Koray wrote:
well now thats interesting

Yeap!! & Weird!!

NothingIsRealButTheGirl wrote:
Bottom layer white

Bottom layer black

I've always come across this and usually end up doing the expand selection as a fix.  I was curious about the opacity and it looks like it caps out at 25%.  Maybe it is an issue with rounding and splitting the difference?



Above, the white mask is actually the combined layers beneath with a mask based on transparency.

Internos Photography wrote:
Above, the white mask is actually the combined layers beneath with a mask based on transparency.

A Photoshop layer mask does a (t)(1 - t) blend with the composite below.



The second, inverted mask doesn't "look upward" to try and fit the fist mask. It looks downward again. In this case to an unseen layer of transparent background.

PANZERWOLF wrote:
i don't see what this setup has to do with how a mask works, or with the initial post
since you have the black layer in each group, you're just adding color values, not transparencies!

if you mask a shape with its own inverted opacity (or add a shape with the inverted mask) you end up with a fringe, ranging from opacity 0% to a maximum of exactly 25% (or 100% to 75%), regardless of your gamma curves

that's because the effect i described above is strongest where the edge is 50%:
1/x=1/0.5+1/0.5 -->
x=1/4 = 0,25 = 25%

Well, yes. Masking an already masked thing is a recipe for matte lines. I wrote about it extensively in those decade-old links above, but it's been known since at least the 80s, and much earlier than that in film opticals.

And multiplying a mask by its inverse to get 25% is something I write about in some detail in this 4-year old post
http://www.digitalartform.com/archives/ … n_out.html

Although I used that amplified 25% 'fringe' effect extensively in the 1993 film 'Demolition Man' to capture, amplify, and color the fringe at the border of the frozen/unfrozen mask in the scenes where we wipe from an unfrozen to a frozen set.

PANZERWOLF wrote:
i don't see what this setup has to do with how a mask works, or with the initial post
since you have the black layer in each group, you're just adding color values, not transparencies!

if you mask a shape with its own inverted opacity (or add a shape with the inverted mask) you end up with a fringe, ranging from opacity 0% to a maximum of exactly 25% (or 100% to 75%), regardless of your gamma curves

that's because the effect i described above is strongest where the edge is 50%:
1/x=1/0.5+1/0.5 -->
x=1/4 = 0,25 = 25%

And there are no 'transparencies.' There are only color values and and extra alpha channel, and what gets done with them generally falls into two categories.

Alpha as a clipping mask (as Photoshop does), and

Alpha as holdback matte, the way CG artists rendering on black generally have worked over the years.

(and sometimes a conversion from the latter to the former using 'unmult' or a divide blend node or what have you to divide all the color channels by the alpha and turn it into an overshot shape that the Photoshop or After Effects mask can clip back again)

and I think to think of this as 'adding transparencies' adds confusion, but what do I know. Although I have been diagnosing people's matte lines for nearly 30 years now -- so long a time that I still call them "matte lines"



Masking already-masked CG gives gray edges.