float precision problem


    Dec 18 2011 | 10:59 am
    I want to calculate distance with gps. I moved code from processing (java) to max patches. Problem is that in processing i have those values for example: dLat: -1.3315805E-7 dLon: 1.7476995E-7
    which is like 0.00000017476995 if i'm correct.
    In max the value is zero, and so after a few calculations i get nan (not a number). What can i do?
    :(
    In case someone know's processing, here is the proceesing code.
    /* //pc testP: gps 5.910325 30. 51.988632 57. 28.372467
    //keuken testP: gps 5.91006 10. 51.988777 19. 29.984245 */
    void setup() { // println(getDistance(51.988632, 5.910325, 51.988777, 5.91006)); // pc > keuken // println(getDistance(51.988777, 5.910006, 51.988739, 5.910056)); /* String[] lines = loadStrings("geo.tsv");
    String[] tokens = split(lines[0], "t"); float lon1 = float(tokens[2]); float lat1 = float(tokens[4]);
    for (int i = 1; i < 2; i++) { //lines.length-1
    tokens = split(lines[i+1], "t"); float lon2 = float(tokens[2]); float lat2 = float(tokens[4]);
    //println(lat2+"t"+lon2); float d = getDistance(lon1, lat1, lon2, lat2); println(d*1000); } */ float d = getDistance(5.910161, 51.988705, 5.910171, 51.988697); println(d*1000);
    }
    float getDistance(float lon1, float lat1, float lon2, float lat2) { int R = 6371; // Radius of the earth in km float dLat = radians(lat2-lat1); float dLon = radians(lon2-lon1);
    println("dLat: "+dLat+"ndLon: "+dLon);
    float a = sin(dLat/2) * sin(dLat/2) + cos(radians(lat1)) * cos(radians(lat2)) * sin(dLon/2) * sin(dLon/2); float c = 2 * atan2(sqrt(a), sqrt(1-a)); float d = R * c; // Distance in km
    return d; }

    • Dec 18 2011 | 11:35 am
      Hi I can't find the corresponding expression in your Processing code (maybe the last 5 lines?); but inside your getDistance subpatch, you are trying to find the sqrt of a negative number (on the right, above [atan2]); this generates a complex number, unrepresentable in Max. That's where the "nan" is generated. Sorry if you already knew this.
      Brendan
    • Dec 18 2011 | 11:50 am
      .....to get it to "work", I inverted the result AFTER the sqrt calculation; probably messes up the result you're expecting but here it is anyway:
      Brendan
    • Dec 18 2011 | 5:23 pm
      Max works with single-precision floating point values, which gives you approximately 6-digit precision in the mantissa and can handle exponents (base 10) of about +/-38.
      This is all considerably less powerful than the double-precision floats used in Java, but should not be a limitation with the values you say you are using.
      Square-roots of negative numbers will NaN no matter what representation you have. From a quick look, it seems that what you have implemented in your max patch is float c = 2 * atan2(sqrt(a), sqrt(0-a)); rather than float c = 2 * atan2(sqrt(a), sqrt(1-a));.
      The problem is further exacerbated by your using send/receive pairs and, apparently, relying on left-to-right order. When you s/r, all bets on execution order are off.
      I've also loadbanged the radius of the earth into your patch.
      The following is only the [p GetDistance] part of your patch, you'll have to manually copy/paste it into your top-level patch.
    • Dec 20 2011 | 12:32 pm
      Thanks peter,
      i allready had it fixed with javascript which is much easier to read for such things (didn't know before that javascript was possible).
      But good to know it is possible.