Hi all,
Not strictly a mapping question, but as this is the only art site I'm a member of...

I'd like to try my hand at drawing/painting interior and exterior perspective pictures of some of the buildings I've created floor plans for, but I'm crap at artwork - can't draw a straight line with a ruler.

What I could do with is some software that will help me to get perspectives right - something that traces in perspective lines, allows me to measure lengths, widths, heights, place my Eye Position, etc. Something that will allow me to place a pair of columns 20 feet high, 40 feet in front of me and ten feet to the left and 8 feet to the right of me.
Or draw a round tower, 30ft dia and 80 ft tall, 60 yards away up a 10 degree incline 15 feet above ground level...

But... I probably still won't be any good at it and I don't want to spend silly money on something I may have to throw out. Free would be good.
(Not much point installing a full church organ in my house if all I can play is Chopsticks...)

Can anyone recommend something that might help me get started? I have Gimp and Inkscape, but I don't see how I can plug numbers into those.