flightgear/docs-mini/README.canvas

Canvas - A 2D Drawing API
=========================

Author: Thomas Geymayer <admin@tomprogs.at>
Revision: 2012/05/18

Introduction
------------

With the increasing complexity of (glass) cockpits the need for a simple API to
draw on a 2D surface without modifying the C++ core increased heavily in the
last time. The 2D canvas is an effort to satisfy this needs. It is now possible
to create offscreen rendertargets only by using the property tree and placing
them on any 3D object on the aircraft by using certain filter criteria.

Currently it is only possible to place text on the canvas but 2d shapes (using
OpenVG) are going to follow.

Creating a canvas
-----------------

A new canvas can be instantiated by creating a node /canvas/texture[<INDEX>]
with at least the following children:

 <size n="0" type="int">       The width of the underlying texture
 <size n="1" type="int">       The height of the underlying texture

 <view n="0" type="int">    The width of the canvas
 <view n="1" type="int">    The height of the canvas

The dimensions of the canvas are needed to be able to use textures with
different resolutions but use the same units for rendering to the canvas.
Therefore you can choose any texture size with the same canvas size and always
get the same results (apart from resolution dependent artifacts).

* Filtering:

 Optionally you can enable mipmapping and/or multisampling (Coverage Sampling
 Antialiasing):

  <mipmapping type="bool">       Use mipmapping (default: false)
  <coverage-samples type="int">  Coverage Samples (default: 0)
  <color-samples type="int">     Color Samples (default: 0, always
                                 have to be <= coverage-samples)
Drawing
-------

Drawing to the canvas is accomplished by creating nodes as childs of the
canvas root node. Every shape has to be a child of a <group> node. Currently
only drawing Text is possible:

* General:
 The following parameters are used by multiple elements:

 Color:
  A color can be specified by the following subtree (NAME is replaced by
  another name depending on the usage of the color)

  <NAME>
    <red type="float">
    <green type="float">
    <blue type="float">
    <alpha type="float">
  </NAME>

* Text:
 Create a <text> node and configure with the following properties:

 <text type="string">        The text to be displayed
 <font type="string">        The font to be used (Searched in
                              1. aircraft-dir/Fonts
                              2. aircraft-dir
                              3. $FG_DATA/Fonts
                              4. Default osg font paths
 <character-size type="float">          The font size (default: 32)
 <character-aspect-ratio type="float">  Ratio between character height and width
                                        (default: 1)
 <tf>               A 3x3 transformation matrix specified by 6 values
                    (child elements <m n="0">, ..., <m n="5"> which equal to a,
                     ...,f used in the SVG standard) See
                    http://www.w3.org/TR/SVG/coords.html#TransformMatrixDefined
                    for details.
                    You can also use shortcuts and use an alternative to
                    specifying six values:
                     - Translation: <t n="0">, <t n="1"> (both default to 0)
                     - Rotation: <rot>
                     - Scale: <s n="0">, <s n="1"> (s[0] is required, s[1]
                                                    defaults to s[0])
 <alginment type="string">   Text alignment (default: "left-baseline") One of:

                              "left-top"
                              "left-center"
                              "left-bottom"
                               
                              "center-top"
                              "center-center"
                              "center-bottom"
                               
                              "right-top"
                              "right-center"
                              "right-bottom"
                               
                              "left-baseline"
                              "center-baseline"
                              "right-baseline"
                               
                              "left-bottom-baseline"
                              "center-bottom-baseline"
                              "right-bottom-baseline"
 <draw-mode type="int">      A bitwise combination of the following values
                             1 (Text - default)
                             2 (Boundingbox)
                             4 (Filled boundingbox)
                             8 (Alignment -> Draw a cross at the position
                               of the text)
 <padding type="float">      Padding between for the boundingbox (default: 0)
 <color>                     Text color
 <color-fill>                Fill color (for the bounding box)

Placement
---------

To place the canvas into the scene one ore more <placement> elements can be
added to the texture node. By setting at least on of the following nodes
the objects where the canvas texture should be placed on are selected:

 <texture type="string">  Match objects with the given texture assigned
 <node type="string">     Match objects with the given name
 <parent type="string">   Match objects with a parent matching the given name
                          (Not necessarily the direct parent)

Example
-------

<canvas>
  <texture>
    <size n="0" type="int">384</size-x>
    <size n="1" type="int">512</size-y>
    <view n="0" type="int">768</view-width>
    <view n="1" type="int">1024</view-height>
    <mipmapping type="bool">false</mipmapping>
    <coverage-samples type="int">0</coverage-samples>
    <color-samples type="int">0</color-samples>
    <color-background>
      <red type="float">0</red>
      <green type="float">0.02</green>
      <blue type="float">0</blue>
      <alpha type="float">1</alpha>
    </color-background>
    <group>
      <text>
        <text type="string">TEST MESSAGE</text>
        <font type="string">helvetica_bold.txf</font>
        <character-size type="float">40</character-size>
        <tf>
          <!-- Translate (18|50) -->
          <tx>18</tx>
          <ty>50</ty>
        </tf>
      </text>
    </group>
    <placement>
      <!-- Place on objects with the texture EICAS.png
           and a parent called HDD 1 -->
      <texture type="string">EICAS.png</texture>
      <parent type="string">HDD 1</parent>
    </placement>
  </texture>
</canvas>