What are these different kinds of masks and how are they used?

"Dale Glaser" wrote in message

Hi,

I have been studying the topic of masks, and in my information gathering have come across reference to many different kinds of masks. Could someone summarize what these masks are, how to create them and how and why to use them?

While layer and gradient masks are perhaps more familiar to people I have included them just to keep my list complete.

I got started on this topic when I came across a keyboard equivalent for selecting the luminosity of a layer (Cmd-Opt-~/Ctrl-Alt-~). I began to wonder what the difference was between using the Magic Wand and using this luminosity mask equivalent, and this led to gathering this list

layer mask
gradient mask
blend mask
edge mask
midtone mask
luminosity mask
highlight mask
contrast mask
shadow mask
density mask

Thanks for any help,

Dale

That’s a lot you’re asking. I’d say RTFM

On 19 Jun 2004 11:51:18 -0700, (Dale Glaser)
wrote:

Hi,

I have been studying the topic of masks, and in my information gathering have come across reference to many different kinds of masks. Could someone summarize what these masks are, how to create them and how and why to use them?

While layer and gradient masks are perhaps more familiar to people I have included them just to keep my list complete.

I got started on this topic when I came across a keyboard equivalent for selecting the luminosity of a layer (Cmd-Opt-~/Ctrl-Alt-~). I began to wonder what the difference was between using the Magic Wand and using this luminosity mask equivalent, and this led to gathering this list

layer mask
gradient mask
blend mask
edge mask
midtone mask
luminosity mask
highlight mask
contrast mask
shadow mask
density mask

Thanks for any help,

Dale

Get a good book and learn how to use Photoshop. Try Photoshop Artistry for whatever version of PS you have. it covers what you’ve asked in about 5 or 6 chapters.

—

Hecate

veni, vidi, reliqui

On 19 Jun 2004 11:51:18 -0700, (Dale Glaser)
wrote:

Hi,

I have been studying the topic of masks, and in my information gathering have come across reference to many different kinds of masks. Could someone summarize what these masks are, how to create them and how and why to use them?

While layer and gradient masks are perhaps more familiar to people I have included them just to keep my list complete.

I got started on this topic when I came across a keyboard equivalent for selecting the luminosity of a layer (Cmd-Opt-~/Ctrl-Alt-~). I began to wonder what the difference was between using the Magic Wand and using this luminosity mask equivalent, and this led to gathering this list

layer mask – an alpha channel that limits the effect of an image or

adjustment layer. probably the most common type of mask used in Photoshop. For a great historical definition of an alpha mask, check out Alvy Ray Smith’s page. For those of you not familiar with Alvy, he’s one of the great figures in computer graphics – he has been credited with the invention of the alpha channel, and was one of the founders of Pixar:

http://www.alvyray.com/Memos/MemosMicrosoft.htm#ImageComposi ting

Alpha channels may also be used as masks in Image>Calculations, and the Quick Mask is a method of creating a mask that may be quickly converted to and from a selection. Selections are a type of temporary mask. The other masks below refer to the type of data in the mask, and not where the data resides, though this is commonly in a layer mask.

gradient mask – a mask generated by the gradient tool. Often used to

compensate for lighting variation, lens vignetting, or any other two dimensionally varying image property.

blend mask – a mask used to merge two images, usually in layers but this

also applies to a feathered floating selection.

edge mask – a mask used to generate a frame or vignette around an image,

or more rarely a mask used to emphasize the edges of objects within the image

midtone mask – a mask with maximum value around 50% image luminance. Not

often used. For layers, it is probably easiest to use the layer blend settings.

luminosity mask – any mask based on the brightness of the image, or an

individual channel.

highlight mask – a mask with max value corresponding to the highlights.

Not often used.

contrast mask – a mask
shadow mask – a mask with max value in the shadows. Often used to remove

noise from dark areas.

density mask – probably the same as luminosity mask

The is another class of mask, also called a matte, which is used to superimpose one image on another, or an object on a background. Photoshop’s extract tool is one source of this type of mask.

Thanks for an interesting question. I may make it into a web page at some point, and will give you a credit with your permission.
—

Mike Russell
www.curvemeister.com
www.geigy.2y.net

Mike,

Thanks for the response…

Couple of questions related to your post…Actually as I wrote this the questions kept increasing. I think there is a major philosophical principle of PS going here that I am not getting and maybe you can ferret it out.

When you talk about a shadow mask being "a mask with max value in the shadows", what do you mean? I assume that a shadow mask masks the shadows so that everything else shows through, but I am having trouble wrapping myself around the "maximum value in the shadows" statement.

In an RGB image, are "maximum values" the blacks with lower rgb values, or the whites or higher rgb values. I assume the latter. This maximum value thing confuses me because I tend to think of whites as no pixels, gray as 50%, and black 100% pixels, which I think is true with CMTK but the opposite with RGB. Do you see my confusion here?

This leads to some much larger questions that I put at the top of my original post:

What is the difference in what is happening between using the Magic Wand in a layer vs. a channel.

And what is actually happening when you Cmd/Ctrl-click in a mask or a channel. As with any selection, are you selecting all pixels with at least 50% of white value. But then different things happen when you do that in each of the channels. So then…in a green channel, do white pixels represent more green in the image than black pixels?

If you use the magic wand in a layer you are selecting by color I think, but I have never been clear about what that actually means. But if you use the magic wand in a channel or on a layer mask, I think you are selecting based on brightness value, or luminecence.

I came across a keyboard command Cmd-Opt/Ctrl-Alt-~ which "loads composite as selection". What does THAT mean. Or Cmd/Ctrl-~ will "load composite channel". What does that mean?

Help! If you could answer even one of my confusions here, I would appreciate it. Or figure out the larger principle at foot here that would be great.

As for permission to make a page up about different masks, go for it, and send me the link!

Dale

Dale Glaser wrote:

Mike,

Thanks for the response…

Couple of questions related to your post…Actually as I wrote this the questions kept increasing. I think there is a major philosophical principle of PS going here that I am not getting and maybe you can ferret it out.

When you talk about a shadow mask being "a mask with max value in the shadows", what do you mean? I assume that a shadow mask masks the shadows so that everything else shows through, but I am having trouble wrapping myself around the "maximum value in the shadows" statement.

It means that the layer or effect controlled by the mask has a max (nominally 255) value for the shadows. For example, using the Dust and Scratches filter with a shadow mask will remove noise from the shadows only, and leave detail in the brighter parts of the image, the midtones and highlights, alone.

In an RGB image, are "maximum values" the blacks with lower rgb values, or the whites or higher rgb values. I assume the latter.

Exactly.

This
maximum value thing confuses me because I tend to think of whites as no pixels, gray as 50%, and black 100% pixels, which I think is true with CMTK but the opposite with RGB. Do you see my confusion here?

In curves it’s true with either one, depending on how you set your curve axis. RGB folks think of 255 as bright, 0 as black. For print folks, 0 is white, 100 is max ink. Internally, Photoshop stores 100% ink as zero.

This leads to some much larger questions that I put at the top of my original post:

What is the difference in what is happening between using the Magic Wand in a layer vs. a channel.

Similar concepts, in that the selection created by the magic wand will limit any subsequent filter or other image operations to the selected area, just as a layer max will limit them to the areas with nonzero mask values. A selection is temporary, and not associated with a layer. A layer mask is permanent, and stays with the layer.

And what is actually happening when you Cmd/Ctrl-click in a mask or a channel. As with any selection, are you selecting all pixels with at least 50% of white value. But then different things happen when you do that in each of the channels. So then…in a green channel, do white pixels represent more green in the image than black pixels?

The cmd/ctrl click loads a the image data or mas as a selection.

If you use the magic wand in a layer you are selecting by color I think, but I have never been clear about what that actually means. But if you use the magic wand in a channel or on a layer mask, I think you are selecting based on brightness value, or luminecence.

If the magic wand is clicked on a color image, then the color and brightness are used to determine the resulting selection. In a monochrome image, channel, or mask, the pixel values are used directly.

I came across a keyboard command Cmd-Opt/Ctrl-Alt-~ which "loads composite as selection". What does THAT mean. Or Cmd/Ctrl-~ will "load composite channel". What does that mean?

That does a per pixel brightness calculation based on the color image, and loads that as a selection.

Help! If you could answer even one of my confusions here, I would appreciate it. Or figure out the larger principle at foot here that would be great.

Try reading Alvy’s article on the alpha channel. I like that article because it describes the original problem that the alpha channel solved, which was to emulate the masking operation use in film for compositing movie images. Photoshop was developed at ILM, and Alvy I’m sure had a hand in getting the alpha channel concept into Photoshop – everyone was using it.

If that doesn’t ring a bell, do a google search for alpha channel, and "layer mask" and find something that makes sense. And play with the stuff. It’s free after all. Make an image with several layers, and play with selections and layer masks.

As for permission to make a page up about different masks, go for it, and send me the link!

Cool!
—

Mike Russell
www.curvemeister.com
www.geigy.2y.net