40% black – not 40% white?

MV
Posted By
Mathias_Vejerslev
Jul 11, 2004
Views
336
Replies
5
Status
Closed
I´m experiencing an unfamiliar problem with Photoshop, perhaps a bug.

I´m assembling a lot of panoramas in Photoshop, and this takes a bit of mask tweaking.

For some areas, I use a soft brush in the Layer Masks at ~40% opacity, and heres the problem:

When painting with black at 40% over an area, and then immediately afterwards with a 40% white brush, it takes several passes with the white brush to make up for the one brush stroke with 40% black in the mask. Why is this?

Mathias
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MV
Mathias_Vejerslev
Jul 11, 2004
Actually, no matter how many times I pass the area with a 40% white brush, the mask never reaches 100% opaqueness. I have to use a 100% white brush to patch the transparency hole.

This is sRGB.

Mathias
Z
Zog
Jul 11, 2004
It sounds to me like a half-life sort of issue, Mathias. 40% gains could never reach 100%.
H
Ho
Jul 11, 2004
Hi Mat,

The best I can achieve with a white brush @ 40% is 1%k after a boatload of strokes. However, as soon as I switch the percentage to 50 or above, I can get 100% white (0%k) with 8 strokes or less. Interestingly, I cannot achieve 100%k by using the 40% opacity black; it tops out at 99%.

What do dis mean, Leon?*

*David Letterman to Leon Spinks, circa 1986.
JA
Jay_Arraich
Jul 11, 2004
Mathias,

That’s how opacity works in Photoshop (brushes and layers). Each additional stroke is that percentage of the remaining opacity. So, for example, after one 20% stoke, there is 80% transparency left. If you do another stroke, you add 20% of that 80% to the 20% that is there from the first stroke. The math is .2 + [.2 x .8]= .36. For the next stroke, the math would be .36 + [.2 x ..64] = .488 or about 49%. The long version of that formula would be .36 +
[.2 x (1 – .36)] = .488

After 9 strokes with a 20% opacity brush you end up with an opacity of only 87% [.8656].

You can test this with the info palette (stroke a path repeatedly).

-Jay Arraich
MV
Mathias_Vejerslev
Jul 12, 2004
That all makes sense.

However, why does it take so many white strokes to make up for one black stroke at the same brush opacity? Its like its not 40% black, but 80% black or similar..

Maybe I´m just not getting it.

Mat

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