[mkgmap-dev] What's d for? (possibly related to clipping issue?)

Sun Feb 22 21:23:13 GMT 2009

>> Why does the line clipper add a tiddly amount (d) in these expressions? 
>>
>>
>> 		double d = 0.00001;
>> 		if (t[0] > 0)
>> 			ends[0] = new Coord((int) (y0 + t[0] * dy + d), (int) (x0 + t[0] * dx + d));
>>
>> 		if (t[1] < 1)
>> 			ends[1] = new Coord((int)(y0 + t[1] * dy + d), (int) (x0 + t[1] * dx + d));
>>     
>
> It for rounding to the nearest map unit.  I remember I was just adjusting
> it until I got something that worked, but can't quite remember what the
> problem I was solving was.
>
> If I'd thought about it more I would have said that the correct value would be:
>
>   360/(2 * 2^24)
>
> which comes to 0.000010728, so perhaps close enough.
>
>   
So I have replaced d=0.00001 it with the exact expression 360.0D / (1 << 
24) / 2;
>> On a similar note, why have the non-zero DELTA here?
>>
>> 	public static int toMapUnit(double l) {
>> 		double DELTA = 0.000001; // TODO check if we really mean this
>>     
>
> That is a much smaller delta and is just protecting against floating point
> numbers that are intended to be an exact value but are just a bit less.
> It is not trying to round to the closest map unit one in this case.
>
> It may not be needed but it is the rouding of the areas in the TDB file
> that was affected if I remember correctly, so if you want to experiment
> without it, or think it is causing trouble elsewhere we need to check the
> TDB areas.
>
>   
I dont fully understand you. Floating points are in this case never 
intended to be 'exact' values. They are lat/lon values.

I have just discovered, that the missing rounding in this place leads to 
a small loss in precision. This means that the LineClipping class 
includes nods and creates end nodes slightly outside the bounding box.
As far as I can see it, this should not cause any problems, but it was a 
little irritating to found nodes included, which are 2m outside the 
bounding box.

So I've replaced d=0.0000001  too with the same expression 360.0D / (1 
<< 24) / 2;
Now clipping works as expected.