OpenVG Scenegraph Adaptation

This is an OpenVG backend for the Qt Quick 2 scenegraph.

Should be feature complete now, but there are still some issues that
could be improved in future commits:

If Rectangle nodes are rendered with a non-affine transform, they will
be rendered incorrectly.  This is because paths expect affine
transformations.

The Glyph cache is a bit cheeky in that it's caching paths, but doing
so per font size.  It shoudln't need to, but right now I've not though
up a good way of getting the transform/scale needed when rendering yet.

Change-Id: Ie3c4f2df35d14279b0f9f55e0e10a873328c025b
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

3 files changed, 83 insertions, 1 deletions

diff --git a/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc b/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc
index 9ce26e1bb8..3fd92177f9 100644
--- a/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc
+++ b/src/quick/doc/src/concepts/visualcanvas/adaptations.qdoc
@@ -58,6 +58,8 @@ The supported backends are the following
 
 \li Direct3D 12 - Requested by the string \c{"d3d12"} or the enum value QSGRendererInterface::Direct3D12.
 
+\li OpenVG - Requested by the string \c{"openvg"} or the enum value QSGRendererInterface::OpenVG.
+
 \endlist
 
 When in doubt which backend is in use, enable basic scenegraph information
@@ -92,6 +94,14 @@ running on Windows 10, both for Win32 and UWP applications. The details for
 this adaptation are available here:
 \l{qtquick-visualcanvas-adaptations-d3d12.html}{Direct3D 12 Adaptation}
 
+\section1 OpenVG
+
+The OpenVG adaptation is an alternative renderer for \l {Qt Quick} 2 that will
+renderer the contents of the scene graph using OpenVG commands to provide
+hardware-acclerated 2D vector and raster graphics. The details for this
+adaptation are available here:
+\l{qtquick-visualcanvas-scenegraph-openvg.html}{OpenVG Adaptation}
+
 */
 
 
@@ -387,3 +397,73 @@ between the frames). By default blocking present is disabled.
 \endlist
 
 */
+
+/*!
+\title Qt Quick OpenVG Adaptation
+\page qtquick-visualcanvas-adaptations-openvg.html
+
+The OpenVG adaptation is an alternative renderer for \l {Qt Quick} 2 that will
+renderer the contents of the scene graph using OpenVG 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.
+
+\section2 EGL Requirement
+Unlike the defualt OpenGL Renderer, there is no built in support for acquiring
+an OpenVG 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 OpenVG renderer.  This means that the OpenVG 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 OpenVG content.
+
+\section2 Renderer
+The OpenVG Renderer works by using the OpenVG 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 OpenVG because these can be represented as paths which
+are stroked and filled. Rendering scenegraph items that would typically involve
+textures are handled in the OpenVG 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.
+
+\section2 Render Loop
+The OpenVG Renderer mirrors the behavior of the Basic render loop and will execute
+all OpenVG commands in a single thread.
+
+See the \l{qtquick-visualcanvas-scenegraph.html}{Scene Graph page} for more
+information on render loops
+
+\section2 Shader Effects
+ShaderEffect components in QtQuick 2 can not be rendered by the OpenVG adaptation.
+While it is possible to user ShaderEffectSource and QML Item Layers (which are both
+offscreen surfaces), it is not actually possible to apply shader effects to them
+via the ShaderEffect item.  This is because OpenVG 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 OpenVG API to get similar effects to
+what is provided by ShaderEffects in custom items. To integrate custom OpenVG
+rendering, use QSGRenderNode in combination with QSGRendererInterface.
+
+\section2 Qt Graphical Effects Module
+\l {Qt Graphical Effects} uses ShaderEffect 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.
+
+\section2 Particle Effects
+It is not possible to render particle effects with the OpenVG adaptation. Whenever
+possible, remove particles completely from the scene. Otherwise they will still
+require some processing, even though they are not visible.
+
+\section2 Rendering Text
+The text rendering with the OpenVG adaptation is based on rendering the glpyh
+paths, and does not use the distance fields technique used by the OpenGL backend.
+
+\section2 Perspective Transforms
+The OpenVG 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 OpenVG
+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.
+
+*/
diff --git a/src/quick/scenegraph/coreapi/qsgrendererinterface.cpp b/src/quick/scenegraph/coreapi/qsgrendererinterface.cpp
index d309044e8f..3b0b2faf97 100644
--- a/src/quick/scenegraph/coreapi/qsgrendererinterface.cpp
+++ b/src/quick/scenegraph/coreapi/qsgrendererinterface.cpp
@@ -77,6 +77,7 @@ QT_BEGIN_NAMESPACE
     \value Software The Qt Quick 2D Renderer is in use
     \value OpenGL OpenGL ES 2.0 or higher
     \value Direct3D12 Direct3D 12
+    \value OpenVG OpenVG via EGL
   */
 
 /*!
diff --git a/src/quick/scenegraph/coreapi/qsgrendererinterface.h b/src/quick/scenegraph/coreapi/qsgrendererinterface.h
index cf8fcf9015..722488201b 100644
--- a/src/quick/scenegraph/coreapi/qsgrendererinterface.h
+++ b/src/quick/scenegraph/coreapi/qsgrendererinterface.h
@@ -53,7 +53,8 @@ public:
         Unknown,
         Software,
         OpenGL,
-        Direct3D12
+        Direct3D12,
+        OpenVG
     };
 
     enum Resource {