proj-imim:
Re: Number of Control Points, was Re:
John Blommers 2001-Jan-06 07:03:33
Let me share the results of a little experiment I just did to try to
understand all this business about how many control points are
needed. I have 4 fish-eye images and this PTPicker script:
p f1 w2400 h1200 v360 u30 n"QTVR"
i f2 w1152 h1152 y0 p0 r0 v183 n"A01.jpg" a0 b0.1 c0 X0 Y0 Z0
i f2 w1152 h1152 y90 p0 r0 v=0 n"A02.jpg" a=0 b=0 c=0 X1 Y0 Z0
i f2 w1152 h1152 y180 p0 r0 v=0 n"A03.jpg" a=0 b=0 c=0 X2 Y0 Z0
i f2 w1152 h1152 y270 p0 r0 v=0 n"A04.jpg" a=0 b=0 c=0 X3 Y0 Z0
v v0 a0 b0 c0
v y1 p1 r1
v y2 p2 r2
v y3 p3 r3
There are 4 images to stitch and 13 variables to optimize. I started
out by picking lots of extra points, then deleted more and more until
I had only two points for each image seam. The results are most
interesting:
Number of control points Average Error after ~2000 iterations
24 3.8
16 4.1
12 4.4
08 points 2.0
The last experiment gave the best stitched result! The optimized v
value for the fish-eye lens came out to 183.2 which is what the
client said it should be. But in the first trial the optimiser came
up with 184.3, slightly higher. More importantly, the last trial's
values for the lens correction parameters (a b c) were a lot smaller
than the first trial. My experience is that the smaller the lens
correction the less distortion in the final stitch. Perhaps "less is
more" works here too.
Let me also point out that each control point represents 4
independent variables since the point is present in 2 images and each
point has an independent (x,y) coordinate. This little fact is
apparent when you look at the definition of a control point:
c n0 N1 x768 y262 X363 Y258
The lesson for me is that maybe I can get by with a lot less control
points than I thought. Perhaps other list readers will share their
experiences in this matter,
- John