Creating an emission texture image from a "normal" image

Question

I have a New York skyline image that I want to use as a background texture image.

Here is the .blend:

I currently can not control the emission value for the individual "lights" in the picture, so is there a way that I can? Using an image for the emission factor would be optimal.

In rendered view with emission node setup for image:

Answer 1

Here's one possible way to do what you want, if I understand your request properly.

The mix shader on the lower left is just the one that comes from the Images as Planes addon. That could just as easily be the direct Color out of the Image node. What I'm doing here is crushing the darker areas so that the image is mostly just the lights and the skyline. This goes into an Emission shader which, by use of the Is Camera Ray output as the Mix Shader factor, lights the scene but doesn't change the appearance of the image.

If this isn't what you're looking for, please update your question to give more information.

Answer 2

There is many possibilities to do separation, but also with so many limits, that make it almost nonsense.

• First limit is 8bit depth of your JPEG, should be 32bit like Open EXR. 32bit gives you much wider color range (especially to separate sky).
• Second limit is bouncing light that make it totaly impossible to separate all indirect light for re-lightning make looks "normal".

Light (color) can be separated by intensity in material like with Power node or by color that is better to work in Compositor with all the Matte nodes or manually by masking some areas. But as I wrote one light contaminates other so there is no simple way in this kind of image.

It always depends what do you want to achieve.

Answer 3

Object Material

Simple control of emission without affecting source appearence. Quite surprised how light wrapped Sphere just from front direction. But object is not lighted from front (black faces in center), because there is not any light behinde camera.
(Same seen in Electric_Wizard's answer).

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World Material

You would probably want to use the image as World material mapped for Camera window (Texture coordinates > Window).

Illusion of environmental light you can quite quickly fake by extending original image by mirroring and a bit of repainting to eliminate sun light from backside (to create like 360 panorama image). Since its just for envi light you don't have to care to much.

Camera Properties > Lens > Focal Lenght 17 brings envi texture into original image horizontal range view. Compensate vertical distortion you can in Texture Coordinates node Scale Z like 6, but lightning looked better without, so I left it.

Note: Pay attention to a node type - for Camera Window its Image texture for envi light Equirectangular texture