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Glow Effect and transparent background mechanics

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Oleg Pirogov
Glow Effect and transparent background mechanics
on Jan 14, 2019 at 1:44:01 pm

Could anyone please give some reference or explain mechanics behind the standard Glow Effect?

I do some basic stuff.
I get myself a star:


And want to make it glow reasonably bright.
So I apply the Glow Effect and try to do the best I can.
But even at ridiculously high values of Radius and Intensity I still have kinda dim star:


I know it has something to do with color depth, so I set it to 32 bpc.
The star is the same:


What gives me the result I expect is adding a background.
I get my ridiculously bright star:


The first problem is that I need a transparent background so I can easily nest the comp with the star.

The second and the main problem is that I have no idea what I am doing. I don't know why background is needed and why out of 6 different set ups ([8 bpc, 16 bpc, 32 bpc] X [w/ bg, w/o bg]) only one works as expected and other 5 are identical.

Any wisdom is kindly appreciated, especially if it is a reference to the exact formula of how the effect works.


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Dave LaRonde
Re: Glow Effect and transparent background mechanics
on Jan 14, 2019 at 4:00:30 pm

What kind of blend mode do you intend to use in the application that gets your work next?

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Oleg Pirogov
Re: Glow Effect and transparent background mechanics
on Jan 15, 2019 at 4:35:59 am

>What kind of blend mode do you intend to use in the application that gets your work next?

I wanted to say "Normal" but then I thought: "Why would I?" Indeed, using Add blending mode accomplishes everything I wanted from the transparent BG.

@Dave LaRonde
@Jeff Kay

Thanks!


Regarding the second question, I want to lay out what I've learned so far.

The three main Glow parameters are the following:
Glow Radius pretty much creates layer's Gaussian blur and adds it to the layer. This is the only parameter which changes alpha of surrounding pixels.
Glow Intensity (GI) and Glow Threshold (GT) affect the brightness of surrounding pixels (namely the pixels from Glow radius). Now, the exact formula is unknown to me and I only tried all-white shapes, but the brightness coefficient seems to be
GI*(GT/100) + GI*16*(1-GT/100).
It further changes across the alpha gradient, so for example, for GI=3 and GT=0% (with expected coeff of 48) for different pixels it can be in some neighborhood of 48 like in [43, 53] range.

I.e. if the glow's original was some white shape of [1,1,1] RGB color, the color of glow pixels around it will be smth like [48, 48, 48]. That is with 32 bpc color depth, of cause.


Now, these simple observations can answer my previous questions:

"Glow effect needs a background to glow", because however bright the glow pixels are, their alphas are still very low and they don't show up. Then a background is added. I will speak of alpha=1 black background and Normal blend, because it simplifies the calculation a lot.
So, alpha=1 black background is is added and normal-blended with the Glow effect:

The resulting alpha will be 1.
The resulting color will be calculated as glowPixelAlpha*glowPixelColor.

Now the brightness kicks in as glowPixelColor is something like that [48, 48, 48]. It can override even (very) low values of glow pixels' alpha's and give us a bright glow we expected.

"This works with 32 bpc color depth, and not with 16 and 8 bpc", because only 32 bpc allow RGB values go beyond 1. At 8 and 16 bps all those super bright [2, 2, 2], [48, 48, 48] etc pixels will be just [1, 1, 1]. When background is added they get alpha=1 and color=(their previous alpha) i.e. the pics w/ and w/o background will be just the same.

I feel a bit more confident, knowing the principles behind Glow. Maybe someone else will also find it useful.


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Jeff Kay
Re: Glow Effect and transparent background mechanics
on Jan 14, 2019 at 4:18:27 pm

Your glow radius is very large. If intensity stays the same and glow radius increases, then the perceptibility of the glow effect will decrease.

What I believe Dave is getting at, is that if you are planning on using the glow comp with a blend mode that works with a solid black background (e.g. screen or add), then putting a solid black behind the glow will correct a lot of issues with the glow effect (this is why adding a background gives you the result you expect).


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Max Haller
Re: Glow Effect and transparent background mechanics
on Jan 15, 2019 at 10:37:15 pm

I think the glow looks like that because after effects needs a layer underneath for the glow to blend with, see how the glow operation is set to "add". With a transparent background there is no layer for the glow to add it's color to.

If you're going to render this to be used in NLE like avid make sure you render it with a straight alpha channel too or the edge on the glow will be ruined. For situations like this i set the comp background color to whatever color makes sense before rendering with alpha. That seems to give us the best results later on when the editors cut it in.

Also keep in mind if you have the star shape layer set to add in after effects, the rendered video won't have that same blending once it's cut in.

Although i just reread your post and see you're just going to be nesting this comp somewhere else in AE but I already typed all this so maybe someone else will find it useful lol.


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Oleg Pirogov
Re: Glow Effect and transparent background mechanics
on Jan 16, 2019 at 11:18:30 pm

As for the final touch for the problem:

I used Add blending mode for a glow with black background instead of making a transparent background. And it was fine for me cause, well, I was blending it with another black bg. In different set ups Add blend (surely) won't be a substitute for a transparent bg.

So here's the actual workaround to get the transparent bg for a glowing object:

Duplicate the layer with a glow and set the black solid layer to track its Luma matter:


If an original layer with glow is put over such bg, it's almost what it was supposed to be, except for it will be a bit less bright, so Glow Intensity of the dummy glow layer should be increased a bit.


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