I'm trying to simulate a centipede game, and I'm doing the snake movement. I have seen that piece of code, but can't get it, I know it gets the different between the current, previou's nodes positions, but what is the point of multiplying by the normal vector ?

for ( int i = 1 ; i < nodes.size(); i++ )

    Vec2f diff =  nodes[i-1].m_Pos-nodes[i].m_Pos;

    float length = diff.length();
    Vec2f norm = diff.normalized();
    nodes[i].m_Pos+=(length - 5)*norm;

| improve this question | | | | |
  • \$\begingroup\$ The normal just gets the direction of the difference. Then it just moves each piece by (length-5) in that direction. mathworld.wolfram.com/UnitVector.html \$\endgroup\$ – mobo Aug 25 '13 at 15:50
  • \$\begingroup\$ is there a better way for doing that ? moving the snake body according to the head? the code actually is like a rope Why the normal should be multiplied? it should be added to the position? not to scale the different by the normal ? \$\endgroup\$ – Mahmoud Aug 25 '13 at 15:52
  • \$\begingroup\$ nodes[i].m_Pos+=(length - 5)*norm; The normal is just the direction the next piece needs to move. The += means at that much to the position. So if head (piece 0) is at [20, 10], and next is at [10,10] (snake going right) then: diff is [10,0]; length is 10, and norm is [1,0]. It moves the piece (10-5)*[1,0] = [5,0] so, 5 pixels to the right. \$\endgroup\$ – mobo Aug 25 '13 at 16:01
  • \$\begingroup\$ you can put it as an answer :) \$\endgroup\$ – Mahmoud Aug 25 '13 at 16:05
  • \$\begingroup\$ I tried to let the first node move and at the screen edge, I give it a negative velocity downwards with x = 0. The problem that the following nodes doesn't move accordingly, I don't know why ? \$\endgroup\$ – Mahmoud Aug 25 '13 at 18:03

In mathematics, a vector is said to be normal to a surface if it is perpendicular. In other words, normal vectors always point directly away from something.

Multiplying a scalar by a normal vector will get you a vector that represents displacement in the direction of the normal. Often normals have a handy property that they are unit-length, so if you want to move 5 units in the normal direction, nothing more than a multiplication is necessary. If it is not the case that the normal is unit-length, then an expensive normalization will be required.

Now, I am concerned by the naming in this code snippet. Norm actually has a different meaning from normal. In many contexts, the norm is actually the length, size or extent. A vector's norm, for instance, is a scalar that represents its length.

The fact that directly above the variable named norm is another variable named length and norm is declared as a vector rather than a scalar give enough context clues to indicate that norm is probably short-hand for normal, but this is poor naming.

| improve this answer | | | | |
  • \$\begingroup\$ Thanks, I have a problem right now. when the first node bounces and changes its velocity, the following nodes don't move at the way the first node moved.. the sample is here gamedev.net/… \$\endgroup\$ – Mahmoud Aug 26 '13 at 6:48
  • \$\begingroup\$ I would submit another question for this, there is not a lot I can do with an executable and no source code / problem description. \$\endgroup\$ – Andon M. Coleman Aug 26 '13 at 19:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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