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Author Topic: adding 4 x 25% opacity is not 100%  (Read 2198 times)

ToxyLT

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adding 4 x 25% opacity is not 100%
« on: December 30, 2015, 02:31:16 am »
Hello,
I have got a polygon filled with a colour. I copied it so that I now have 4 polygons. I reduced the opacity in the Transparency panel to 25% for each of the polygons. The transparency mode is "normal". I shifted and rotated each polygon a little so that I got a kind of stepped gradient (1, 2, 3 or 4 layers over each other).
But where all 4 polygons overlap I expected the same colour as in a polygon with 100% opacity. But it is a lot lighter and still transparent (objects under the 4 polygons are still visible). Only with opacity values of 60-70% for all 4 polygons I get the full colour. Changing the transparency mode changes sometimes the colours but I never get 4x25%=100%.
Why is this so and can I get the effect that I want?

Thank you for your help.
ToxyLT

utnik

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Re: adding 4 x 25% opacity is not 100%
« Reply #1 on: December 30, 2015, 10:45:13 am »
hi toxylt

…where all 4 polygons overlap I expected the same colour as in a polygon with 100% opacity. But it is a lot lighter and still transparent (objects under the 4 polygons are still visible).

it's not scribus' fault if you expect something out of the usual – transparencies never work like that, not in the real world and not in scribus.
  • the first layer lets 75% pass
  • the second one 75% of this 75% (which is 56.25%)
  • with four layers of 25% opacity the result will be 31.64% transparency (0.75↑4) or 68.36% opacity.
i hope this makes it clear. otherwise try two real world 50% gray filters – they will darken the image, but never block it…

utnik
« Last Edit: December 30, 2015, 10:47:01 am by utnik »

ToxyLT

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Re: adding 4 x 25% opacity is not 100%
« Reply #2 on: December 30, 2015, 03:36:36 pm »
Hello utnik,
thank you for the explanation. I didn't blame Scribus, I just didn't understand the behaviour. But with your example of the filter and the calculation it is clear now.
But I still wonder, if it is possible to get the desired effect. The attached picture shows a (coloured) photo behind four transparent "waves". I want the photo to be covered completely where all 4 waves overlap, but below the text "In 100 Tagen" you can see the lower border of the underlying photo. Is there a way to accomplish this?

Best regards
ToxyLT

utnik

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Re: adding 4 x 25% opacity is not 100%
« Reply #3 on: December 30, 2015, 05:41:43 pm »
hi toxylt

…I still wonder, if it is possible to get the desired effect.

it's possible, but not that easy. you need to set different opacity values for the different areas.

if the lower shapes would be fully overlapped by the upper ones, like in my screenshot, the balanced opacity values for four shapes would be 25%, 33%, 50% and 100% (from top to bottom).

yours are different, they overlap partially. to be exact, you need to cut them in areas of 1, 2, 3 and 4 overlapping layers and give them the correct values.
but an easier approach would be to set the individual shapes to something like 50% opacity and just add another one with 100% opacity which covers the four layer overlapping area.

btw: i'd like to read the full travelling stuff, not only some ten lines…

utnik



« Last Edit: December 30, 2015, 05:44:19 pm by utnik »

ToxyLT

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Re: adding 4 x 25% opacity is not 100%
« Reply #4 on: January 02, 2016, 02:40:53 am »
but an easier approach would be to set the individual shapes to something like 50% opacity and just add another one with 100% opacity which covers the four layer overlapping area.
Hello utnik,
first I wish you and all the readers a happy new year!
I followed your tip and copied the 4 bezier curves into a new document to export them as svg-file to inkscape. Here I selected all curves and reduced them to the area where all 4 overlap with the command path/intersection (Pfad/Überschneidung). The result found its way back into scribus as svg-file to be put over the 4 existing curves.
Unfortunately scribus imports the svg only with a width of 105mm instead of the real 109mm (paper + bleed) but that  can be fixed manually same as the positioning.
I attached the result. It does what I wanted though I'm still not fully satisfied with the design, but that is another matter...
(The page is the backside of the flyer for the yearly travelogues presented by the ADFC Lübeck (bicycle club).
Greetings
ToxyLT
« Last Edit: January 02, 2016, 02:45:33 am by ToxyLT »

 

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