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latest news
Text formating
Sep 02, 2011
Now that Google Summer of Code is over I'd like to show you some of theresults:
I already posted information about the
Improving text formating was the second half of my project.
Currently when you want to label a peak like this
Elevation
| <TextSymbolizer name="[name]" face-name="DejaVu Sans Bold" size="12" placement="point" dy="3" wrap-width="35" /> | |
| <TextSymbolizer name="'('+[ele]+')'" face-name="DejaVu Sans Oblique" size="9" placement="point" dy="15" /> |
This usually works, but it has some limitations. For example let's see what happens when the name gets a little longer:
Mapnik breaks the long name and the elevation disappears, because the offset is not correct.
Now let's have a look at how it is done with the new placement system:You only define one TextSymbolizer but with a different syntax:
| <TextSymbolizer face-name="DejaVu Sans Bold" size="12" placement="point" dy="3" wrap-width="35" allow-overlap="0"> | |
| [name]+' '<Format face-name="DejaVu Sans Oblique" size="9">'('+[ele]+')'</Format> | |
| </TextSymbolizer> |
As you can see Mapnik automatically takes care of correct line spacing, no need to calculate offsets manually.
Backwards incompatible changes
To make this work some backwards incompatibilities had to be introduced.
- Line spacing is no longer defined by the largest character in a certain text, but by the line spacing value defined in the font file. This should result in a nice line spacing by default, but if you have defined additional line spacing in your style you have to remove it or at least reduce it to get the same effect as before.
- The text rendered by a TextSymbolizer is no longer given in the
name attribute, but as a text node.<TextSymbolizer name="[abc]" />
becomes<TextSymbolizer>[abc]</TextSymbolizer>
Parameters you can change
You can change the following text attributes with the
- face-name
- size
- character-spacing
- line-spacing
- opacity
- wrap-character
- text-transform
- fill
- halo-fill
- halo-radius
But you are not limited to using a fixed format. You can implement your own class in C++ to do the formating. It's very simple:
| class my_format: public abstract_formating_token | |
| { | |
| public: | |
| void apply(char_properties &properties, Feature const& feature) | |
| { | |
| //Modify properties here | |
| } | |
| }; |
| class my_text : public abstract_text_token | |
| { | |
| public: | |
| UnicodeString to_string(Feature const& feature) | |
| { | |
| return "some text here"; | |
| } | |
| }; |
Next steps
The code is ready, now it's required to test it on larger styles to find any remaining bugs. Python bindings also have to be updated to fully support the new features. When this is done the code will be released as Mapnik 2.1 as it's to late in the release cycle to include it in Mapnik 2.0.