I understand PS has this, but have never used it. Is it actually useful and, if so, possible to implement in Qimage? I suspect a lot less is out of gamut then people think, and this would prove it one way or another.
edit- I'd be just as happy to see the image outline on the CIE color diagram as highlights on the actual image, if that's easier.

Wowza! I learned some things in the first few minutes of trying it. Thanks!

I agree with Terry completely.
Just wanted to add a point that I think is significant.
For may many many, years I had Epson Printers, using Epson paper primarily.
According to expectations, the matte paper was wishy washy.  That means, the blacks never got really black, and the colors were wishy washy....
When my last Epson went to the big gamut in the sky, I took advantage of a perticularly great sale, and bought a Canon Pro 100.
I started buying Canon paper which included a cheap matte (MP101) and a good matte (PM 101 (Premium matte). I was going to dismiss the paper (came in a sampler pack) based on matte experience with Epson.  Oh well free paper. I'll try it.
Holy Moley Batman. The prints jump off the page.
So I learned that the paper makes a great difference in gamut. The blacks are deep and rich again.

Fred

FWIW, I use a Canon PRO-100 and factory ink. Not sure what it's capable of printing in terms of color map, but it's certainly well known that using matte paper is going to kill the blacks and limit the range. I mostly use Red River UltraPro Gloss 2.0 and their supplied profiles. Monitor is a Viewsonic VP-2468 that isn't wide gamut, just sRGB and a maybe tad more. Calibration by eye. Camera is my antique Nikon D200.

I haven't typically shot raw, but am trying to do more. I'd like to use the larger color spaces, but I'm just a babe in the woods with that. It may be a fools errand; the reality is that a properly handled jpeg does everything I need most of the time. What I did find is I seem to be capable of processing images to have significantly out of gamut colors, even if they started out OK. Some things I shoot at work with colored plastic stock boxes and flash are also just wildly out of gamut. I think the information from the gamut warning is useful for choosing the rendering intent for a given image. Soft proofing works quite well for me, though in all honesty I usually just hit the print icon and am happy.

Anyway, I'm very happy to have the feature, though I hope it doesn't cause more questions and instill more fear than it's worth. Certainly a page of explanation would be useful, emphasizing that what it shows isn't always trouble and gets handled in the rendering process.

Already doing it, and I'm finding much of my editing is moving out of my editing program, and into Qimage. It's just a heck of a program.

My philosophy on the monitor is probably too casual, but I put priority on the printer. If everything is correct, the various reference images on the 'net will print beautifully. And they do. If the monitor displays the correct grey-scale, and looks like those printed images color-wise, I'm happy. And it does! My environment around the monitor is uncontrolled  and varies a lot, so I don't know how much I'd gain from doing an instrumented calibration.