Support Canvas.antialiasing without multisample support.

FBO based rendering relies on framebuffer multisampling to do
antialiasing, which is often not available on OpenGL ES and even on
some desktop chips. As a high-level API, it is quite bad that Canvas
users (on embedded in particular) have to choose between quality
(Image) or performance (FBO).

This change implements super sampling, rendering the content
at twice the size and then scaling it down.

[ChangeLog][QtQuick][Canvas] Implement antialiasing on
FramebufferObject based render targets through super-sampling (SSAA)
when framebuffer multisampling is not available.

Change-Id: I373f3a645342dac157506b746c1e39b0f3f3f9f2
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

1 files changed, 19 insertions, 4 deletions

diff --git a/src/quick/items/context2d/qquickcontext2dtexture.cpp b/src/quick/items/context2d/qquickcontext2dtexture.cpp
index fbfd3df324..7e88e18c65 100644
--- a/src/quick/items/context2d/qquickcontext2dtexture.cpp
+++ b/src/quick/items/context2d/qquickcontext2dtexture.cpp
@@ -227,7 +227,7 @@ void QQuickContext2DTexture::paintWithoutTiles(QQuickContext2DCommandBuffer *ccb
 
     p.setCompositionMode(QPainter::CompositionMode_SourceOver);
 
-    ccb->replay(&p, m_state);
+    ccb->replay(&p, m_state, scaleFactor());
     endPainting();
     markDirtyTexture();
 }
@@ -268,7 +268,7 @@ void QQuickContext2DTexture::paint(QQuickContext2DCommandBuffer *ccb)
             QQuickContext2D::State oldState = m_state;
             foreach (QQuickContext2DTile* tile, m_tiles) {
                 if (tile->dirty()) {
-                    ccb->replay(tile->createPainter(m_smooth, m_antialiasing), oldState);
+                    ccb->replay(tile->createPainter(m_smooth, m_antialiasing), oldState, scaleFactor());
                     tile->drawFinished();
                     tile->markDirty(false);
                 }
@@ -418,6 +418,14 @@ QQuickContext2DFBOTexture::~QQuickContext2DFBOTexture()
         QOpenGLContext::currentContext()->functions()->glDeleteTextures(2, m_displayTextures);
 }
 
+QVector2D QQuickContext2DFBOTexture::scaleFactor() const
+{
+    if (!m_fbo)
+        return QVector2D(1, 1);
+    return QVector2D(m_fbo->width() / m_fboSize.width(),
+                     m_fbo->height() / m_fboSize.height());
+}
+
 QSGTexture *QQuickContext2DFBOTexture::textureForNextFrame(QSGTexture *lastTexture)
 {
     QSGPlainTexture *texture = static_cast<QSGPlainTexture *>(lastTexture);
@@ -500,7 +508,7 @@ void QQuickContext2DFBOTexture::grabImage(const QRectF& rf)
     QImage grabbed;
     {
         GLAcquireContext ctx(m_context->glContext(), m_context->surface());
-        grabbed = m_fbo->toImage().mirrored().copy(rf.toRect());
+        grabbed = m_fbo->toImage().scaled(m_fboSize, Qt::IgnoreAspectRatio, Qt::SmoothTransformation).mirrored().copy(rf.toRect());
     }
 
     m_context->setGrabbedImage(grabbed);
@@ -561,7 +569,14 @@ QPaintDevice* QQuickContext2DFBOTexture::beginPainting()
         } else {
             QOpenGLFramebufferObjectFormat format;
             format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
-            m_fbo = new QOpenGLFramebufferObject(m_fboSize, format);
+            QSize s = m_fboSize;
+            if (m_antialiasing) { // do supersampling since multisampling is not available
+                GLint max;
+                QOpenGLContext::currentContext()->functions()->glGetIntegerv(GL_MAX_TEXTURE_SIZE, &max);
+                if (s.width() * 2 <= max && s.height() * 2 <= max)
+                    s = s * 2;
+            }
+            m_fbo = new QOpenGLFramebufferObject(s, format);
         }
     }