Easy peasy.
Use curves in the editor and move the point top right (255) to 0 and move the bottom lef point (0 (zero) up to 255 so the line now slopeas in the opposite direction.
See attached screen shot.
Terry

Thanks to Terry for the answer!  There's a second way to do it too (which is really the same way, just another way to get there)...

You can right click on a thumbnail or thumbnails, select "Predefined Filter", and then select the "negative" filter.

Regards,
Mike

Well, FWIW, I tried flipping the curve and doing all my editing in Qimage, modifying the curve slightly, applying DFS and cropping. The results were better than what I got using multiple programs. I'm mostly working with medium format black and white negatives I shot in the early '70s in high school. My scanner (Epson 4490) lets me select negative film and does the flip for me, but the results seem better if I stick with the positive film setting and work with the resulting negative image in Qimage. Tests with a step tablet confirm better range and noise. I start with a very large 16-bit tiff image and export the result as a down-sampled jpg. 

Right, and possibly an orange mask filter for color negatives there too, the input based on a mask only part of the color neg film and a strength slider that goes down to zero for B&W negatives. 

I use scanners and cameras for that work and Vuescan allows export of a DNG packed inverted negative scan or take that has no other editing done in Vuescan. It is basically not a RAW file though but a Tiff in disguise. From there I can do the job further with several image editing programs. A true RAW but "positive" file might give even more editing possibilities.

Met vriendelijke groet, Ernst Dinkla

https://www.pigment-print.com/spectralplots/spectrumviz_1.htm

"For every complex problem there is an answer that is clear, simple, and wrong." H. L. Mencken

Correct, I have even studied the making of film masks for film separations in offset printing processes. Where the inks had an even worse representation of pure cyan, magenta, yellow for color mixing while the color negative to start from had a construction that aimed at the use of color print paper with its chromogenic photo dyes. I mentioned the strength slider to neutralize at least a part of the mask. The rest should be done with a profile to compensate the shifts per color mix area but even there problems appear as the development of the color negative film has never been that stable and by that the mask density changes per batch too. On top of that is fading of the negatives, especially for the 1970's color film it can be horrible. Vuescan has some choices to compensate for all that but you still have to do color editing.

There is another issue; the RB spectral sensibilities of most camera and scanner sensors fit quite good for the Cyan and Yellow dye densities in developed film but the Green filtered pixels do not fit as well spectrally to the Magenta dye of color films.

It would be easier today to make color negatives and use an X-rite Color Checker to make a RAW or ICC profile and use that for the output of that batch of developed color negative film. Not to mention the maskless color negative films as used in air photography. Slide material is even easier and digital takes are most convenient. However what I have in color negative film is 40 to 50 years old. So you struggle to get it right and RAW editing tools offer more in my experience. ACR accepted the inverted DNG files exported from Vuescan.

BTW, a good link you added there.


Met vriendelijke groet, Ernst Dinkla

https://www.pigment-print.com/spectralplots/spectrumviz_1.htm

"For every complex problem there is an answer that is clear, simple, and wrong." H. L. Mencken