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170 lines
6.4 KiB
170 lines
6.4 KiB
Canvas - A 2D Drawing API
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=========================
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Author: Thomas Geymayer <admin@tomprogs.at>
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Revision: 2012/05/18
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Introduction
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------------
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With the increasing complexity of (glass) cockpits the need for a simple API to
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draw on a 2D surface without modifying the C++ core increased heavily in the
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last time. The 2D canvas is an effort to satisfy this needs. It is now possible
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to create offscreen rendertargets only by using the property tree and placing
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them on any 3D object on the aircraft by using certain filter criteria.
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Currently it is only possible to place text on the canvas but 2d shapes (using
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OpenVG) are going to follow.
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Creating a canvas
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-----------------
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A new canvas can be instantiated by creating a node /canvas/texture[<INDEX>]
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with at least the following children:
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<size n="0" type="int"> The width of the underlying texture
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<size n="1" type="int"> The height of the underlying texture
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<view n="0" type="int"> The width of the canvas
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<view n="1" type="int"> The height of the canvas
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The dimensions of the canvas are needed to be able to use textures with
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different resolutions but use the same units for rendering to the canvas.
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Therefore you can choose any texture size with the same canvas size and always
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get the same results (apart from resolution dependent artifacts).
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* Filtering:
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Optionally you can enable mipmapping and/or multisampling (Coverage Sampling
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Antialiasing):
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<mipmapping type="bool"> Use mipmapping (default: false)
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<coverage-samples type="int"> Coverage Samples (default: 0)
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<color-samples type="int"> Color Samples (default: 0, always
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have to be <= coverage-samples)
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Drawing
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-------
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Drawing to the canvas is accomplished by creating nodes as childs of the
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canvas root node. Every shape has to be a child of a <group> node. Currently
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only drawing Text is possible:
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* General:
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The following parameters are used by multiple elements:
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Color:
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A color can be specified by the following subtree (NAME is replaced by
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another name depending on the usage of the color)
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<NAME>
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<red type="float">
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<green type="float">
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<blue type="float">
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<alpha type="float">
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</NAME>
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* Text:
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Create a <text> node and configure with the following properties:
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<text type="string"> The text to be displayed
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<font type="string"> The font to be used (Searched in
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1. aircraft-dir/Fonts
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2. aircraft-dir
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3. $FG_DATA/Fonts
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4. Default osg font paths
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<character-size type="float"> The font size (default: 32)
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<character-aspect-ratio type="float"> Ratio between character height and width
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(default: 1)
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<tf> A 3x3 transformation matrix specified by 6 values
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(child elements <m n="0">, ..., <m n="5"> which equal to a,
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...,f used in the SVG standard) See
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http://www.w3.org/TR/SVG/coords.html#TransformMatrixDefined
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for details.
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You can also use shortcuts and use an alternative to
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specifying six values:
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- Translation: <t n="0">, <t n="1"> (both default to 0)
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- Rotation: <rot>
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- Scale: <s n="0">, <s n="1"> (s[0] is required, s[1]
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defaults to s[0])
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<alginment type="string"> Text alignment (default: "left-baseline") One of:
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"left-top"
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"left-center"
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"left-bottom"
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"center-top"
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"center-center"
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"center-bottom"
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"right-top"
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"right-center"
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"right-bottom"
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"left-baseline"
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"center-baseline"
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"right-baseline"
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"left-bottom-baseline"
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"center-bottom-baseline"
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"right-bottom-baseline"
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<draw-mode type="int"> A bitwise combination of the following values
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1 (Text - default)
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2 (Boundingbox)
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4 (Filled boundingbox)
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8 (Alignment -> Draw a cross at the position
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of the text)
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<padding type="float"> Padding between for the boundingbox (default: 0)
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<color> Text color
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<color-fill> Fill color (for the bounding box)
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Placement
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---------
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To place the canvas into the scene one ore more <placement> elements can be
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added to the texture node. By setting at least on of the following nodes
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the objects where the canvas texture should be placed on are selected:
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<texture type="string"> Match objects with the given texture assigned
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<node type="string"> Match objects with the given name
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<parent type="string"> Match objects with a parent matching the given name
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(Not necessarily the direct parent)
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Example
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-------
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<canvas>
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<texture>
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<size n="0" type="int">384</size-x>
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<size n="1" type="int">512</size-y>
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<view n="0" type="int">768</view-width>
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<view n="1" type="int">1024</view-height>
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<mipmapping type="bool">false</mipmapping>
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<coverage-samples type="int">0</coverage-samples>
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<color-samples type="int">0</color-samples>
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<color-background>
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<red type="float">0</red>
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<green type="float">0.02</green>
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<blue type="float">0</blue>
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<alpha type="float">1</alpha>
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</color-background>
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<group>
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<text>
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<text type="string">TEST MESSAGE</text>
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<font type="string">helvetica_bold.txf</font>
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<character-size type="float">40</character-size>
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<tf>
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<!-- Translate (18|50) -->
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<tx>18</tx>
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<ty>50</ty>
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</tf>
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</text>
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</group>
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<placement>
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<!-- Place on objects with the texture EICAS.png
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and a parent called HDD 1 -->
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<texture type="string">EICAS.png</texture>
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<parent type="string">HDD 1</parent>
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</placement>
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</texture>
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</canvas>
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