I'm trying to implement a mathematical formula in a program I'm making, but while the programming is no problem I'm having trouble with some of the math. I need to calculate $\sin(\alpha(x,y))$ with $\alpha(x,y)$ the local tilt angle in $(x,y)$.

I have $2$-dimentional square grid, with at each point the height, representing a $3$-dimentional terrain. To find the tilt in a point I can use the height of its direct neighbors. So $h(x+1,y)$ can be used, however $h(x+2,y)$ not. I also know the distance between two neighboring points ($dx$). By tilt I mean the angle between the normal at a point on the terrain and a vector pointing straight up.

This seems like a not too hard problem, but I can't seem to figure out how to do it. Anyone got a good way to do this?


  • $\begingroup$ Not sure what you mean by "local tilt angle" ... are we talking about a two-dimensional surface or a one-dimensional curve? $\endgroup$ – Zubin Mukerjee Nov 29 '14 at 19:38
  • $\begingroup$ I have a two-dimentional height field, representing a three-dimentional terrain. By "local tilt angle" I mean the angle between a vector pointing straight up and the normal of the terrain in a point (x,y). $\endgroup$ – The Oddler Nov 29 '14 at 19:42
  • $\begingroup$ Okay. Makes sense now, thanks :) $\endgroup$ – Zubin Mukerjee Nov 29 '14 at 19:42

A helpful construct here would be the normal vector to our terrain.

Our terrain is modeled by the equation $$ z = h(x,y) $$ Or equivalently, $$ z - h(x,y) = 0 $$ We can define $g(x,y,z) = z - h(x,y)$. It turns out that the vector normal to this level set is given by $$ \operatorname{grad}(g) = \newcommand{\pwrt}[2]{\frac{\partial #1}{\partial #2}} \left\langle \pwrt{g}{x},\pwrt gy, \pwrt gz \right \rangle = \left\langle -\pwrt{h}{x},-\pwrt hy, 1 \right \rangle := v(x,y) $$ We can calculate the angle between this normal and the vertical $\hat u = \langle 0,0,1 \rangle$ using the formula $$ \cos \theta = \frac{u \cdot v}{\|u\| \|v\|} $$ in particular, we find that $$ \cos \theta = \frac{\hat u \cdot v}{\|\hat u\| \|v\|} = \frac{1}{\sqrt{1 + \left( \pwrt hx \right)^2 + \left( \pwrt hy \right)^2}} $$ We may approximate $$ \pwrt hx(x,y) \approx \frac{h(x+dx,y) - h(x-dx,y)}{2(dx)}\\ \pwrt hy(x,y) \approx \frac{h(x,y+dy) - h(x,y-dy)}{2(dy)} $$

Note: since you have to calculate $\sin \theta$, you find $$ \sin \theta = \sqrt{1 - \cos^2 \theta} = \frac{\sqrt{\left( \pwrt hx \right)^2 + \left( \pwrt hy \right)^2}}{\sqrt{1 + \left( \pwrt hx \right)^2 + \left( \pwrt hy \right)^2}} $$

  • $\begingroup$ I love how you call it 'our terrain', like we're really working on it together :D Which, even though we're probably thousands of miles appart, is actually true. Thanks for this very clear explanation! $\endgroup$ – The Oddler Nov 29 '14 at 20:12
  • $\begingroup$ Whoops, I mean... ahem... I guess you can keep it.... Anyway, in response to your other comment, we have $$ \sin(\arccos(x)) = \sqrt{1 - x^2} $$ (and you're welcome) $\endgroup$ – Omnomnomnom Nov 29 '14 at 20:16
  • $\begingroup$ Slight error before, see my edit. $\endgroup$ – Omnomnomnom Nov 29 '14 at 20:18
  • $\begingroup$ Thanks a lot mate! That is indeed a much better way to calculate $\sin(\arccos(x))$. $\endgroup$ – The Oddler Nov 29 '14 at 20:23

Option 1: estimate the partial derivatives using the finite difference scheme $\frac{h(x+1,y)-h(x-1,y)}{2\Delta x}$, $\frac{h(x,y+1)-h(x,y-1)}{2\Delta x}$ and use the normalvector $(h_x',h_y',-1)\to\cos\theta=1/\sqrt{h_x^2+h_y^2+1}$.

Option 2: fit a least squares plane $z=ax+by+c$ in the 3x3 neighborhood and use $(a,b,-1)\to\cos\theta=1/\sqrt{a^2+b^2+1}$.

  • $\begingroup$ Option 1 seems perfect. However, since the code is done literally millions of times per seconds I would like not to use $\sin(\arccos(...))$. Is there a better way to get $\sin\theta$ from this? $\endgroup$ – The Oddler Nov 29 '14 at 20:10
  • $\begingroup$ $\sin^2\theta=1-\cos^2\theta=(a^2+b^2)/(a^2+b^2+1)$. Are you sure it is the sine that you need ? $\endgroup$ – Yves Daoust Nov 30 '14 at 9:11

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