Is that all you need to do now....just get a bunch of pixels on that outline?
If so, we've tackled something similar before....try this and see if it fits with your circumstances.
Make a copy of your original layer before anything else.
It looks as if you have the means to create a marquee around the vehicles so you're halfway there already......and that's where to start.....with a good clean marquee.
1. Copy and paste the cars to a new layer, or remove the background.....whichever is easier for you...just as long as you end up with transparency around the area you want to 'extend'.
2. Apply a 'Stroke' fx to those pixels.....make it Black, 1 or 2 px at most and set to 'Inside'.
3. Set the 'FILL' of that layer to zero.....you should now have just the stroke showing.
4. Make a marquee from the stroke......you could use any channel to do this....they will all be the same.
5. Then turn OFF the stroke layer.
6. Turn ON your original layer........make sure its the active layer.
7. Copy the pixels and paste to a new layer.....or Ctrl + J.
That will give you a 1 or 2px outline with exactly the same colours as the original.
It sounds as if you've got the rest covered so hopefully this is all you need now.
Regards.
MrToM.
