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<p>Hi Gerd,</p>
<p><br>
</p>
<p>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?<br>
</p>
<p><br>
</p>
<p>Does this make sense? Are you taking this into account when
choosing the best encoding?</p>
<p><br>
</p>
<p>Best regards,</p>
<p><br>
</p>
<p> Alexandre<br>
</p>
<br>
<div class="moz-cite-prefix">On 18/07/2016 04:22, Gerd Petermann
wrote:<br>
</div>
<blockquote
cite="mid:HE1PR08MB0764905B900B8C1464D2AFFB9E360@HE1PR08MB0764.eurprd08.prod.outlook.com"
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<p>Let's try with a real world example:</p>
<p><a moz-do-not-send="true" id="LPlnk607062"
href="https://www.openstreetmap.org/way/245112355"
class="x_OWAAutoLink">https://www.openstreetmap.org/way/245112355</a></p>
<p><a moz-do-not-send="true" id="LPlnk607062"
href="https://www.openstreetmap.org/way/245112355"
class="x_OWAAutoLink"><br>
</a></p>
<p>This highway has 6 points, it starts in the north, the
x-delta values (in resolution 24) are
<br>
</p>
<p>4, -19, -5, 0, 3</p>
<p>The largest absolute value is 19 which requires 5 bits:
10011.</p>
<p>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)</p>
<p>So, the first iteration gives:</p>
<p>6 bit words : 4 = 000100 , -19 = 110011, -5 = 111011, 0
= 000000, 3 = 000011 --> 30 bits</p>
<p>Now, the optimization process starts trying to use the
"trick" with 5 bits. WIth the trick,</p>
<p>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)</p>
<p>All other values fit into 5 bits, so the 2nd iteration
gives:<br>
</p>
<p><span>5<span> bit words</span>: 4 = 00100 , -19 = <span>10000
11100</span>, -5 = 11011, 0 = 00000, 3 = 00011
--> 30 bits</span></p>
<p><span></span>The next step is to try with only 4 bits.
Again, only the value -19 needs the trick, this time we
get - 19 = 1000
<span>1000 </span>1011 (7 +7 + 5 made negative)<br>
</p>
<p><span><span>4 bit words</span> : 4 = 0100 , -19 = <span>1000
1000 1011</span>, -5 = 1011, 0 = 0000, 3 = 0011
--> 24 bits</span></p>
<p><span></span></p>
<div>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
<span><span><span>100 100 100 100 100 100 111</span></span></span>
(3 + 3 + 3 + 3 + 3+ 3 + 1 made negative)<br>
The result:<br>
<p><span><span>3 bit words</span> : 4 = 100 001 , -19 = <span>100
100 100 100 100 100 111</span>, -5 = 100 110, 0 =
000, 3 = 011 --> 39 bits</span></p>
<p><span><br>
</span></p>
<p><span>So, we see that the trick is used too often now
and the resulting bit stream is much longer than the
one before.
<br>
</span></p>
<p><span><br>
</span></p>
<p><span>So, using the trick with 4 bit words saves 6 bits
for the x values. <br>
</span></p>
<p><span><br>
</span></p>
<p><span>ciao,<br>
</span></p>
<p><span>Gerd<br>
</span></p>
</div>
P.S. To be complete:<br>
The y-deltas are all negative:<br>
-3, -7,- 3,- 5, -4<br>
This allows to encode only the absolute values which all fit
into 3 bits, the trick doesn't help here.<br>
<span></span><br>
</div>
<hr tabindex="-1" style="display:inline-block; width:98%">
<div id="x_divRplyFwdMsg" dir="ltr"><font
style="font-size:11pt" face="Calibri, sans-serif"
color="#000000"><b>Von:</b> mkgmap-dev
<a class="moz-txt-link-rfc2396E" href="mailto:mkgmap-dev-bounces@lists.mkgmap.org.uk"><mkgmap-dev-bounces@lists.mkgmap.org.uk></a> im Auftrag
von nwillink <a class="moz-txt-link-rfc2396E" href="mailto:osm@pinns.co.uk"><osm@pinns.co.uk></a><br>
<b>Gesendet:</b> Sonntag, 17. Juli 2016 16:59:19<br>
<b>An:</b> <a class="moz-txt-link-abbreviated" href="mailto:mkgmap-dev@lists.mkgmap.org.uk">mkgmap-dev@lists.mkgmap.org.uk</a><br>
<b>Betreff:</b> Re: [mkgmap-dev] [Patch v1] improve
LinePreparer to reduce img size</font>
<div> </div>
</div>
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<div class="PlainText">Many thanks for that Gerd<br>
<br>
I hope this makes it a bit clearer? <br>
<br>
The adjective 'crystal' does not immediately spring to
mind but I understand<br>
what<br>
you are getting at.<br>
<br>
Presumably the patch now checks every line for the need to
apply the trick.<br>
<br>
<br>
<br>
<br>
--<br>
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