yep, agree with @Rick on the GIMP suggestion, there are many ways you could do it, but the most obvious to me looks like this:
- Duplicate the layer
- Desaturate it
- Use Levels to crush everything to black expect the highlights you want the halation on
- Use Curves to really crush that to pure black and white
- Set it to screen
- Gaussian Blur to taste
- Use the channel mixer to change the colour by boosting Red in Red Channel and reducing green and blue in their respective channels. I like a slightly orange tinge.
When I do this in Resolve for video I do the blur in a linear colour space for a better result, but not sure if that’s possible in GIMP or not.
Anyway, here’s a before and after:
notice it on the pillars in the back right as well. Also, I actually had to manually paint black over the arrows on the ground as they were caught in the selection too and it looked a bit odd.
EDIT: as for getting your image looking filmy in the first place, I have another slightly peculiar workflow I use in darktable that’s something of a port of my Resolve workflow - but an easier way to get your feet wet would be to download the RawTherapee haldCLUT pack for the lut3D module in darktable.