**diff options**

author | Peter Hutterer <peter.hutterer@who-t.net> | 2011-04-20 11:49:57 +1000 |
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committer | Peter Hutterer <peter.hutterer@who-t.net> | 2011-04-21 13:48:41 +1000 |

commit | df90db892d16143742eb3c09e048f5b76457a62e (patch) | |

tree | 50626f57c749f3d6c4918127e102fc2daf818668 | |

parent | 7513190ec7bc08db1629b04d3f88f86ea2b1588e (diff) |

dix: document DoGetDirection's maths

This is the best explanation I can come up with, but it seems to hold true
for my example values.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Jamey Sharp <jamey@minilop.net>
Reviewed-by: Simon Thum <simon.thum@gmx.de>

-rw-r--r-- | dix/ptrveloc.c | 12 |

1 files changed, 10 insertions, 2 deletions

diff --git a/dix/ptrveloc.c b/dix/ptrveloc.c index 3bf871658..ab2bca766 100644 --- a/dix/ptrveloc.c +++ b/dix/ptrveloc.c @@ -483,8 +483,16 @@ DoGetDirection(int dx, int dy){ #else r = atan2(dy, dx); #endif - /* find direction. We avoid r to become negative, - * since C has no well-defined modulo for such cases. */ + /* find direction. + * + * Add 360° to avoid r become negative since C has no well-defined + * modulo for such cases. Then divide by 45° to get the octant number, + * e.g. 0 <= r <= 1 is [0-45]° + * 1 <= r <= 2 is [45-90]° + * etc. + * But we add extra 90° to match up with our N, S, etc. defines up + * there, rest stays the same. + */ r = (r+(M_PI*2.5))/(M_PI/4); /* this intends to flag 2 directions (45 degrees each), * except on very well-aligned mickeys. */ |