Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

I have co-ordinates (x,y,z) for three markers placed on the leg, and I need to calculate the position of a 'fourth' marker which appears in the first frame of my recording and then disappears. Together the four markers make up a rectangle shape, and the markers are fixed (one marker is on the inside of the ankle, a second is on the outside of the ankle, the third is on the outside of the knee, and the one I need to find is on the inside of the knee). I wonder if there is a way to calculate the co-ordinates of the 'fourth' point based on the co-ordinates of the others. I think this should be relativley simple but I don't know where to start!

Thanks!

share|improve this question
add comment

2 Answers 2

inside of knee equals inside of ankle plus outside of knee minus outside of ankle.

share|improve this answer
    
+1 what a nice equation! –  draks ... Feb 28 '12 at 12:14
add comment

Based on Gerry Myerson's answer:

$$x_{ik} = x_{ia} + x_{ok} - x_{oa}$$ $$y_{ik} = y_{ia} + y_{ok} - y_{oa}$$ $$z_{ik} = z_{ia} + z_{ok} - z_{oa}$$

This works even if the rectangle becomes a parallelogram.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.