Noob alpha premultiply/divide confusion

Hi,

I have a flame effect rendered in Phoenix fd which is premultiplied.

sourceFile.png (289.93 KiB) Viewed 5050 times

When I load it into Fusion it looks how I'd expect. The alpha is pretty solid (over the checkers) and the flames are bright and orange/yellow.

inFusion.png (98.55 KiB) Viewed 5050 times

But when I export it from Fusion and try to lay it over an image in photoshop, any semi transparent pixels are almost completely transparent, (this includes any glow or other effects from fusion, which almost completely transparent)

fusionOutput.png (175.93 KiB) Viewed 5050 times

Is there any way to get the outputted png file to contain the same sort of alpha information that it appears to have in the Fusion editor?
I've tried every combination of Pre-divide/post-multiply I can find. But nothing seems to do the trick. I thought I'd got it when I added a pre-divide node before the save node, but that just seems to desaturate the flame.
Any help would be very gratefully appreciated.

How does your alpha channel look if you watch it before the saver?
Transparency and alpha are 2 different things in Photoshop.
Maybe also try another image format, like tiff

PNG's specification dictates unpremultiplied alpha. Fusion will let you use a premultiplied PNG with no problem, even though it's wrong, but Photoshop will "helpfully" premultiply it again for you.

Section 12.8: https://www.w3.org/TR/PNG-Rationale.html

I am not sure if there is a way to unmultiply an image in Photoshop. There is no Divide option in the Calculation menu.

Thanks for the reply guys.

I think mentioning photoshop was misleading. I was only using that as an example. I'm actually rendering sprites for games. Whatever I view the final png in, photoshop, gimp, the game engine, windows picture viewer, the flame is too transparent.

@Michael Here's the alpha as viewed in fusion. It doesn't matter what file format I output, it always looks the same.

fusion-Alpha.png (8.16 KiB) Viewed 5008 times

I get a similar effect when I try to output semi-transparent effects like glow or highlight from Fusion.

For example, here's a coin to which I've added vector motion blur and a highlight effect. In the Fusion viewport, the highlights are brightly visible.

fusionCoin.png (84.48 KiB) Viewed 5001 times

But as soon as I try to use the outputted image, I can hardly see the output at all and there's a dark fringe like it's premultiplied.

What is the Fusion viewport doing that isn't happening when I render!?!?

as you can see your alpha is very weak in the area of the flames, seems that it will be saved with the wrong/double premultiplication.

the viewport shows the image as it is but when you save it might get premultiplied again. in this case everything without the alpha will be lost (multiply by 0 = 0)

PNG's specification dictates unpremultiplied alpha. Fusion will let you use a premultiplied PNG with no problem, even though it's wrong, but Photoshop will "helpfully" premultiply it again for you.

Section 12.8: https://www.w3.org/TR/PNG-Rationale.html

I am not sure if there is a way to unmultiply an image in Photoshop. There is no Divide option in the Calculation menu.

Thanks Bryan, I think this where I'm likely to find the solution.
Do you know of any way to unpremultipy a premultiplied png image in fusion?
I've tried toggling pre-divide by alpha in the saver.
I've added an AlphaDivide node after the loader
I've toggled Post multiply alpha in the loader.
And various combinations of all of the above.

But, nothing seems to work. When I render out the png the flames are too transparent and/or undersaturated.

What do the RGB channels look after doing alphadivide?

You could test if your saver puts out correct image by a simple setup:
- create a circle shape in random color
- blur it
- do an alphadivide

Result should have identical RGB values everywhere where alpha is not zero. Image is in unpremultiplied state now. If RGB values fall off towards the edge, something is wrong with alphadivide. Now if you write it out to png, does it look as the blurred version in second step when you open it in PS?

But I suspect there is something else going on, most probably you have some stuff in RGB channels while alpha is zero. It is not a problem by itself, actually it is the only way to produce emissive elements when you must use over, not add operation, but these values are lost wheneven the alpha premult operation is applied. If you can't turn off the premultiplication for png files in your game engine, try to use some format which is handled as already premultiplied.

PS. I guess the loss of color is due to very low alpha values and heavy rounding/clipping due to that when doing alphadivide.