Nested layers, layer level comp-op and opacity

[talaj]

Enables grouped compositing and improves layer organization.

Layers are rendered recursively, creating it's own buffer if comp-op or opacity properties are set. Otherwise, rendering buffer is inherited from parent layer.

All tests are passing except some SVGs with changes in grouping SVG elements per Layer.

Here is a simple example.

<Map background-color="darkblue" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
    <Parameters>
        <Parameter name="sizes">512,256</Parameter>
        <Parameter name="bbox">-20037508.3427892476320267,1283343.6938826972618699,20037508.3427892439067364,13365151.3630704730749130</Parameter>
    </Parameters>
    <Style name="world">
        <Rule>
            <PolygonSymbolizer fill="white"/>
        </Rule>
    </Style>
    <Style name="text">
        <Rule>
            <TextSymbolizer size="120" dy="-50" halo-radius="8" face-name="DejaVu Sans Bold" fill="white" vertical-alignment="top" placement="point">WORLD</TextSymbolizer>
        </Rule>
    </Style>

    <Layer name="world" comp-op="src-over" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
        <StyleName>world</StyleName>
        <Datasource>
            <Parameter name="file">../../data/shp/world_merc</Parameter>
            <Parameter name="type">shape</Parameter>
        </Datasource>

        <Layer name="text" comp-op="dst-in" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
            <StyleName>text</StyleName>
            <Datasource>
                <Parameter name="type">csv</Parameter>
                <Parameter name="inline">
                    x, y
                    0, 0
                </Parameter>
            </Datasource>
        </Layer>
    </Layer>
</Map>

[talaj]

I have tried to add buffer recycling. Some performance benefit ~0.1 millisecond was measurable on the following style.

Times before and after the change:

 $ test/visual/run --agg -s 1 -d -i 1000 nested-layers-perf
.
Visual rendering: 0 failed / 1 passed / 0 overwritten / 0 errors
agg:    29675 milliseconds
total:  29675 milliseconds

 $ test/visual/run --agg -s 1 -d -i 1000 nested-layers-perf
.
Visual rendering: 0 failed / 1 passed / 0 overwritten / 0 errors
agg:    29585 milliseconds
total:  29585 milliseconds

<Map background-color="white">
    <Parameters>
        <Parameter name="sizes">256, 256</Parameter>
        <Parameter name="bbox">-1, -1, 1, 1</Parameter>
    </Parameters>

    <Datasource name="d">
        <Parameter name="type">csv</Parameter>
        <Parameter name="inline">
            x, y
            0, 0
        </Parameter>
    </Datasource>

    <Style name="s">
        <Rule>
            <MarkersSymbolizer file="../../data/images/marker.png" width="256" height="256" ignore-placement="true" />
        </Rule>
    </Style>

    <Layer comp-op="multiply"><StyleName>s</StyleName><Datasource base="d"></Datasource>
    <Layer comp-op="multiply"><StyleName>s</StyleName><Datasource base="d"></Datasource>
    <Layer comp-op="multiply"><StyleName>s</StyleName><Datasource base="d"></Datasource>
    </Layer></Layer></Layer>

    <Layer comp-op="multiply"><StyleName>s</StyleName><Datasource base="d"></Datasource>
    <Layer comp-op="multiply"><StyleName>s</StyleName><Datasource base="d"></Datasource>
    <Layer comp-op="multiply"><StyleName>s</StyleName><Datasource base="d"></Datasource>
    </Layer></Layer></Layer>
</Map>

[tds4u]

How is the progress? Would be awesome to have it. It's like object oriented layering, short OOL ;-)

[talaj]

@tds4u You can use it in downstream.

[artemp]

Noto Color Emoji + Nested layers

[talaj]

Cool monkeys :-). It's rebased.

I would like to add some rationale behind this. From design perspective I would prefer some component separated from Mapnik as described in this Compositing Ideas. That would also be more flexible for adding new features or optimizations like drawing layers concurrently. On the other side that requires to at least slightly modify all components using Mapnik. Given the simplicity of this implementation in Mapnik, naturalness of nested layers design and backward compatibility, I decided to do it in Mapnik. But have to say I'm still not absolutely certain whether it was a good idea.

[artemp]

@talaj - Lets give it a go in 3.1.x and see if this implementation plays well. I'm going to merge, thanks!

[artemp]

Hmm.. there are some serialisation issues.. WIP

[artemp]

a9aa1c6