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<td >Qt 5.12</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"><a href="qtquick-index.html">Qt 5.12.6 Reference Documentation</a></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 renders the contents of the scene graph using <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> commands to provide hardware accelerated 2D vector and raster graphics. Much like the <a href="qtquick-visualcanvas-adaptations-software.html">Software Adaptation</a>, 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 id="egl-requirement">EGL Requirement</h3>
<p>Unlike the default OpenGL Renderer, there is no built-in support to acquire an <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> context. This means that the renderer is responsible for requesting and managing the the current context. To do this, you use EGL directly in the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> renderer. Consequently, the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> renderer can only be used with platform plugins that support creating QWindows with support for QSurfaceFormat::OpenVG. From this window, the renderer can get an EGLSurface which can then be used with an EGLContext to render <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> content.</p>
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<h3 id="renderer">Renderer</h3>
<p>The <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> Renderer uses the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> API to send commands and data to a Vector GPU that renders the scene graph in an accelerated manner, offloading graphics rendering from the CPU. Many operations like the rendering of rectangles and font glyphs are ideal for <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> because they can be represented as paths which are stroked and filled. Rendering scene graph items that would typically involve textures are handled in the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> renderer using VGImage. Additionally, when you render to offscreen surfaces (like with Layers), the scene subtree is rendered to a VGImage which can be reused in the scene.</p>
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<h3 id="render-loop">Render Loop</h3>
<p>The <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> Renderer mirrors the behavior of the Basic render loop and it runs all <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> commands in a single thread.</p>
<p>For more information on render loops, see <a href="qtquick-visualcanvas-scenegraph.html">Qt Quick Scene Graph</a>.</p>
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<h3 id="shader-effects">Shader Effects</h3>
<p><a href="qml-qtquick-shadereffect.html">ShaderEffect</a> components in <a href="qtquick-module.html">QtQuick</a> 2 can't be rendered by the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> adaptation. While it's possible to use <a href="qml-qtquick-shadereffectsource.html">ShaderEffectSource</a> and QML Item Layers (which are both offscreen surfaces), it's not 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. However, you may be able to take advantage of Image Filter operations in the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> API to get effects that are similar to what ShaderEffects provides 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 id="qt-graphical-effects-module">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 shouldn't hide the source item so that the original item can still be rendered.</p>
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<h3 id="particle-effects">Particle Effects</h3>
<p>It's 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'll still require some processing, even though they are not visible.</p>
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<h3 id="rendering-text">Rendering Text</h3>
<p>Text rendering with the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> adaptation is based on rendering the glyph paths, and doesn't use the distance fields technique, unlike with the OpenGL backend.</p>
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<h3 id="perspective-transforms">Perspective Transforms</h3>
<p>The <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> API doesn't allow paths to be transformed with non-affine transforms, but it's possible with Qt Quick. Consquently, when you render components using paths like Rectangles and Text while applying perspective transforms, the <a href="qtquick-visualcanvas-adaptations.html#openvg">OpenVG</a> backend first renders to a VGImage before applying transformations. This behavior uses more memory at runtime and takes more time; avoid it if possible.</p>
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