Getting the light from an image

[Deckonym]

Hello guys!

I've got two computer rendered images. In one of the a light is turned on, thats the only difference. Now I want to get the light out of the image, into a new one. Basically I want to be able to turn on the light, by showing the new layer, with only the light, on top of the image, in which there is no light.

Thanks

[firstdefence]

Welcome to the forum @Deckonym

You might be able to get something from this video with regard to luminosities.

 

[JIPJIP]

I'm not sure I undestand well what you want.

But if I understand, I already did that with substractive blending mode between your two rendered images :

Substract the layer without light to the layer with light and you get only the light result.

[JIPJIP]

In fact I did the inverse in my case.

I had a light the director didn't want. It was faster to compute the scene with this light only than to compute the whole scene without this light.

So I computed the scene with this light only and substracted it in compositing from the original scene. It worked perfectly.

[Deckonym]

Not quite working for me... I need the light to be transparent. So that I can add up multiple lights, just by new layers on top.

Those are the two pictures. If I substract them I get this:

and I only want the light.

[JIPJIP]

3 hours ago, Deckonym said:

I need the light to be transparent

Still not sure what you mean by transparent. What for do you need an alpha channel here ?

For what I think I undestand you want to do, you should,  if your 3D software allows it, do render passes (or whatever name it is called in your 3D software - what 3D software do you use ?) and separate each light and environment lighting in different passes. Then recompose them after that, so you could turn on and off each light as you need when compositing. In this case layers should be composited additively. (Assuming your base rendering (with all lights) is as you want, as it seems to be). (For me your light in the small room seems stronger and burns the walls around her - or you show this image in linear mode and have not yet apllied a transfer curve - which should be done with a 32bit per channel linear render to keep values precise enough).

I think you want to be able to turn on/off each light as needed, is this what you want to do ?

[Komatös]

Your plan is doomed to failure, because you cannot switch lights tranzparently.

Try the Live Filter Layer 'Lightning' with the original image.

[JIPJIP]

@Deckonym

You should explain what you want to do with your layers. It seems you have to adapt your 3d workflow first. (3d in my trade since about 30 years).

Most 3D softwares are able to render lot of different output buffers that allow many possibilities in compositing.

[RichardMH]

I may be being naive but would lighten blend mode help? (Or screen or lighter colour?) Then play with opacity.