Masks and Channels: Adobe Classroom in a Book Lesson

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brushit
Oct 1, 2003
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I am following a tutorial in Adobe Classroom in a Book Lesson 6 on Masks and Channels:

One of the last steps is to create a new channel, and draw a gradient from top to bottom on the channel, then load the gradient channel as a selection and a selection appears in the bottom part of the image.

1. Why does the gradient that I created on the channel turn into a selection that takes up the bottom part of the image?

2. What exactly is occurring when you load a gradient as a selection?
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LenHewitt
Oct 1, 2003
1. Why does the gradient that I created on the channel turn into a
selection that takes up the bottom part of the image?<<

You are creating a selection mask, but only the areas of the mask that are over 50% will show within the "marching ants" selection area. However any actions you perform with the selection loaded will extend beyond the visible selection area.

What exactly is occurring when you load a gradient as a selection?<<

Where the alpha channel is 100% white, any action you perform on the image will be applied at 100%. Where the alpha cahnnel is 100% black, no action will be performed. Between those two extremes, the effect of the action will be proportional to the ‘lightness’of the alpha channel.
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brushit
Oct 1, 2003
Thanks for the response.

When you say:

"only the areas of the mask that are
over 50% will show within the "marching ants" selection area."

What do you mean by over 50%?

So anything over this 50% determines the selection area?
GA
George_Austin
Oct 1, 2003
Brushit,

A selection "mask" differs from a "hard" selection.

In a hard selection, a pixel is either selected or it is not selected. The marching ants separate the selected from the unselected.

In a layer mask, ALL pixels in the entire image are selected, but not to the same DEGREE—yes, a pixel can be partially selected. The marching ants, now marking the locus of pixels that are "half" selected, serve a less useful purpose in a mask than in a hard selection.

The extent to which a pixel is selected is determined by its opacity. In a hard selection, selected pixels are 100% opaque, unselected are zero percent opaque (100% transparent).

In a layer mask, each pixel has its own opacity attribute and that fully describes the degree to which it has been selected. The pixels in a masked image can range in opacity over the full 0-100% opacity range rather than being confined to zero or one hundred percent as in a hard selection. At the half-way point where the marching ants appear, the opacity is 50%.

In a simple linear gradient of opacity, you know that the opacity is greater than 50% on one side of the marching ant contour and less than 50% on the other side—for whatever that’s worth to you. But in more complex patterns, it is easy to lose sight of where the high and low opacities are—which is why I said the ants are of less use to you than in a hard selection.

George
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LenHewitt
Oct 2, 2003
What do you mean by over 50%?<<

Over 50 % white or in RGB values 128,128,128 (a mid-tone grey)

So anything over this 50% determines the selection area?<<

No. Anything over 50% will be enclosed within the "marching ants" but any action will still affect the whole image area to a degree determined by the density of the alpha channel at that point

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