Improve the Canvas threading model

The canvas classes were mixing scene graph resources and GL
content across threads. This led to a number of potential crashes
in addition to that the FBO based rendering had significant
potential for stalling.

QQuickContext2DTexture is no longer a QSGTexture with ambiguous
ownership. Instead we use textureForNextFrame which is called
on the render thread while the GUI is locked to synchronize
state from the Context2D's "texture" into the actual QSGTexture.
This means that cleanup of the QQuickContext2DTexture and
the QSGTexture used for display is no longer in conflict.

QQuickPixmap no longer contains a QSGTexture either as these
are strictly for use on the scene graph thread. The Images are
anyway loaded explicitly as QImage files in QQuickContext2DContext
and uploaded again for every Canvas, so relying on the GL paint
engine to do the caching will give us the same with less code.

I also changed the default strategy to Immediate as that one
supports the full API (cooperative does not support readback)
and because cooperative is pretty bad for performance since the
rendering happens in the sync() step.

Task-number: QTBUG-34268
Task-number: QTBUG-31052
Task-number: QTBUG-21935
Task-number: QTBUG-30689
Task-number: QTBUG-29007

Change-Id: Ic540b22d5faa1188e21e56a3beee24191d13f423
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

1 files changed, 34 insertions, 30 deletions

diff --git a/src/quick/items/context2d/qquickcanvasitem.cpp b/src/quick/items/context2d/qquickcanvasitem.cpp
index ba3592986c..19a161a605 100644
--- a/src/quick/items/context2d/qquickcanvasitem.cpp
+++ b/src/quick/items/context2d/qquickcanvasitem.cpp
@@ -44,8 +44,10 @@
 #include <private/qquickitem_p.h>
 #include <private/qquickcanvascontext_p.h>
 #include <private/qquickcontext2d_p.h>
+#include <private/qquickcontext2dtexture_p.h>
 #include <qsgsimpletexturenode.h>
 #include <QtQuick/private/qquickpixmapcache_p.h>
+#include <QtGui/QGuiApplication>
 
 #include <qqmlinfo.h>
 #include <private/qqmlengine_p.h>
@@ -58,19 +60,15 @@
 
 QT_BEGIN_NAMESPACE
 
-QQuickCanvasPixmap::QQuickCanvasPixmap(const QImage& image, QQuickWindow *window)
+QQuickCanvasPixmap::QQuickCanvasPixmap(const QImage& image)
     : m_pixmap(0)
     , m_image(image)
-    , m_texture(0)
-    , m_window(window)
 {
 
 }
 
-QQuickCanvasPixmap::QQuickCanvasPixmap(QQuickPixmap *pixmap, QQuickWindow *window)
+QQuickCanvasPixmap::QQuickCanvasPixmap(QQuickPixmap *pixmap)
     : m_pixmap(pixmap)
-    , m_texture(0)
-    , m_window(window)
 {
 
 }
@@ -78,8 +76,6 @@ QQuickCanvasPixmap::QQuickCanvasPixmap(QQuickPixmap *pixmap, QQuickWindow *windo
 QQuickCanvasPixmap::~QQuickCanvasPixmap()
 {
     delete m_pixmap;
-    if (m_texture)
-        m_texture->deleteLater();
 }
 
 qreal QQuickCanvasPixmap::width() const
@@ -105,18 +101,6 @@ bool QQuickCanvasPixmap::isValid() const
     return !m_image.isNull();
 }
 
-QSGTexture *QQuickCanvasPixmap::texture()
-{
-    if (!m_texture) {
-        if (m_pixmap) {
-            Q_ASSERT(m_pixmap->textureFactory());
-            m_texture = m_pixmap->textureFactory()->createTexture(m_window);
-        } else {
-            m_texture = m_window->createTextureFromImage(m_image, QQuickWindow::TextureCanUseAtlas);
-        }
-    }
-    return m_texture;
-}
 QImage QQuickCanvasPixmap::image()
 {
     if (m_image.isNull() && m_pixmap)
@@ -194,7 +178,7 @@ QQuickCanvasItemPrivate::QQuickCanvasItemPrivate()
     , hasCanvasWindow(false)
     , available(false)
     , renderTarget(QQuickCanvasItem::Image)
-    , renderStrategy(QQuickCanvasItem::Cooperative)
+    , renderStrategy(QQuickCanvasItem::Immediate)
 {
     antialiasing = true;
 }
@@ -246,10 +230,14 @@ QQuickCanvasItemPrivate::~QQuickCanvasItemPrivate()
 
     The Canvas.FramebufferObject render target utilizes OpenGL hardware
     acceleration rather than rendering into system memory, which in many cases
-    results in faster rendering.
+    results in faster rendering. Canvas.FramebufferObject relies on the
+    OpenGL extensions \c GL_EXT_framebuffer_multisample and
+    \c GL_EXT_framebuffer_blit for antialiasing. It will also use more
+    graphics memory when rendering strategy is anything other than
+    Canvas.Cooperative.
 
     The default render target is Canvas.Image and the default renderStrategy is
-    Canvas.Cooperative.
+    Canvas.Immediate.
 
     \section1 Tiled Canvas
     The Canvas item supports tiled rendering by setting \l canvasSize, \l tileSize
@@ -489,7 +477,7 @@ void QQuickCanvasItem::setCanvasWindow(const QRectF& rect)
     context will choose appropriate options and Canvas will signal the change
     to the properties.
 
-    The default render target is \c Canvas.FramebufferObject.
+    The default render target is \c Canvas.Image.
 */
 QQuickCanvasItem::RenderTarget QQuickCanvasItem::renderTarget() const
 {
@@ -531,7 +519,7 @@ void QQuickCanvasItem::setRenderTarget(QQuickCanvasItem::RenderTarget target)
     the GUI thread.  Selecting \c Canvas.Cooperative, does not guarantee
     rendering will occur on a thread separate from the GUI thread.
 
-    The default value is \c Canvas.Cooperative.
+    The default value is \c Canvas.Immediate.
 
     \sa renderTarget
 */
@@ -689,6 +677,15 @@ void QQuickCanvasItem::updatePolish()
     }
 }
 
+class QQuickCanvasNode : public QSGSimpleTextureNode
+{
+public:
+    ~QQuickCanvasNode()
+    {
+        delete texture();
+    }
+};
+
 QSGNode *QQuickCanvasItem::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *)
 {
     Q_D(QQuickCanvasItem);
@@ -698,9 +695,9 @@ QSGNode *QQuickCanvasItem::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData
         return 0;
     }
 
-    QSGSimpleTextureNode *node = static_cast<QSGSimpleTextureNode*>(oldNode);
+    QQuickCanvasNode *node = static_cast<QQuickCanvasNode*>(oldNode);
     if (!node)
-        node = new QSGSimpleTextureNode;
+        node = new QQuickCanvasNode();
 
     if (d->smooth)
         node->setFiltering(QSGTexture::Linear);
@@ -712,8 +709,15 @@ QSGNode *QQuickCanvasItem::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData
         d->context->flush();
     }
 
-    node->setTexture(d->context->texture());
-    node->markDirty(QSGNode::DirtyMaterial);
+    QQuickContext2D *ctx = qobject_cast<QQuickContext2D *>(d->context);
+    QQuickContext2DTexture *factory = ctx->texture();
+    QSGTexture *texture = factory->textureForNextFrame(node->texture());
+    if (!texture) {
+        delete node;
+        return 0;
+    }
+
+    node->setTexture(texture);
     node->setRect(QRectF(QPoint(0, 0), d->canvasWindow.size()));
     return node;
 }
@@ -916,7 +920,7 @@ void QQuickCanvasItem::loadImage(const QUrl& url)
     if (!d->pixmaps.contains(fullPathUrl)) {
         QQuickPixmap* pix = new QQuickPixmap();
         QQmlRefPointer<QQuickCanvasPixmap> canvasPix;
-        canvasPix.take(new QQuickCanvasPixmap(pix, d->window));
+        canvasPix.take(new QQuickCanvasPixmap(pix));
         d->pixmaps.insert(fullPathUrl, canvasPix);
 
         pix->load(qmlEngine(this)