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QCPLayer Class Reference

A layer that may contain objects, to control the rendering order. More...

Inherits QObject.

Public Types

enum  LayerMode

Public Functions

 QCPLayer (QCustomPlot *parentPlot, const QString &layerName)
QCustomPlotparentPlot () const
QString name () const
int index () const
QList< QCPLayerable * > children () const
bool visible () const
LayerMode mode () const
void setVisible (bool visible)
void setMode (LayerMode mode)
void replot ()

Protected Functions

void draw (QCPPainter *painter)
void drawToPaintBuffer ()
void addChild (QCPLayerable *layerable, bool prepend)
void removeChild (QCPLayerable *layerable)

Detailed Description

A layer that may contain objects, to control the rendering order.

The Layering system of QCustomPlot is the mechanism to control the rendering order of the elements inside the plot.

It is based on the two classes QCPLayer and QCPLayerable. QCustomPlot holds an ordered list of one or more instances of QCPLayer (see QCustomPlot::addLayer, QCustomPlot::layer, QCustomPlot::moveLayer, etc.). When replotting, QCustomPlot goes through the list of layers bottom to top and successively draws the layerables of the layers into the paint buffer(s).

A QCPLayer contains an ordered list of QCPLayerable instances. QCPLayerable is an abstract base class from which almost all visible objects derive, like axes, grids, graphs, items, etc.

Default layers

Initially, QCustomPlot has six layers: "background", "grid", "main", "axes", "legend" and "overlay" (in that order). On top is the "overlay" layer, which only contains the QCustomPlot's selection rect (QCustomPlot::selectionRect). The next two layers "axes" and "legend" contain the default axes and legend, so they will be drawn above plottables. In the middle, there is the "main" layer. It is initially empty and set as the current layer (see QCustomPlot::setCurrentLayer). This means, all new plottables, items etc. are created on this layer by default. Then comes the "grid" layer which contains the QCPGrid instances (which belong tightly to QCPAxis, see QCPAxis::grid). The Axis rect background shall be drawn behind everything else, thus the default QCPAxisRect instance is placed on the "background" layer. Of course, the layer affiliation of the individual objects can be changed as required (QCPLayerable::setLayer).

Controlling the rendering order via layers

Controlling the ordering of layerables in the plot is easy: Create a new layer in the position you want the layerable to be in, e.g. above "main", with QCustomPlot::addLayer. Then set the current layer with QCustomPlot::setCurrentLayer to that new layer and finally create the objects normally. They will be placed on the new layer automatically, due to the current layer setting. Alternatively you could have also ignored the current layer setting and just moved the objects with QCPLayerable::setLayer to the desired layer after creating them.

It is also possible to move whole layers. For example, If you want the grid to be shown in front of all plottables/items on the "main" layer, just move it above "main" with QCustomPlot::moveLayer.

The rendering order within one layer is simply by order of creation or insertion. The item created last (or added last to the layer), is drawn on top of all other objects on that layer.

When a layer is deleted, the objects on it are not deleted with it, but fall on the layer below the deleted layer, see QCustomPlot::removeLayer.

Replotting only a specific layer

If the layer mode (setMode) is set to lmBuffered, you can replot only this specific layer by calling replot. In certain situations this can provide better replot performance, compared with a full replot of all layers. Upon creation of a new layer, the layer mode is initialized to lmLogical. The only layer that is set to lmBuffered in a new QCustomPlot instance is the "overlay" layer, containing the selection rect.

Member Enumeration Documentation

§ LayerMode

Defines the different rendering modes of a layer. Depending on the mode, certain layers can be replotted individually, without the need to replot (possibly complex) layerables on other layers.

See also

Layer is used only for rendering order, and shares paint buffer with all other adjacent logical layers.


Layer has its own paint buffer and may be replotted individually (see replot).

Constructor & Destructor Documentation

§ QCPLayer()

QCPLayer::QCPLayer ( QCustomPlot parentPlot,
const QString &  layerName 

Creates a new QCPLayer instance.

Normally you shouldn't directly instantiate layers, use QCustomPlot::addLayer instead.

It is not checked that layerName is actually a unique layer name in parentPlot. This check is only performed by QCustomPlot::addLayer.

Member Function Documentation

§ index()

int QCPLayer::index ( ) const

Returns the index this layer has in the QCustomPlot. The index is the integer number by which this layer can be accessed via QCustomPlot::layer.

Layers with higher indices will be drawn above layers with lower indices.

§ children()

QList< QCPLayerable * > QCPLayer::children ( ) const

Returns a list of all layerables on this layer. The order corresponds to the rendering order: layerables with higher indices are drawn above layerables with lower indices.

§ setVisible()

void QCPLayer::setVisible ( bool  visible)

Sets whether this layer is visible or not. If visible is set to false, all layerables on this layer will be invisible.

This function doesn't change the visibility property of the layerables (QCPLayerable::setVisible), but the QCPLayerable::realVisibility of each layerable takes the visibility of the parent layer into account.

§ setMode()

void QCPLayer::setMode ( QCPLayer::LayerMode  mode)

Sets the rendering mode of this layer.

If mode is set to lmBuffered for a layer, it will be given a dedicated paint buffer by the parent QCustomPlot instance. This means it may be replotted individually by calling QCPLayer::replot, without needing to replot all other layers.

Layers which are set to lmLogical (the default) are used only to define the rendering order and can't be replotted individually.

Note that each layer which is set to lmBuffered requires additional paint buffers for the layers below, above and for the layer itself. This increases the memory consumption and (slightly) decreases the repainting speed because multiple paint buffers need to be joined. So you should carefully choose which layers benefit from having their own paint buffer. A typical example would be a layer which contains certain layerables (e.g. items) that need to be changed and thus replotted regularly, while all other layerables on other layers stay static. By default, only the topmost layer called "overlay" is in mode lmBuffered, and contains the selection rect.

See also

§ replot()

void QCPLayer::replot ( )

If the layer mode (setMode) is set to lmBuffered, this method allows replotting only the layerables on this specific layer, without the need to replot all other layers (as a call to QCustomPlot::replot would do).

If the layer mode is lmLogical however, this method simply calls QCustomPlot::replot on the parent QCustomPlot instance.

QCustomPlot also makes sure to replot all layers instead of only this one, if the layer ordering has changed since the last full replot and the other paint buffers were thus invalidated.

See also

§ draw()

void QCPLayer::draw ( QCPPainter painter)

Draws the contents of this layer with the provided painter.

See also
replot, drawToPaintBuffer

§ drawToPaintBuffer()

void QCPLayer::drawToPaintBuffer ( )

Draws the contents of this layer into the paint buffer which is associated with this layer. The association is established by the parent QCustomPlot, which manages all paint buffers (see QCustomPlot::setupPaintBuffers).

See also

§ addChild()

void QCPLayer::addChild ( QCPLayerable layerable,
bool  prepend 

Adds the layerable to the list of this layer. If prepend is set to true, the layerable will be prepended to the list, i.e. be drawn beneath the other layerables already in the list.

This function does not change the mLayer member of layerable to this layer. (Use QCPLayerable::setLayer to change the layer of an object, not this function.)

See also

§ removeChild()

void QCPLayer::removeChild ( QCPLayerable layerable)

Removes the layerable from the list of this layer.

This function does not change the mLayer member of layerable. (Use QCPLayerable::setLayer to change the layer of an object, not this function.)

See also

The documentation for this class was generated from the following files: