Advantages of YSLD??

There are many reasons to use the YSLD styling markup language over the SLD markup language:

Easier to read??

Compare the following style directives, both of which specify the fill color of a feature to be red.

SLD:

<CssParameter name="fill">#ff0000</CssParameter>

YSLD:

fill-color: '#FF0000'

In SLD, the XML-based nature of the content obscures the important aspects of the directive in the middle of the line. With YSLD, the attribute and the value are clearly marked and associated with no extraneous information, making comprehension easier.

More compact??

Individual style files of a sufficient complexity can easily grow to dozens of rules. (For example, a file that has ten different attribute-based rules at ten different scales will have one hundred rules.) Therefore the length of each individual rule can drastically affect the length of the entire style.

Compare the follow style rules, which both specify the a point layer to be styled as a red circle with 8-pixel diameter and a 2-pixel black stroke:

SLD:

<Rule>
  <PointSymbolizer>
    <Graphic>
      <Mark>
        <WellKnownName>circle</WellKnownName>
        <Fill>
          <CssParameter name="fill">#ff0000</CssParameter>
        </Fill>
        <Stroke>
          <CssParameter name="stroke">#000000</CssParameter>
          <CssParameter name="stroke-width">2</CssParameter>
        </Stroke>
      </Mark>
      <Size>8</Size>
    </Graphic>
  </PointSymbolizer>
</Rule>

YSLD:

rules:
- symbolizers:
  - point:
      size: 8
      symbols:
      - mark:
          shape: circle
          fill-color: '#FF0000'
          stroke-color: '#000000'
          stroke-width: 2

While the SLD comes in at 300 characters, the YSLD equivalent comes in at about half that. Also, by not using an XML-based markup language, the removal of open and close tags make the document to look much simpler and be much more compact.

In addition, SLD is a formally structured document designed to configure a set of WMS layers. This produces a document with additional XML that is not required when styling a single layer in isolation.

The following are the headers in a typical SLD:

<?xml version="1.0" encoding="ISO-8859-1"?>
<StyledLayerDescriptor version="1.0.0"
    xsi:schemaLocation="http://www.opengis.net/sld StyledLayerDescriptor.xsd"
    xmlns="http://www.opengis.net/sld"
    xmlns:ogc="http://www.opengis.net/ogc"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <NamedLayer>
    <Name>Default Point</Name>
    <UserStyle>
      <Title>A boring default style</Title>
      <Abstract>A sample style that just prints out a purple square</Abstract>
...

YSLD does not require a representation of StyledLayerDescriptor, NamedLayer and UserStyle, so all of the above are unnecessary when compared with YSLD.

More flexible syntax??

SLD, being an XML-based markup language, has a schema to which any style file needs to adhere. This means that not only are certain tags required, but the order of those tags are significant. This can cause confusion when the correct directives happen to be in the wrong order.

For example, take the following fill and stroke directives for a symbolizer. In SLD, this is valid:

<Fill>
  <CssParameter name="fill">#ff0000</CssParameter>
</Fill>
<Stroke>
  <CssParameter name="stroke">#000000</CssParameter>
</Stroke>

while this is invalid:

<Stroke>
  <CssParameter name="stroke">#000000</CssParameter>
</Stroke>
<Fill>
  <CssParameter name="fill">#ff0000</CssParameter>
</Fill>

YSLD, by contrast, does not require any of the directives to be ordered, so long as they are contained in the proper block.

For example, the following are both equally valid:

fill-color: '#FF0000'
stroke-color: '#000000'

and:

stroke-color: '#000000'
fill-color: '#FF0000'

Contains variables for reusable code??

In SLD, if you have content that needs to be reused from rule to rule, you must manually generate the directives for each rule over and over. YSLD eliminates the need for redundant directives by introducing the ability to create variables that can take the place of the same content.

For example, in SLD, multiple rules must share much of the same content:

<Rule>
  <MinScaleDenominator>35000</MinScaleDenominator>
  <PointSymbolizer>
    <Graphic>
      <Mark>
        <WellKnownName>circle</WellKnownName>
        <Fill>
          <CssParameter name="fill">#ff0000</CssParameter>
        </Fill>
        <Stroke>
          <CssParameter name="stroke">#000000</CssParameter>
          <CssParameter name="stroke-width">2</CssParameter>
        </Stroke>
      </Mark>
      <Size>6</Size>
    </Graphic>
  </PointSymbolizer>
</Rule>
<Rule>
  <MaxScaleDenominator>35000</MaxScaleDenominator>
  <PointSymbolizer>
    <Graphic>
      <Mark>
        <WellKnownName>circle</WellKnownName>
        <Fill>
          <CssParameter name="fill">#ff0000</CssParameter>
        </Fill>
        <Stroke>
          <CssParameter name="stroke">#000000</CssParameter>
          <CssParameter name="stroke-width">3</CssParameter>
        </Stroke>
      </Mark>
      <Size>8</Size>
    </Graphic>
  </PointSymbolizer>
</Rule>

In YSLD, all the directives that occur multiple times can be replaced with a variable:

define: &variable
  shape: circle
  fill-color: '#FF0000'
  stroke-color: '#000000'

rules:
- name: rule1
  scale: [35000,max]
  symbolizers:
  - point:
      size: 6
      symbols:
      - mark:
          <<: *variable
          stroke-width: 2
- name: rule2
  scale: [min,35000]
  symbolizers:
  - point:
      size: 8
      symbols:
      - mark:
          <<: *variable
          stroke-width: 3

Note the definition of variable at the top, and the variable substitution in the line <<: *variable.

Direct match with SLD??

In addition to all of these advantages, YSLD directly aligns with SLD concepts. This allows existing SLD files to be converted into YSLD representation.

Note

While YSLD and SLD share the core concepts, several YSLD features are modified during use.

  • Comments are removed
  • Zoom parameters are converted to scale parameters
  • Variables are evaluated