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<td >Qt 5.9</td><td ><a href="qtquick-index.html">Qt Quick</a></td><td >Qt Quick OpenVG Adaptation</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.9.4 Reference Documentation</td>
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<h3><a name="toc">Contents</a></h3>
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<li class="level2"><a href="#egl-requirement">EGL Requirement</a></li>
<li class="level2"><a href="#renderer">Renderer</a></li>
<li class="level2"><a href="#render-loop">Render Loop</a></li>
<li class="level2"><a href="#shader-effects">Shader Effects</a></li>
<li class="level2"><a href="#qt-graphical-effects-module">Qt Graphical Effects Module</a></li>
<li class="level2"><a href="#particle-effects">Particle Effects</a></li>
<li class="level2"><a href="#rendering-text">Rendering Text</a></li>
<li class="level2"><a href="#perspective-transforms">Perspective Transforms</a></li>
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<h1 class="title">Qt Quick OpenVG Adaptation</h1>
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<p>The <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> adaptation is an alternative renderer for <a href="qtquick-index.html">Qt Quick</a> 2 that will renderer the contents of the scene graph using <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> commands to provide hardware-acclerated 2D vector and raster graphics. Much like the Software adaptation, some features and optimizations are no longer available. Most Qt Quick 2 applications will run without modification though any attempts to use unsupported features will be ignored.</p>
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<h3 >EGL Requirement</h3>
<p>Unlike the defualt OpenGL Renderer, there is no built in support for acquiring an <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> context. This means that the renderer has the responsbility of requesting and managing the the current context. To do this EGL has to be used directly in the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> renderer. This means that the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> renderer is only usable with platform plugins that support creating QWindows with support for QSurfaceFormat::OpenVG. From this window, the renderer can get an EGLSurface which can be used with an EGLContext to render <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> content.</p>
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<h3 >Renderer</h3>
<p>The <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> Renderer works by using the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> API to send commands and data to a Vector GPU which will render the scenegraph in an accelerated manner, offloading graphics rendering from the CPU. Many operations like the rendering of rectangles and fonts glyphs ideal for <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> because these can be represented as paths which are stroked and filled. Rendering scenegraph items that would typically involve textures are handled in the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> renderer by using VGImage. In addition when rendering to offscreen surfaces (like when using Layers), the scene subtree is rendered to a VGImage which can be reused in the scene.</p>
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<h3 >Render Loop</h3>
<p>The <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> Renderer mirrors the behavior of the Basic render loop and will execute all <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> commands in a single thread.</p>
<p>See the <a href="qtquick-visualcanvas-scenegraph.html">Scene Graph page</a> for more information on render loops</p>
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<h3 >Shader Effects</h3>
<p><a href="qml-qtquick-shadereffect.html">ShaderEffect</a> components in <a href="qtquick-module.html">QtQuick</a> 2 can not be rendered by the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> adaptation. While it is possible to user <a href="qml-qtquick-shadereffectsource.html">ShaderEffectSource</a> and QML Item Layers (which are both offscreen surfaces), it is not actually possible to apply shader effects to them via the <a href="qml-qtquick-shadereffect.html">ShaderEffect</a> item. This is because <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> lacks an API for applying per vertex and per fragment shader operations. It may be possible however to take advantage of Image Filter operations in the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> API to get similar effects to what is provided by ShaderEffects in custom items. To integrate custom <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> rendering, use <a href="qsgrendernode.html">QSGRenderNode</a> in combination with <a href="qsgrendererinterface.html">QSGRendererInterface</a>.</p>
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<h3 >Qt Graphical Effects Module</h3>
<p>Qt Graphical Effects uses <a href="qml-qtquick-shadereffect.html">ShaderEffect</a> items to render effects. If you use graphical effects from this module, then you should not hide the source item so that the original item can still be rendered.</p>
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<h3 >Particle Effects</h3>
<p>It is not possible to render particle effects with the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> adaptation. Whenever possible, remove particles completely from the scene. Otherwise they will still require some processing, even though they are not visible.</p>
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<h3 >Rendering Text</h3>
<p>The text rendering with the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> adaptation is based on rendering the glpyh paths, and does not use the distance fields technique used by the OpenGL backend.</p>
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<h3 >Perspective Transforms</h3>
<p>The <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> API does not allow paths to be transformed with non-affine transforms, while it is possible with Qt Quick. This means that rendering components using paths like Rectangles and Text, when applying perspective transforms the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> backend will first render to a VGImage before applying transformations. This uses more memory at runtime and is a slower path so avoid doing this if necessary.</p>
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