Canvas SVG parser

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Vector Image Support

The SVG parser basically just maps the xml structure to the property tree. It is implemented in Nasal on top of the XML parsing facilities already provided by FlightGear. Using separate canvas elements instead of a single image will always be slower as every little piece of the canvas hat to be triangulated and afterwards rendered every time the canvas gets updated instead of just copying an image. On the other hand you if you use the canvas you can dynamically update the contents of the image and also get (theoretically) unlimited resolution, even changeable at runtime. For SVG, already existing tools (Inkscape) can be used to create images and then just load them via Nasal and add some dynamic features to them.


Basic example

For loading a SVG file onto a Canvas we first need to create a Canvas instance (See Howto:Add_a_2D_canvas_instrument_to_your_aircraft). Afterwards we can load a SVG by just using the function canvas.parsesvg from the Canvas API, also the API now supports retrieving elements by id which enables the following simple code snippet for changing the text and color in an instrument:

# Create a group for the parsed elements
var eicas = my_canvas.createGroup();

# Parse an SVG file and add the parsed elements to the given group
canvas.parsesvg(eicas, "Aircraft/C-130J/Instruments/EICAS.svg");

# Get a handle to the element called "ACAWS_10" inside the parsed
# SVG file...
var msg = eicas.getElementById("ACAWS_10");

# ... and change it's text and color
msg.setText("THE NEW API IS COOL!");
msg.setColor(1,0,0);

Also the API now supports retrieving elements by id which enables the following simple code snippet for changing the text and color in an instrument: You can lookup any type of element you want and modify them how you want (Add transformations, change colors/texts/coordinates etc.). You can also lookup an parent element and afterwards some of is child elements. By this you can use the same id multiple times but are still able to get access to every element (eg. Engine 1/Dial N1, Engine 2/Dial N1, etc.).


The result will look somehow like in the following image. The screen on the left side has been created by using the code snippet above and the screen on the right side is just a statically rendered version of the EICAS:

Simple EICAS example (Notice our warning message)

Supported SVG features

The SVG file used for this demo has been created using Inkscape. Using paths (also with linestipple/dasharray), text, groups and cloning is supported, but don't try to use more advanced features like gradients, as the SVG parser doesn't interpret every part of the SVG standard. (You can always have a look at the implementation and also improve it if you want ;-) )


NOTE: As of 08/2012, the Canvas system also provides support for raster images, however the SVG parser has not yet been extended to also support raster images via the "image" tag.

Advanced usage

Font settings

By default every text element uses "LiberationFonts/LiberationMono-Bold.ttf" for rendering. If you want to use another font you can pass a function as an additional option to canvas.parsesvg:

# There are two arguments passed to the function. The first contains
# the font-family and the second one the font-weight attribute value
var font_mapper = func(family, weight)
{
  if( family == "Ubuntu Mono" and weight == "bold" )
    # We have to return the name of the font file, which has to be
    # inside either the global Font directory inside fgdata or a
    # Font directory inside the current aircraft directory.
    return "UbuntuMono-B.ttf";

  # If we don't return anything the default font is used
};

# Same as before...
canvas.parsesvg
(
  eicas,
  "Aircraft/C-130J/Instruments/EICAS.svg",
  # ... but additionally with our font mapping function
  {'font-mapper': font_mapper}
);