Add samples property for Item.layer and ShaderEffectSource

[ChangeLog][Qt Quick] Added the properties ShaderEffectSource.samples
and Item.layer.samples to allow requesting MSAA rendering of an item
subtree, without enabling multisampling for the entire scene.

Task-number: QTBUG-58945
Change-Id: I9102cfabba10d4dc1e7ad2aa0b258ada6d9a5a47
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 40 insertions, 0 deletions

diff --git a/src/quick/items/qquickshadereffectsource.cpp b/src/quick/items/qquickshadereffectsource.cpp
index 1b37a746d3..c782ddb5f7 100644
--- a/src/quick/items/qquickshadereffectsource.cpp
+++ b/src/quick/items/qquickshadereffectsource.cpp
@@ -189,6 +189,7 @@ QQuickShaderEffectSource::QQuickShaderEffectSource(QQuickItem *parent)
     , m_sourceItem(0)
     , m_textureSize(0, 0)
     , m_format(RGBA)
+    , m_samples(0)
     , m_live(true)
     , m_hideSource(false)
     , m_mipmap(false)
@@ -582,6 +583,44 @@ void QQuickShaderEffectSource::setTextureMirroring(TextureMirroring mirroring)
 }
 
 /*!
+    \qmlproperty int QtQuick::ShaderEffectSource::samples
+    \since 5.10
+
+    This property allows requesting multisampled rendering.
+
+    By default multisampling is enabled whenever multisampling is enabled for
+    the entire window, assuming the scenegraph renderer in use and the
+    underlying graphics API supports this.
+
+    By setting the value to 2, 4, etc. multisampled rendering can be requested
+    for a part of the scene without enabling multisampling for the entire
+    scene. This way multisampling is applied only to a given subtree, which can
+    lead to significant performance gains since multisampling is not applied to
+    other parts of the scene.
+
+    \note Enabling multisampling can be potentially expensive regardless of the
+    layer's size, as it incurs a hardware and driver dependent performance and
+    memory cost.
+
+    \note This property is only functional when support for multisample
+    renderbuffers and framebuffer blits is available. Otherwise the value is
+    silently ignored.
+ */
+int QQuickShaderEffectSource::samples() const
+{
+    return m_samples;
+}
+
+void QQuickShaderEffectSource::setSamples(int count)
+{
+    if (count == m_samples)
+        return;
+    m_samples = count;
+    update();
+    emit samplesChanged();
+}
+
+/*!
     \qmlmethod QtQuick::ShaderEffectSource::scheduleUpdate()
 
     Schedules a re-rendering of the texture for the next frame.
@@ -683,6 +722,7 @@ QSGNode *QQuickShaderEffectSource::updatePaintNode(QSGNode *oldNode, UpdatePaint
     m_texture->setHasMipmaps(m_mipmap);
     m_texture->setMirrorHorizontal(m_textureMirroring & MirrorHorizontally);
     m_texture->setMirrorVertical(m_textureMirroring & MirrorVertically);
+    m_texture->setSamples(m_samples);
 
     if (m_grab)
         m_texture->scheduleUpdate();