[mkgmap-dev] [Patch v1] improve LinePreparer to reduce img size

Sun Jul 24 20:54:01 BST 2016

Hi Gerd,

I agree. The speed improvement, if existent, should be very small.  But 
if you multiply that by the thousands in each map, and multiply by the 
number of time the map is loaded, and multiply by the number of map 
users, the gain should be worth it.  Besides that, I always strive for 
the better solution... ;)

But I see that you already pushed this fix, so kudos to you (again!).

Best regards,

     Alexandre

On 20/07/2016 13:03, Gerd Petermann wrote:
>
> Hi Alexandre,
>
>
> in fact the img size is only reduced when the bit stream length in 
> bytes in shorter.
>
> The new algo will prefer the trick, so if you are right and the trick 
> costs runtime
>
> in the device I should change that. The effect is probably very small 
> though.
>
>
> Gerd
>
> ------------------------------------------------------------------------
> *Von:* mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag 
> von Alexandre de Menezes <afmlistas at terra.com.br>
> *Gesendet:* Mittwoch, 20. Juli 2016 17:55:30
> *An:* Development list for mkgmap
> *Betreff:* Re: [mkgmap-dev] [Patch v1] improve LinePreparer to reduce 
> img size
>
> Hi Gerd,
>
>
> I am not familiar with the format, but from your explanation I am 
> guessing this will be at some point stored in full bytes (or words or 
> dwords).  Assuming bytes, there will be indeed a size gain going from 
> 30bits to 24bits as in your example, but not in going from 32 to 26, 
> for example.  In this case, wouldn't the original encoding be simpler 
> (faster) to decode?
>
>
> Does this make sense?  Are  you taking this into account when choosing 
> the best encoding?
>
>
> Best regards,
>
>
>      Alexandre
>
>
> On 18/07/2016 04:22, Gerd Petermann wrote:
>>
>>
>> Let's try with a real world example:
>>
>> https://www.openstreetmap.org/way/245112355
>>
>>
>> <https://www.openstreetmap.org/way/245112355>
>>
>> This highway has 6 points, it starts in the north, the x-delta values 
>> (in resolution 24) are
>>
>> 4, -19, -5, 0,  3
>>
>> The largest absolute value is 19 which requires 5 bits: 10011.
>>
>> Since both positive and negative values exist, we have to encode the 
>> sign for each single delta value, so we have to use 6 bits (the 
>> leftmost gives the sign)
>>
>> So, the first iteration gives:
>>
>> 6 bit words : 4 = 000100  , -19 = 110011, -5 = 111011, 0 = 000000, 3 
>> = 000011   --> 30 bits
>>
>> Now, the optimization process starts trying to use the "trick" with 5 
>> bits. WIth the trick,
>>
>> the value 19 doesn't fit into 5 bits, so the trick is used (it stands 
>> for the binary value 1111) and the result for -19 is 10000 11100  (15 
>> + 4  made negative)
>>
>> All other values fit into 5 bits, so the 2nd iteration gives:
>>
>> 5bit words: 4 = 00100  , -19 = 10000 11100, -5 = 11011, 0 = 00000, 3 
>> = 00011 --> 30 bits
>>
>> The next step is to try with only 4 bits. Again, only the value -19 
>> needs the trick, this time we get - 19 = 1000 1000 1011 (7 +7 + 5 
>> made negative)
>>
>> 4 bit words : 4 = 0100  , -19 = 1000 1000 1011, -5 = 1011, 0 = 0000, 
>> 3 = 0011 --> 24 bits
>>
>> The next step is to try with only 3 bits. This time, more values need 
>> the trick (4 , -5 and -19). For -19 we get 100 100 100 100 100 100 
>> 111 (3 + 3 + 3 + 3 + 3+ 3 + 1 made negative)
>> The result:
>>
>> 3 bit words : 4 = 100 001 , -19 = 100 100 100 100 100 100 111, -5 = 
>> 100 110, 0 = 000, 3 = 011   --> 39 bits
>>
>>
>> So, we see that the trick is used too often now and the resulting bit 
>> stream is much longer than the one before.
>>
>>
>> So, using the trick with 4 bit words saves 6 bits for the x values.
>>
>>
>> ciao,
>>
>> Gerd
>>
>> P.S. To be complete:
>> The y-deltas are all negative:
>> -3, -7,- 3,- 5, -4
>> This allows to encode only the absolute values which all fit into 3 
>> bits, the trick doesn't help here.
>>
>> ------------------------------------------------------------------------
>> *Von:* mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag 
>> von nwillink <osm at pinns.co.uk>
>> *Gesendet:* Sonntag, 17. Juli 2016 16:59:19
>> *An:* mkgmap-dev at lists.mkgmap.org.uk
>> *Betreff:* Re: [mkgmap-dev] [Patch v1] improve LinePreparer to reduce 
>> img size
>> Many thanks for that Gerd
>>
>> I hope this makes it a bit clearer?
>>
>> The adjective 'crystal' does not immediately spring to mind but I 
>> understand
>> what
>> you are getting at.
>>
>> Presumably the patch now checks every line for the need to apply the 
>> trick.
>>
>>
>>
>>
>> --
>> View this message in context: 
>> http://gis.19327.n5.nabble.com/Patch-v1-improve-LinePreparer-to-reduce-img-size-tp5878545p5878569.html
>> Sent from the Mkgmap Development mailing list archive at Nabble.com.
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>
>
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