View Full Version : transparency issues

01 January 2006, 07:19 PM
I have 2 semitransparent images that overlap and I need to fade from one to the next.

The only way I can think of to do it is to fade one in and fade the other out at the exact same time.

I did this, and every frame the transparencies add up perfectly to 100%. Since it always adds up right Why then does it get darker and lighter during this process.

Does anyone else know how to transition 2 semitransparent objects, from 1 to the next?

The images are actually shadows of objects. We were hoping that it would look like a perfectly smooth transition, but it gets darker in the middle because the shadows look like they are multiplied onto one another, but neither is using multiply or any other overlay function.

Anyone have an idea of how to do that?

01 January 2006, 07:29 PM
dont know how your images look like, but try the blending mode "alpha add" for the upper one. please tell me if it helped.
good luck,

01 January 2006, 01:25 PM
oh so close.

That solved one problem. Since the transparencies didn't line up, the solid object was getting darker (because the background is darker).

I've got shadows comming off of that, they are pretty much the same for each file, but if they overlap they get darker because they are both semi-transparent.

The alpha add fixed the solid objects and it made it easier because I only had to animate one transparency. The shadows still overlap which of course causes it to get darker as soon as it overlaps in the timeline.

Other than masking it out are the any suggestions.

One thought I had was to precompose each one with the background attatched in each so it actually didn't use any alpha. That way it should just be a straight fade.

01 January 2006, 01:49 PM
Adjust the T transparency for the overlayed images as they fade out.
Do not alter the underlayed images.
For example:
If you have 15 layers and want to make a slideshow with fades, line them up in the timeline.Select all>
Either go to keyframe assistant> sequence layers and check overlay fade as much as you want, or:
Select all> Press T. Make a fade out at their outer ends> one keyframe 100 and at the end "O" change the value to 0.
This affects all layers since their selected.
Then go to keyframe assistant> sequence layers overlap but dont check fade..

01 January 2006, 02:05 PM
The alpha add fixed the solid objects and it made it easier because I only had to animate one transparency.

as far as i know it should only work if you animate the transparency of both layers. but my phantasy isnīt able to imagine the exact problem that you have right now. i cant give any more suggestions without seeing screenshots...

03 March 2006, 11:58 AM
On the layer you are fading from:
Effect > Channel > Blend

select layer to blend to
animate the amount


03 March 2006, 12:50 PM
Agreed. Use Channel->Blend instead of animating transparency, avoid the crossfading, animate only the topmost layer. If you have to crossfade use precomposed layers.

03 March 2006, 06:22 PM
Or try masking the shadows out where they overlap.

01 January 2007, 04:06 PM
unfortunately I can't post any pictures. I'm sure you all are familiar with that.

BUT, I figured a super easy way you can see what I am talking about.

Create any old composition. Make the background white.
Create a black solid. Duplicate it.
Key opacity on the top layer somewhere in the middle from 100 to 0 percent.
Now change the bottom layer from 0 to 100 percent at the same locations.

If you play this back you will notice grey instead of staying black. I would really like to have someone explain that to me actually. I would think that 30% grey plus 70% grey could equal 100% (or black).
But it doesn't. Here's what i did to help it, but it's not perfect either. Select the first key and make it ease in (F9 will do the trick just to make things quicker). Select the second key frame for the bottom layer and do the same thing.

Notice this helps a lot, but it still fades a bit.

Now, I don't have the shadows on their own layer so it is really had to mask them off since everything is moving. This example shows only the shadows, but the entire image does this effect.

Does anyone know how to avoid this effect? I want to cross fade and have the percentages add up properly so it doesn't appear like the image is going to actually fade away completely and then return to the proper value.

01 January 2007, 05:43 PM
why dont you use alpha add blending mode (for the upper layer), like i said a year ago???

01 January 2007, 12:02 PM
I would really like to have someone explain that to me actually. I would think that 30% grey plus 70% grey could equal 100% (or black).
But it doesn't.
That's because both layers are transparent, so they can't block 100%. Think of it as a real-life example, where you have two plastic sheets that are both 50% transparent: That won't block 100% either because:

First sheet blocks 50% of the light, leading 50% of the light along to the next sheet.
Next sheet then block 50% of the remaining 50%, meaning 25% of the original.
So, two 50% opacity layers block 75%, not 100%.
And with the 70/30 example:

1st layer blocks 70%, leaving 30%
2nd layer blocks 30% of the 30% passed along from the 1st layer, equaling 9%.
That totals 79% blocking, not 100%. So it will still be transparent.
(and you get the same result if you swap the order of the 70/30 layers)

It's just like statistics on flipping a coin:

The propability for getting "tails" is 100/2 = 50%.
But by flipping the coin twice, the propability for getting "tails" once is not:
(100/2) + (100/2) = 50+50=100
(100/2)+(the remains/2) = 50+50/2 = 50 + 25 = 75
...etc. :)

As for the solution to your problem, I always just animate the opacity of the top layer, it's easy and fast. No need to do anything fancier than that. But try the channel blend as the others suggest.

- Jonas

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