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Merge sibling <g> nodes with identical attributes

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In some cases, gnuplot generates a very suboptimal SVG content of the
following pattern:

        <g color="black" fill="none" stroke="currentColor">
        <path d="m82.5 323.3v-4.1" stroke="#000"/>
        </g>
        <g color="black" fill="none" stroke="currentColor">
        <path d="m116.4 323.3v-4.1" stroke="#000"/>
        </g>
        ... repeated 10+ more times here ...
        <g color="black" fill="none" stroke="currentColor">
        <path d="m65.4 72.8v250.5h420v-250.5h-420z" stroke="#000"/>
        </g>

A more optimal pattern would be:

        <g color="black" fill="none" stroke="#000">
        <path d="m82.5 323.3v-4.1"/>
        <path d="m116.4 323.3v-4.1"/>
        ... 10+ more paths here ...
        <path d="m65.4 72.8v250.5h420v-250.5h-420z"/>
        </g>

This patch enables that optimization by handling the merging of two
sibling <g> entries that have identical attributes.  In the above
example that does not solve the rewrite from "currentColor" to "#000"
for the stroke attribute.  However, the existing code already handles
that automatically after the <g> elements have been merged.

This change provides comparable results to --create-groups as shown by
the following diagram while being a distinct optimization:

       +----------------------------+-------+--------+
       |           Test             | Size  |  in %  |
       +----------------------------+-------+--------+
       | baseline                   | 17961 |  100%  |
       | baseline + --create-groups | 17418 |  97.0% |
       | patched                    | 16939 |  94.3% |
       | patched + --create-groups  | 16855 |  93.8% |
       +----------------------------+-------+--------+

The image used in the size table above was generated based on the
instructions from https://bugs.debian.org/858039#10 with gnuplot 5.2
patchlevel 2.  Beyond the test-based "--create-groups", the following
scour command-line parameters were used:
      --enable-id-stripping --enable-comment-stripping \
      --shorten-ids --indent=none

Note that the baseline was scour'ed repeatedly to stablize the image
size.

Signed-off-by: Niels Thykier <niels@thykier.net>
This commit is contained in:
Niels Thykier 2018-03-20 21:34:20 +00:00
parent 40753af88a
commit cdf5e479a6
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg xmlns="http://www.w3.org/2000/svg">
<desc>Produced by GNUPLOT 5.2 patchlevel 8</desc>
<rect width="900" height="600" fill="none"/>
<g color="black" fill="none">
<path d="m88.5 564h9m777.5 0h-9" stroke="#000"/>
<g transform="translate(80.2,567.9)" fill="#000" font-family="Arial" font-size="12" text-anchor="end">
<text><tspan font-family="Arial">0</tspan></text>
</g>
</g>
<g color="black" fill="none">
<path d="m88.5 473h9m777.5 0h-9" stroke="#000"/>
<g transform="translate(80.2,476.9)" fill="#000" font-family="Arial" font-size="12" text-anchor="end">
<text><tspan font-family="Arial">5000</tspan></text>
</g>
</g>
<g color="black" fill="none">
<path d="m88.5 382h9m777.5 0h-9" stroke="#000"/>
<g transform="translate(80.2,385.9)" fill="#000" font-family="Arial" font-size="12" text-anchor="end">
<text><tspan font-family="Arial">10000</tspan></text>
</g>
</g>
<g color="black" fill="none">
<path d="m88.5 291h9m777.5 0h-9" stroke="#000"/>
<g transform="translate(80.2,294.9)" fill="#000" font-family="Arial" font-size="12" text-anchor="end">
<text><tspan font-family="Arial">15000</tspan></text>
</g>
</g>
</svg>
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