We already know that images are made up of three channels—Red, Green and Blue. But every once in a while, you will hear the words “Alpha” or “Alpha Channel” used in the context of digital imagery.
What are Alpha channels?
All channels are greyscale images that basically represent the intensity of that particular colour. For example, white areas in a Red Channel mean that those areas have full (100 per cent) red intensity Similarly, black areas mean that those areas have no red in them.
The Alpha channel, too, behaves this way, only instead of colourit represents transparency, or rather, the presence of the image itself So black areas in an alpha channel mean that the image is fully transparent (or not there) in those areas, while the image will show through the white areas. When you create a mask for a layer, you’re actually creating an Alpha channel for it.
Why Masks?
A good deal of what can be accomplished by using masks is also possible with a few well-made selections here and there, so why use them? Well, for one thing, masks are treated just the same as any other layer—any effect you can apply to a layer can also be applied to the mask, giving you the ability to create some incredible effects that would take days to accomplish with just selections. Moreover, masks allow for completely non-destructive editing—that is, you don’t need to make any alterations to the layer you’re working on, and getting back to where you started is as simple as shutting off or deleting the mask.
Getting Started With Masks
Now that we’ve got an idea of what masks are, it’s time to see how we can create and edit them.
Creating a mask
To create a mask for a layer, ,click on the Create Layer Mask button at the bottom of the Layers Palette. By default, Photoshop creates a “Reveal All” mask, which is filled with white. The name is quite suggestive—the Reveal All mask lets the layer appear as is, and it is only after you paint on black areas in the mask that the underlying layer shows through.
To create the exact opposite, use the “Hide All” mask, holding down [Alt] before clicking on the Create Layer Mask button. You will now see a mask that is filled with black instead of white.
Alternatively, you could go to Layer > Add Layer Mask > Reveal All or Hide All to create a new layer mask.
Switching between an active mask and an active layer
As soon as you create your mask, it is made active—you can now paint on it and apply filters to it. You will notice that the colours in the toolbox have changed to black and white, and the thumbnail for the default colours now show white as the foreground with black as the background. In the Layers palette, the thumbnail for the mask has a thicker border than the main layer.
If you’d like to edit the layer and not its mask, click on its thumbnail. Now, the layer’s thumbnail will have the thicker border, the colours in the toolbox will revert to whatever they were before
you started with the mask, and the default colours will once again be black foreground and white background.
Linking and unlinking masks
By default, layers and their masks are linked to each other—if you use the move tool to move a mask, the layer moves with it and vice-versa. The chain-link icon between the two thumbnails
indicates this linkage.
Mask and layer are linked in the image above
If you’d like to move or transform the layer and mask independently of each other, you can unlink them by clicking on the chain-link icon.
Mask and layer are not linked in the image above
Temporarily disabling the mask
If you want to see what an image looks like without the mask, hold down [Shift] and click on the mask’s thumbnail. A red “X” will appear on the mask’s thumbnai and the effect of the mask will now vanish from the layer. To get it back [Shift] + left-click the thumbnail once again.
The layer mask in the main document window
We’ve discussed how layer masks create new Alpha channels for the image—you can see this new channel in the Channels palette. By default, the channel’s visibility is turned off (there is no “eye” icon next to the thumbnail). However, it sometimes becomes a little annoying to have to edit masks and judge the outcome based on the corresponding layer.
To see the mask itself in the document window, turn on the visibility of the channel in the Channels palette. The mask will now appear in the main document window as a 50% transparent
red layer. You can change this appearance if you want by right-clicking on the mask’s thumbnail and selecting Layer Mask Options.
Gradient masks
Like we said before, the great thing about masks is that they can be treated like normal layers, and this means that we can even apply gradients to them. When you apply a gradient to a mask, you end up with a very interesting fade-out effect for the layer—the white part is completely opaque, the black part is completely transparent, and the intermediate areas fade according to the intensity of white or black.
Creating a gradient mask is as simple as using the gradient tool—just configure it as you would do so for any other purpose, and instead of filling the layer with the gradient, just fill the mask.
Adobe Photoshop Tutorial
Masks and Mattes
The Transparency Selection Method
The Color Selection Method
The Channel Selection Method
The Path Selection Method
The Extract Filter
Noise and Grain
Explaining Grain
Show Me the Noise
Testing, Testing
Turning Down the Volume
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