Sorry, me again:
3rd person crosshair -> has any one done it?

What I mean is: the camera sits behind the player somewhere, and a little above. if you shoot, the mouse cannot give a unique 3d position, instead it gives you an infinite line of points that the player could shoot at.

If a target is selected then it's no problem: the distance between camera and the targeted enemy is the depth of the vec_for_screen function. But what if no enemy is targeted?

c_trace won't do, as I'm talking about a space shooter. there'll be nothing that can be hit, in most directions. And since the weapons are not all instant-hit-weapons, the player needs to aim somewhere infront of the moving enemy ship, not directly at the ship, in order to hit him.

Any suggestions?

Try a three-dimensional crosshair. Use a sort of translucent wireframe box (not actually wireframe, but use narrow boxes to form the edges) to make a sort of futuristic space crosshair.

I've used a 3rd person crosshair before, though not in a space shooter. I made a helicopter game once that had two views onscreen. One was a view behind the helicopter, and took up most of the screen. In the corner a smaller window had the view from right next to the helicopter's gatling gun. I used c_trace to determine the distance ahead of the gun and used vec_to_screen to place the crosshair over the object.

try a double crosshair - it works to a point. 1 at x distance, and a second at x*2 distance. Or add a line through the crosshair, at least twice the size of the crosshair, that goes along the path of firing. This way the player gets more of an idea of the path than having to project the line themselves in their head.

Don
have a great day

both ideas are great. thank you!