red channel.
To Original Alpha, To Original Red, To Original Blue, To Original Green:
If the current image has an active selection (from doing a Paste or a Place or a select and drag operation), the intensity levels for each pixel in the selection will be sent to the designated channel in the current image.
Otherwise (no active selection), the intensity levels for each pixel in the current image will be sent to the designated channel in the original image (the one obtained by doing Revert To Original).
Invert Alpha, Invert Red, Invert Green, Invert Blue
This function inverts the specified channel. For example, value 0 goes to 255, value 1 goes to 254, etc.
Clear Alpha
This function set the alpha channel to all zeroes.
Non-Background Mask To Alpha
This function takes the current image and makes a mask where white represents every pixel in the image which is non-background color, and black represents every pixel in the image which is background color. It then places this mask into the current image's alpha channel. Be sure to set the Background Color before doing this function via the Palette:Options:Dithering & Background Color… dialog.
Restore Image Where Alpha Is 0
This function copies pixels from the previous version of this image to the current version of this image wherever the alpha channel in the current image is 0. Restore only works if the current image has an alpha channel and is the same size and bit depth as the previous image.
Restore Image Where Alpha Is Not 0
This function blends pixels from the previous version of this image into the current version of this image wherever the alpha channel in the current image is not 0, using the alpha channel value as the blend factor (0 for no change, 128 for 50% from each, and 255 for full replace). Restore only works if the current image has an alpha channel and is the same size and bit depth as the previous image.
