Post Reply 

Opacity Not 50%?

Feb 22, 2012, 08:41 (This post was last modified: Feb 22, 2012 08:58 by SerahFarron.)
Post: #1
Opacity Not 50%?
So sorry if this has been asked but I haven't really seen anything yet for this problem...

But anyway, I'll try to explain the best I can.

So I want to make a colour 50% opacity. Basically, I'm doing markings for a creature and they need to be able to be different opacities. I have no problem getting it to actually be exactly 50% when I put #000000 on #FFFFFF.

[Image: 100Percent.png]
[Image: 50Percent.png]

So that's all good. But now I want a marking that's a mix of 50% of #000000 and 100% #000000 on the same layer:

[Image: 100PercentCat.png]

To get the lighter spots, I put the 'brush' to 50% opacity and didn't overlap, the colour being #000000. I would expect it does the same thing as just putting the layer itself at 50% opacity.

Looks fine at first, but when I pull the layer down to 50%, I would expect the dark spots to be 50% and the light spots to be pure #FFFFF white. But that's not the case. This is what happens:

[Image: 50PercentCat.png]

So yeah...If my explanation is right, is there any way to fix this? Or am I doing something horribly wrong? I'm not brand new to gimp, but I am still reasonably new so bear with me.

Thanks so much in advance!!
Find all posts by this user
Quote this message in a reply
Feb 22, 2012, 10:03
Post: #2
RE: Opacity Not 50%?
Alpha-compositing isn't additive but multiplicative. If you have a full black layer at 50% opacity over white, you indeed get a 50% gray. But if you superpose two such layers, you don't get full black, but 50%x50°%=25% gray (which is quite dark, but not black).

Also, a layer with RGBA=(0,0,0,.5) (black half transparent) isn't the same as one which is RGBA=(.5,.5,.5,1.) (gray opaque). Move them to a non-white background to convince yourself of the difference. And when you use a 50% opaque paintbrush on a 100% opaque layer, you get (.5,.5,.5,1.), not (0,0,0,.5).

The full math is expounded on Wikipedia but the article also carries a nice "squares" explanation.

Now answering your questions on
https://Gimp-Forum.net
Visit this user's website Find all posts by this user
Quote this message in a reply
Feb 22, 2012, 22:36 (This post was last modified: Feb 22, 2012 22:37 by SerahFarron.)
Post: #3
RE: Opacity Not 50%?
Ah okay. I think I understood that. So, how would I go about having a layer with both 100% and 50% at the same time, and then change the opacity properly when I want to?

(Sorry if you explained in that, I only half understood. xD Again still somewhat new to this kinda thing...)

*edit*

Also, the picture with the cat both the 50% and 100% are on the same layer. Just thought I'd mention that.
Find all posts by this user
Quote this message in a reply
Feb 23, 2012, 07:37 (This post was last modified: Feb 23, 2012 07:38 by ofnuts.)
Post: #4
RE: Opacity Not 50%?
(Feb 22, 2012 22:36)SerahFarron Wrote:  So, how would I go about having a layer with both 100% and 50% at the same time, and then change the opacity properly when I want to?

This is exactly what the "layer masks" are meant for. The layer mask lets you "paint" the transparency of the layer (white is opaque, black is transparent, grays are translucent). Google for tutorials and video. This one is a good example:

http://puteraaladin.blogspot.com/2008/08...p-for.html

Now answering your questions on
https://Gimp-Forum.net
Visit this user's website Find all posts by this user
Quote this message in a reply
Feb 25, 2012, 12:45
Post: #5
RE: Opacity Not 50%?
Thanks! I think I get it now! =D
Find all posts by this user
Quote this message in a reply
Post Reply 


Possibly Related Threads...
Thread: Author Replies: Views: Last Post
  Layer-mask opacity effects not clear veek 2 150 Jun 3, 2017 04:29
Last Post: veek
  How to set gradually changing opacity of a layer? adamail 2 277 Sep 4, 2014 07:30
Last Post: adamail
  Using "Bucket Fill Tool" to modify opacity? SuperCactus 2 310 Feb 13, 2014 18:20
Last Post: SuperCactus
  Mask not 100% opacity Dustin Hobbs 1 465 Aug 4, 2013 22:28
Last Post: ofnuts
  Opacity bar not working jocelynrenee 10 908 Jul 7, 2013 16:30
Last Post: ofnuts

Forum Jump:


GIMP ForumPortalArchiveContactTermsRSS