Why are two 50% transparency layers not 100% when stacked?

[George-Frazee]

Simple question, probably a simple answer but I couldn't find a thread that answered it.

I noticed while working on something, that two layers when set to 50% transparency don't "add up" to 100% transparency?

Example: 

 

All three shapes have a #000000 fill, with the two selected having opacity set to 50%. Intuitively I would think the overlap area would be 100% opacity. If I use the dropper tool the "single layer" tools have RGB levels of 128 (makes sense) and the overlap is 64.

I don't think this is a bug, it doesn't affect me since I usually set transparency by eye, but I am curious what the process is here to end up with this result. All the blend modes seem to have similar results as well.

 

[Old Bruce]

50% of 50% is 25%. 0.5 x 0.5 = 0.25. We multiply, we don't add. Consider having three shapes at 50% opacity, would it be 150% opaque?

6 minutes ago, George-Frazee said:

I am curious what the process is here to end up with this result. All the blend modes seem to have similar results as well.

 

[George-Frazee]

20 minutes ago, Old Bruce said:

We multiply, we don't add

That makes sense. 

 

20 minutes ago, Old Bruce said:

Consider having three shapes at 50% opacity, would it be 150% opaque

Just figured it would cap out at 100%, but I get it now.

[NotMyFault]

The blend formula is actually a1 + a2*(1-a1), leading to 0.75 in this case.

yo can’t rech full opacity unless one of the layers has full opacity. (Leaving aside rounding effects)

https://en.wikipedia.org/wiki/Alpha_compositing

 

Edited December 29, 2024 by NotMyFault
Added rounding exception

[NotMyFault]

1 hour ago, Old Bruce said:

50% of 50% is 25%. 0.5 x 0.5 = 0.25. We multiply, we don't add. Consider having three shapes at 50% opacity, would it be 150% opaque?

 

This is not correct. See my other post. You can always check this by using info panel, or color picker. 

[George-Frazee]

1 hour ago, NotMyFault said:

yo can’t rech full opacity unless one of the layers has full opacity.

Even more interesting. I was just thinking that, that if I stack these up they would never reach 100, if I wanted that I'd have to do some manual stuff with booleans to create the fully opaque layer.

[NotMyFault]

The easiest way to regain 100 opacity is to either add a fully opaque fill layer on bottom of the layer stack, or to add an adjustment layer like curves, channels, levels which can impact the alpha channel.

the effect you are observing has one unwanted consequence: it creates light seams when 2 vector shapes face perfectly to each other.

 

[Alfred]

15 minutes ago, NotMyFault said:

add a fully opake fill layer

*opaque

Methinks someone has been eating too much Christmas caque!

[thomaso]

5 hours ago, Alfred said:

*opaque

*opaquity 😉

early 15c., opake, (…), from Latin opacus "shaded, in the shade, shady, dark, darkened, obscure," of unknown origin. Spelling influenced after c. 1650 by French opaque (c. 1500), from the Latin.  https://www.etymonline.com/word/opaque   (german: opak)

[Alfred]

16 hours ago, thomaso said:

https://www.etymonline.com/word/opaque   (german: opak)

Danke vielmals! 👍

[NotMyFault]

Full disclosure: I use my iPad with language settings to English as primary, German as secondary, and QWERTZ German keyboard, and autocorrections active. I many cases the autocorrection gets it wrong, sometimes changing English words (with typos) to German. My most frequent error is swapping letters, and not getting the space key but instead one of the keys directly above space key. Autocorrection is always unable to compensate this simple typing errors, either changes to unrelated words or providing no correction. The lack of physical cursor keys on iPad hurts me, as touch gestures for cursor are just slow and painful. Mostly I save the post first, then edit again to manually correct. But this one slipped through as opaque / opacity/ opak / have too many variations over c, k, q after the common „opa“ prefix.

Does anybody know an app or safari extension which can be trained to my specific typing errors to fix them better than regular OS / browser extensions?

[Alfred]

6 minutes ago, NotMyFault said:

the common „opa“ prefix

The granddaddy of them all??

[NotMyFault]

5 minutes ago, Alfred said:

The granddaddy of them all??

What else could it be?

[lepr]

On 12/27/2024 at 7:21 PM, NotMyFault said:

yo can’t rech full opacity unless one of the layers has full opacity.

That's not true in practice. You have forgotten to take quantisation into account. For example, a pair of 96% opaque objects will give a 100% opaque result when alpha is 8 bits deep.

In your own words: 

On 12/27/2024 at 7:25 PM, NotMyFault said:

You can always check this by using info panel, or color picker. 

 

[lacerto]

If you want to have predictable stacked transparency calculation (addition and subtraction), you can use appropriate blend modes, below demonstrated both in CMYK and RGB:

 

 

[thomaso]

1 hour ago, NotMyFault said:

Does anybody know an app or safari extension which can be trained to my specific typing errors to fix them better than regular OS / browser extensions?

Perhaps it helps to avoid/reduce unwanted autocorrections if you add English to your iOS keyboards in your German iOS?

https://support.apple.com/guide/ipad/ipad1aa5a19a/

https://www.softwareenthusiasta.de/sprache-der-autokorrektur-von-iphone-amp-ipad-umstellen-mini-tipp

"Die Autokorrektur kann seit ein paar Jahren, genau gesagt seit iOS 10, die Sprache automatisch anpassen, sprich: Beginnt man einen Satz in einer Fremdsprache, sollten die Vorschläge nach wenigen Worten wechseln."

An additional keyboard language would add an extra button (🌐) and enable you to click-hold-toggle between two languages when typing at least.
(On a previous Android phone I had a keyboard app that toggled between two keyboard languages simply by swiping the space key)

On my MacBook under macOS -> "Firefox", the forum post editing seems to only accept English and marks any German text in the spell check, although German is set as the current keyboard. This does not happen for me under iOS -> "Via" browser, so I'm wondering if the browser affects autocorrect in its own way and possibly differently than "Safari" (I haven't tried it).

[R C-R]

8 hours ago, NotMyFault said:

The lack of physical cursor keys on iPad hurts me, as touch gestures for cursor are just slow and painful.

It may not be convenient but you can pair a BlueTooth keyboard with your iPad. I use an inexpensive one sold by Anker for this. I'm not sure if it is offered in a different version than English or if it is available outside the US.

[NotMyFault]

thank you all for replying.

I used an iPad hardware keyboard until it died just after warranty expired. It was too heavy and expensive so this is no longer an option.

i have configured both EN and DE keyboards, it does not help and keyboard settings are effectively independent from input languages and spelling correction.

The problem is that all spell checker are dumb and unable to correct the specific spelling mistakes I create. Their cannot correct swapped letters or accidentally hitting keys above the space key. They only correct „classical“ misspelled words, not the case where hit a neighboring key.

This spell checker discusses is off topic, I would like to close it for good.

 

 

[R C-R]

1 hour ago, NotMyFault said:

I used an iPad hardware keyboard until it died just after warranty expired. It was too heavy and expensive so this is no longer an option.

FWIW, the Anker one I have cost around $20 US & is compact & lightweight, about 181 g.