Give Item::layer documentation some much needed love.

Change-Id: I31e038d961d3aa09a36db0c091c4e4910e395c2e
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

10 files changed, 460 insertions, 2 deletions

diff --git a/src/quick/doc/images/qml-blending-layered.png b/src/quick/doc/images/qml-blending-layered.png
new file mode 100644
index 0000000000..fa1c24a98e
--- /dev/null
+++ b/src/quick/doc/images/qml-blending-layered.png
diff --git a/src/quick/doc/images/qml-blending-nonlayered.png b/src/quick/doc/images/qml-blending-nonlayered.png
new file mode 100644
index 0000000000..265abf6530
--- /dev/null
+++ b/src/quick/doc/images/qml-blending-nonlayered.png
diff --git a/src/quick/doc/images/qml-shadereffect-layereffect.png b/src/quick/doc/images/qml-shadereffect-layereffect.png
new file mode 100644
index 0000000000..44b05a9949
--- /dev/null
+++ b/src/quick/doc/images/qml-shadereffect-layereffect.png
diff --git a/src/quick/doc/images/qml-shadereffect-nolayereffect.png b/src/quick/doc/images/qml-shadereffect-nolayereffect.png
new file mode 100644
index 0000000000..f21e3f0c4e
--- /dev/null
+++ b/src/quick/doc/images/qml-shadereffect-nolayereffect.png
diff --git a/src/quick/doc/images/qml-shadereffect-opacitymask.png b/src/quick/doc/images/qml-shadereffect-opacitymask.png
new file mode 100644
index 0000000000..e056aa763b
--- /dev/null
+++ b/src/quick/doc/images/qml-shadereffect-opacitymask.png
diff --git a/src/quick/doc/snippets/qml/layerblending.qml b/src/quick/doc/snippets/qml/layerblending.qml
new file mode 100644
index 0000000000..0ef23465a5
--- /dev/null
+++ b/src/quick/doc/snippets/qml/layerblending.qml
@@ -0,0 +1,111 @@
+/****************************************************************************
+**
+** Copyright (C) 2014 Gunnar Sletta <gunnar@sletta.org>
+** Contact: http://www.qt-project.org/legal
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** You may use this file under the terms of the BSD license as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of Digia Plc and its Subsidiary(-ies) nor the names
+**     of its contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick 2.0
+
+Item {
+    width: 300
+    height: 200
+
+    // The checkers background
+    ShaderEffect {
+        id: tileBackground
+        anchors.fill: parent
+
+        property real tileSize: 10
+        property color color1: Qt.rgba(0.9, 0.9, 0.9, 1);
+        property color color2: Qt.rgba(0.85, 0.85, 0.85, 1);
+
+        property size pixelSize: Qt.size(width / tileSize, height / tileSize);
+
+        fragmentShader:
+            "
+            uniform lowp vec4 color1;
+            uniform lowp vec4 color2;
+            uniform highp vec2 pixelSize;
+            varying highp vec2 qt_TexCoord0;
+            void main() {
+                highp vec2 tc = sign(sin(3.14152 * qt_TexCoord0 * pixelSize));
+                if (tc.x != tc.y)
+                    gl_FragColor = color1;
+                else
+                    gl_FragColor = color2;
+            }
+            "
+    }
+
+
+    Row {
+        height: 100
+        anchors.centerIn: parent
+        spacing: 50
+
+//! [non-layered]
+Item {
+    id: nonLayered
+
+    opacity: 0.5
+
+    width: 100
+    height: 100
+
+    Rectangle { width: 80; height: 80; border.width: 1 }
+    Rectangle { x: 20; y: 20; width: 80; height: 80; border.width: 1 }
+}
+//! [non-layered]
+
+//! [layered]
+Item {
+    id: layered
+
+    opacity: 0.5
+
+    layer.enabled: true
+
+    width: 100
+    height: 100
+
+    Rectangle { width: 80; height: 80; border.width: 1 }
+    Rectangle { x: 20; y: 20; width: 80; height: 80; border.width: 1 }
+}
+//! [layered]
+
+    }
+}
diff --git a/src/quick/doc/snippets/qml/layerwitheffect.qml b/src/quick/doc/snippets/qml/layerwitheffect.qml
new file mode 100644
index 0000000000..8cfac0bb19
--- /dev/null
+++ b/src/quick/doc/snippets/qml/layerwitheffect.qml
@@ -0,0 +1,111 @@
+/****************************************************************************
+**
+** Copyright (C) 2014 Gunnar Sletta <gunnar@sletta.org>
+** Contact: http://www.qt-project.org/legal
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** You may use this file under the terms of the BSD license as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of Digia Plc and its Subsidiary(-ies) nor the names
+**     of its contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick 2.2
+
+Rectangle {
+    id: root
+    width: 320
+    height: 320
+
+    gradient: Gradient {
+        GradientStop { position: 0; color: "steelblue" }
+        GradientStop { position: 1; color: "black" }
+    }
+
+    property real cx: width / 2;
+    property real cy: height / 2;
+    property real boxSize: root.width * 0.2;
+    property real radius: width / 4;
+
+
+//! [1]
+Item {
+    id: layerRoot
+    layer.enabled: true
+    layer.effect: ShaderEffect {
+        fragmentShader: "
+            uniform lowp sampler2D source; // this item
+            uniform lowp float qt_Opacity; // inherited opacity of this item
+            varying highp vec2 qt_TexCoord0;
+            void main() {
+                lowp vec4 p = texture2D(source, qt_TexCoord0);
+                lowp float g = dot(p.xyz, vec3(0.344, 0.5, 0.156));
+                gl_FragColor = vec4(g, g, g, p.a) * qt_Opacity;
+            }"
+    }
+//! [1]
+
+        anchors.fill: parent
+
+
+        Repeater {
+            id: repeater
+            model: 200
+
+            Rectangle {
+                id: box
+
+                property real t: index / (repeater.model - 1);
+
+                width: 0
+                height: 0
+                x: root.cx - Math.sin(Math.PI * 2 * t) * root.radius;
+                y: root.cy - Math.cos(Math.PI * 2 * t) * root.radius;
+
+                Rectangle {
+                    width: root.boxSize
+                    height: root.boxSize
+                    anchors.centerIn: parent
+                    color: Qt.hsla(box.t, 0.5, 0.5);
+                    border.color: "white"
+                    antialiasing: true;
+                    rotation: box.t * 360;
+//                    RotationAnimator on rotation {
+//                        from: box.t * 360;
+//                        to: box.t * 360 + 360;
+//                        duration: 7592;
+//                        loops: Animation.Infinite
+//                    }
+                }
+            }
+        }
+    }
+}
diff --git a/src/quick/doc/snippets/qml/opacitymask.qml b/src/quick/doc/snippets/qml/opacitymask.qml
new file mode 100644
index 0000000000..e458f810ea
--- /dev/null
+++ b/src/quick/doc/snippets/qml/opacitymask.qml
@@ -0,0 +1,114 @@
+/****************************************************************************
+**
+** Copyright (C) 2014 Gunnar Sletta <gunnar@sletta.org>
+** Contact: http://www.qt-project.org/legal
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** You may use this file under the terms of the BSD license as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of Digia Plc and its Subsidiary(-ies) nor the names
+**     of its contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick 2.0
+
+Item {
+    id: root
+    width: 480
+    height: 320
+
+    // The checkers background
+    ShaderEffect {
+        id: tileBackground
+        anchors.fill: parent
+
+        property real tileSize: 20
+        property color color1: Qt.hsla(0, 0, 0.1);
+        property color color2: Qt.hsla(0, 0, 0.2);
+
+        property size pixelSize: Qt.size(width / tileSize, height / tileSize);
+
+        fragmentShader:
+            "
+            uniform lowp vec4 color1;
+            uniform lowp vec4 color2;
+            uniform highp vec2 pixelSize;
+            varying highp vec2 qt_TexCoord0;
+            void main() {
+                highp vec2 tc = sign(sin(3.14152 * qt_TexCoord0 * pixelSize));
+                if (tc.x != tc.y)
+                    gl_FragColor = color1;
+                else
+                    gl_FragColor = color2;
+            }
+            "
+    }
+
+//! [1]
+    Rectangle {
+        id: gradientRect;
+        width: 10
+        height: 10
+        gradient: Gradient {
+            GradientStop { position: 0; color: "white" }
+            GradientStop { position: 1; color: "steelblue" }
+        }
+        visible: false; // should not be visible on screen.
+        layer.enabled: true;
+        layer.smooth: true
+    }
+
+    Text {
+        id: textItem
+        font.pixelSize: 48
+        text: "Gradient Text"
+        anchors.centerIn: parent
+        layer.enabled: true
+        // This item should be used as the 'mask'
+        layer.samplerName: "maskSource"
+        layer.effect: ShaderEffect {
+            property var colorSource: gradientRect;
+            fragmentShader: "
+                uniform lowp sampler2D colorSource;
+                uniform lowp sampler2D maskSource;
+                uniform lowp float qt_Opacity;
+                varying highp vec2 qt_TexCoord0;
+                void main() {
+                    gl_FragColor =
+                        texture2D(colorSource, qt_TexCoord0)
+                        * texture2D(maskSource, qt_TexCoord0).a
+                        * qt_Opacity;
+                }
+            "
+        }
+    }
+//! [1]
+}
diff --git a/src/quick/items/qquickitem.cpp b/src/quick/items/qquickitem.cpp
index 38ae0ed3c8..b8362c9dac 100644
--- a/src/quick/items/qquickitem.cpp
+++ b/src/quick/items/qquickitem.cpp
@@ -1838,6 +1838,101 @@ void QQuickItemPrivate::updateSubFocusItem(QQuickItem *scope, bool focus)
     their layouts.
 
     See LayoutMirroring for more details.
+
+    \section1 Item Layers
+
+    An Item will normally be rendered directly into the window it
+    belongs to. However, by setting \l layer.enabled, it is possible
+    to delegate the item and its entire subtree into an offscreen
+    surface. Only the offscreen surface, a texture, will be then drawn
+    into the window.
+
+    If it is desired to have a texture size different from that of the
+    item, this is possible using \l layer.textureSize. To render only
+    a section of the item into the texture, use \l
+    layer.sourceRect. It is also possible to specify \l
+    layer.sourceRect so it extends beyond the bounds of the item. In
+    this case, the exterior will be padded with transparent pixels.
+
+    The item will use linear interpolation for scaling if
+    \l layer.smooth is set to \c true and will use mipmap for
+    downsampling if \l layer.mipmap is set to \c true. Mipmapping may
+    improve visual quality of downscaled items. For mipmapping of
+    single Image items, prefer Image::mipmap.
+
+    \section2 Layer Opacity vs Item Opacity
+
+    When applying \l opacity to an item hierarchy the opacity is
+    applied to each item individually. This can lead to undesired
+    visual results when the opacity is applied to a subtree. Consider
+    the following example:
+
+    \table
+    \row
+      \li \inlineimage qml-blending-nonlayered.png
+      \li \b {Non-layered Opacity} \snippet qml/layerblending.qml non-layered
+    \endtable
+
+    A layer is rendered with the root item's opacity being 1, and then
+    the root item's opacity is applied to the texture when it is
+    drawn. This means that fading in a large item hierarchy from
+    transparent to opaque, or vice versa, can be done without the
+    overlap artifacts that the normal item by item alpha blending
+    has. Here is the same example with layer enabled:
+
+    \table
+    \row
+      \li \image qml-blending-layered.png
+      \li \b {Layered Opacity} \snippet qml/layerblending.qml layered
+    \endtable
+
+    \section2 Combined with ShaderEffects
+
+    Setting \l layer.enabled to true will turn the item into a \l
+    {QQuickItem::isTextureProvider}{texture provider}, making it
+    possible to use the item directly as a texture, for instance
+    in combination with the ShaderEffect type.
+
+    It is possible to apply an effect on a layer at runtime using
+    layer.effect:
+
+    \snippet qml/layerwitheffect.qml 1
+
+    In this example, we implement the shader effect manually. The \l
+    {Qt Graphical Effects} module contains a suite of ready-made
+    effects for use with Qt Quick.
+
+    See ShaderEffect for more information about using effects.
+
+    \note \l layer.enabled is actually just a more convenient way of using
+    ShaderEffectSource.
+
+
+    \section2 Memory and Performance
+
+    When an item's layer is enabled, the scene graph will allocate memory
+    in the GPU equal to \c {width x height x 4}. In memory constrained
+    configurations, large layers should be used with care.
+
+    In the QPainter / QWidget world, it is some times favorable to
+    cache complex content in a pixmap, image or texture. In Qt Quick,
+    because of the techniques already applied by the \l {Qt Quick
+    Scene Graph Renderer} {scene graph renderer}, this will in most
+    cases not be the case. Excessive draw calls are already reduced
+    because of batching and a cache will in most cases end up blending
+    more pixels than the original content. The overhead of rendering
+    to an offscreen and the blending involved with drawing the
+    resulting texture is therefore often more costly than simply
+    letting the item and its children be drawn normally.
+
+    Also, an item using a layer can not be \l {Batching} {batched} during
+    rendering. This means that a scene with many layered items may
+    have performance problems.
+
+    Layering can be convenient and useful for visual effects, but
+    should in most cases be enabled for the duration of the effect and
+    disabled afterwards.
+
 */
 
 /*!
@@ -7486,12 +7581,15 @@ void QQuickItemLayer::setMipmap(bool mipmap)
     allow you to save some texture memory.
 
     \list
-    \li ShaderEffectSource.Alpha - GL_ALPHA
+    \li ShaderEffectSource.Alpha - GL_ALPHA;
     \li ShaderEffectSource.RGB - GL_RGB
     \li ShaderEffectSource.RGBA - GL_RGBA
     \endlist
 
-    \note Some OpenGL implementations do not support the GL_ALPHA format.
+    \note ShaderEffectSource.RGB and ShaderEffectSource.Alpha should
+    be used with caution, as support for these formats in the underlying
+    hardare and driver is often not present.
+
  */
 
 void QQuickItemLayer::setFormat(QQuickShaderEffectSource::Format f)
diff --git a/src/quick/items/qquickshadereffect.cpp b/src/quick/items/qquickshadereffect.cpp
index fb5dac1080..0358495a3b 100644
--- a/src/quick/items/qquickshadereffect.cpp
+++ b/src/quick/items/qquickshadereffect.cpp
@@ -641,10 +641,34 @@ void QQuickShaderEffectCommon::propertyChanged(QQuickItem *item, int mappedId,
     corner. For non-linear vertex transformations, like page curl, you can
     specify a fine grid of vertices by specifying a \l mesh resolution.
 
+    \section1 ShaderEffect and Item Layers
+
+    The ShaderEffect type can be combined with \l {Item Layers} {layered items}.
+
+    \table
+    \row
+      \li \b {Layer with effect disabled} \inlineimage qml-shadereffect-nolayereffect.png
+      \li \b {Layer with effect enabled} \inlineimage qml-shadereffect-layereffect.png
+      \li \snippet qml/layerwitheffect.qml 1
+    \endtable
+
+    It is also possible to combine multiple layered items:
+
+    \table
+    \row
+      \li \inlineimage qml-shadereffect-opacitymask.png
+      \li \snippet qml/opacitymask.qml 1
+    \endtable
+
+    The \l {Qt Graphical Effects} module contains several ready-made effects
+    for using with Qt Quick applications.
+
     \note Scene Graph textures have origin in the top-left corner rather than
     bottom-left which is common in OpenGL.
 
     For information about the GLSL version being used, see \l QtQuick::OpenGLInfo.
+
+    \sa {Item Layers}
 */
 
 QQuickShaderEffect::QQuickShaderEffect(QQuickItem *parent)