Brush GUI Design with Lager

Krita controls overview

In Krita we have a really complicated system of brush settings, so in the beginning it would be nice to make a short overview of what we have


PaintOp is a brush engine that can load a brush preset and paint on canvas


Option is a high-level property of the brush. E.g. “Size”, “Opacity” or “Smudge Rate”. In the GUI an option is represented as a single page full of smaller settings. Most of Krita options also have a curve that links option’s value to the stylus sensors.


Sensor represents a single sensor available in the stylus.

Overview of brush editor controls

Problem Definition

The building block of any brush engine GUI is a PaintOpOption. When building a configuration widget for a PaintOp we just compose a set of independent options, pass them the brush preset (in a form of KisPropertiesConfiguration object) and show the result to the user.

Each option has four responsibilities:

  1. read/write serialized XML or properties data

  2. define dependencies between properties of the option and other options, for example

  • Brush Application widget is available only for RGB brushes. For all standard brushes it should be grayed out and set to “Mask” mode

    Mask mode is forced for non-RGB brushes
  • Lightness Strength option is available only when an RGB brush is selected and “Lightness Map” mode is enabled

    Lightness strength is disabled for non-lightness brushes
  1. show options in the GUI as Qt’s widgets

  2. apply the actual effect of the option to the stroke on the canvas

The problem of our current implementation is that all four responsibilities are packed either in one (sometimes two) classes (see e.g. KisAirbrushOptionWidget, or KisSmudgeOption + KisSmudgeOptionWidget). And the dependencies logic is usually implemented in Widget part of the pack, which makes it extremely hard to debug and maintain (not speaking about porting to QML).

What is Lager?

Lager is a C++ library to assist value-oriented design by implementing the unidirectional data-flow architecture. It is heavily inspired by Elm and Redux, and enables composable designs by promoting the use of simple value types and testable application logic via pure functions.

What does it mean for us?

Value-oriented design

Value oriented design means that the library operates with immutable “value types”. We don’t “edit” any model. When we want to change something we just replace the whole “state”.

For Krita it means that we have a C++ structure for each part of the brush settings and can manipulate it easily. See, for example, KisAirbrushOptionData which represents the corresponding option:

struct KisAirbrushOptionData
    : boost::equality_comparable<KisAirbrushOptionData>
    inline friend bool operator==(const KisAirbrushOptionData &lhs,
                                  const KisAirbrushOptionData &rhs);

    bool isChecked {false};
    qreal airbrushRate {50.0};
    bool ignoreSpacing {false};

    bool read(const KisPropertiesConfiguration *setting);
    void write(KisPropertiesConfiguration *setting) const;

KisAirbrushOptionData is a simple structure without any constructor, destructor or virtual functions. It is assignable and comparable. One can also write or read its value to a KisPropertiesConfiguration object.

The main benefit of having such representation of the option is that now we can compare old and new value of the option and if the value hasn’t changed, don’t issue any update. It solves the problem of cycling updates that we have in the old implementation. The old implementation stores all the options in a single KisPropertiesConfiguration, so we cannot split or compare it.

Unidirectional data-flow architecture

The original idea of Lager is that the system would be implemented in a fully “functional programming” approach. That is, there is a single “state” and the GUI calling “pure functions” to replace this state. We cannot use this “functional” design fully right now, but we can use other composing tools lager provides for our benefit.

Basically, Lager provides tools for building tree-like structures of values that depend on each other in uni-directional way.

Let’s consider the following simplified example of a scatter option:

struct KisSensorData
    KoID id;
    QString curve;

struct KisCurveOptionData
    bool isChecked {false};
    qreal strength {1.0};

    KisSensorData pressureSensor;
    KisSensorData rotationSensor;
    KisSensorData fuzzySensor;

struct KisScatterOptionData
    bool scatterAxisX {true};
    bool scatterAxisY {true};

    KisCurveOptionData curveOption;

You can see that the scatter option is composed of a curve option and a few own properties, like scatterAxisX and scatterAxisY.

The whole GUI is represented as a graph. Each node of this graph knows its value (and has a representation as a plain C++ struct).

Graph of the scatter option

Since each node knows its current value, when an update comes, the node can compare the new value against the current one and cancel update propagation in case the value haven’t changed. It allows us to avoid the problem of cycling updates, since a lot of Qt’s widgets emit updates even when the value doesn’t change.

Graph of the scatter option

What Lager provides?

Lager library consists of four main classes:

  • lager::state<> is the single source of truth in the system. It stores the actual data and always represents the root of the graph.

  • lager::cursor<> is a node of the graph. A cursor connects to the state and track all of its updates. One can read or write into the cursor and the value will be propagated up the tree:

    // create state with automatic updates
    lager::state<KisScatterOptionData, lager::automatic_tag> optionState;
    // connect to one specific subvalue of the state
    lager::cursor<qreal> strength =
    // read the linked value
    // write the linked value
    // subscribe to the linked value updates
    // (please note that lager also has a way to connect via
    // native Qt signals)
  • lager::reader<> and lager::writer<> work in the same way as cursors, but for read-only and write-only access types

On-the-fly value transformations

When creating a node with a cursor one can not only access member variables, but also do transformations on the fly!

lager::state<KisScatterOptionData, lager::automatic_tag> optionState;

// connect to one specific subvalue of the state
lager::cursor<qreal> strength =

// create a cursor that automatically scales the strength value from 0...1 range
// to 0...100
lager::cursor<qreal> scaledStrength =

Here we use a .zoom() expression with a lens that implements conversion of the value in both directions. That is, when scaledStrength value is read, the lens multiplies the source value by 100.0. When scaledStrength is written, it automatically divides the new value by 100.0 before writing into the source.

Value aggregation and “effectiveValue” pattern

In some cases one needs to combine multiple cursors coming from different sources. For example, Lightness Strength option’s checked state depends on the two separate values:

  • whether the user checked it using the checkbox

  • whether Lightness Strength is actually supported by the brush

When the brush does not support Lightness Strength, then the option is unchecked and disabled. That can be written in Lager using the lager::with() expression:

lager::state<KisLightnessStrengthOptionData, lager::automatic_tag> optionState;

// the cursor provided by the brush option externally
lager::cursor<bool> allowedByTheBrush = ...;

// connect to the user-set value
lager::cursor<bool> isCheckedByUser =

// combine the two cursors using logical-and operator into
// an "effective" isChecked value;
lager::reader<bool> effectiveIsChecked =

    // `lager::with()` expression combines multiple cursors into one tuple

    lager::with(allowedByTheBrush, isCheckedByUser)

    // `.map()` expression accepts a standard function or functor which is used to
    // transform the source cursor on-the-fly


We use such “effectiveValue” design a lot. It is the main tool against the cycling dependencies. The point is, we cannot assign anything to isCheckedByUser from within the update, it would create a cycling dependency:

// piping one cursor into another creates loops, don't do this!

Such design has a small complication though. This “effective” value is no longer serialized by KisScatterOptionData automatically, since it is not present in KisScatterOptionData. To overcome this issue we use the process of “baking” the model into the data. This process will be explained later.

Combining value transformations

Lager performs value transformations via so called transducers. Transducer is a special form of a lambda expression that allows combining multiple operations into a single C++ entity, which can be manipulated later. Standard transducers for Lager are provided by zug library (check official documentation for zug). Krita also provides a set of useful transducers in KisZug.h.

Let’s check an example from KisPredefinedBrushModel.h. Our brightness adjustment is stored in a form of a qreal value with range 0…1, but the GUI widget shows it as an integer percentage value in range 0…100. Here is an example of how we can link these values with Lager:

struct PredefinedBrushData
    // source value is `qreal`!
    qreal brightnessAdjustment {0.0};

// destination value is `int`!
lager::cursor<int> brightnessAdjustment =


        // `xform` expression accepts two transducers that transform the expression
        // on-the-fly. The first transducer is a "getter", the second is a "setter"


            // getter: multiply the value by 100.0 and then round it to the nearest
            //         integer

            kiszug::map_mupliply<qreal>(100.0) | kiszug::map_round,

            // setter: cast integer into a `qreal` and scale back into 0...1 range

            kiszug::map_static_cast<qreal> | kiszug::map_mupliply<qreal>(0.01));

Extending value types

The value oriented design has one non-obvious complication. Since we want all the values to be easily assignable and comparable, we can use no polymorphism. Basically, virtual functions are prohibited in the “values” we operate with.

Consequently, if we need to extend some type, e.g. KisCurveOptionData, we cannot do that by overriding virtual methods (what we would do in the old design). Instead we should combine KisCurveOptionData with extra data using composition or inheritance. Here is an example of how we do that for KisScatterOptionData:

// Define the scatter-specific options in a separate mixin class that
// implements all standard operations: equality comparison, read and write

struct KisScatterOptionMixIn
    : boost::equality_comparable<KisScatterOptionMixInImpl>
    friend bool operator==(const KisScatterOptionMixInImpl &lhs,
                                const KisScatterOptionMixInImpl &rhs);

    bool axisX {true};
    bool axisY {true};

    bool read(const KisPropertiesConfiguration *setting);
    void write(KisPropertiesConfiguration *setting) const;

// Combine this mixin class with KisCurveOptionData and manually forward
// all the main operators to the parent classes

struct KisScatterOptionData
    : KisCurveOptionData,
    , KisScatterOptionMixIn
    , boost::equality_comparable<KisScatterOptionData>
        : KisCurveOptionData(KoID("Scatter", i18n("Scatter")))

    friend bool operator==(const KisScatterOptionMixInImpl &lhs,
                           const KisScatterOptionMixInImpl &rhs)
        return static_cast<const KisCurveOptionData&>(lhs) ==
               static_cast<const KisCurveOptionData&>(rhs)
               static_cast<const KisScatterOptionMixIn&>(lhs) ==
               static_cast<const KisScatterOptionMixIn&>(rhs);

    bool read(const KisPropertiesConfiguration *setting) {
        return KisCurveOptionData::read(setting) &&
    void write(KisPropertiesConfiguration *setting) const {

In this example we manually define a class that combines our scatter-specific mixin class with the base KisCurveOptionData. You see it requires a lot of boiler-plate code. Hence there is a special tool to do such composition automatically :)

// Combine the mixin class with KisCurveOptionData using a special tool class
// KisOptionTuple. It inherits from all its template parameters and automatically
// implements equality comparison, read and write operators.

struct KisScatterOptionData : KisOptionTuple<KisCurveOptionData,
        : KisOptionTuple<KisCurveOptionData,
                        KisScatterOptionMixIn>(KoID("Scatter", i18n("Scatter")))


Even though virtual function are prohibited, we still use them in one place, KisDynamicSensor. KisDynamicSensor is a representation of a single sensor in KisCurveOptionData and it is somewhat polymorphic. But these polymorphic sensors are fully contained inside a single curve option. They are created internally and none of their pointers are ever exposed to the outer world.

Official documentation

How all this applies to Krita?

From the previous chapters you know that each option in Krita has four responsibilities:

  1. read/write serialized XML or properties data

  2. define dependencies between properties of the option and other options, for example

  3. show options in the GUI as Qt’s widgets

  4. apply the actual effect of the option to the stroke on the canvas

The problem of the old implementation was that all of them were implemented in a single class, which was hard to maintain and extent.

In the Lager-based implementation each option now has five different entities that map to these responsibilities cleanly:

  1. Data reads/writes to/from XML or properties; has no logic inside!

  2. State — the single source of truth of the system. It just wraps Data into lager::state<Data> and brings it into the world of Lager.

  3. Model models all dependencies between brush settings and other options; it implements all the logic of the option.

  • a model is connected to its state via lager::cursor<>

  • a model creates a Qt Property for each brush setting so we could connect it either to a widget or QML control

  1. Widget implements an actual widget for the option

  • a widget connects to model’s Qt Properties using KisWidgetConnectionUtils. In the future QML controls will be connected to these properties directly.

  • widgets have no logic inside!

  1. Option is used by KisPaintOp to apply the actual effect to the brush stroke. Options do not depend on any Lager or GUI classes, they only use Data objects to actually read the data.

A complete example from Krita

Let’s consider KisPaintingModeOption as a simple example. This option is used to select brush painting mode and has only one setting that can flip between two values: build-up and wash.

Brush painting mode selection in the GUI

‘Data’ for “painting mode” option

First define a Data structure that implements equality comparison, read and write operators:

enum class enumPaintingMode {

struct KisPaintingModeOptionData
    : boost::equality_comparable<KisPaintingModeOptionData>
    inline friend bool operator==(const KisPaintingModeOptionData &lhs,
                                  const KisPaintingModeOptionData &rhs);

    enumPaintingMode paintingMode { enumPaintingMode::BUILDUP };

    bool read(const KisPropertiesConfiguration *setting);
    void write(KisPropertiesConfiguration *setting) const;

‘Model’ for “painting mode” option

Now let’s implement a model for this option. Painting mode has a minor complication: it is available only when masking brush feature is disabled. When the user enables masking brush feature, the painting mode option becomes disabled and selects WASH mode automatically.


The code below uses LAGER_QT_CURSOR macro. It defines a cursor of the provided type, creates a Qt Property with the provided name and links it to the cursor. To access the cursor later we should write LAGER_QT(propertyName).

namespace {
int calcEffectivePaintingMode(enumPaintingMode mode, bool maskingBrushEnabled) {
    return static_cast<int>(maskingBrushEnabled ? enumPaintingMode::WASH : mode);

class KisPaintingModeOptionModel : public QObject

    // declare cursors of the model

    lager::cursor<KisPaintingModeOptionData> optionData;
    lager::reader<bool> maskingBrushEnabled;

    // Define option settings and create Qt Properties for them:

    // paintingMode is the mode selected by the user in the GUI

    LAGER_QT_CURSOR(int, paintingMode);

    // effectivePaintingMode is the actual mode used by the brush
    // calculated from the combination of user selection and the
    // masking brush presence

    LAGER_QT_READER(int, effectivePaintingMode);

    // A special property type that updates a state (isEnabled + currentIndex)
    // of a button group in a single signal call. It is useful to avoid partial
    // updates that can lead to cycles in some cases.

    LAGER_QT_READER(ButtonGroupState, paintingModeState);

    // The constructor of the model accepts two cursors. `optionData` is stored in
    // an external 'state'; `maskingBrushEnabled` cursor is provided by masking
    // brush option

    KisPaintingModeOptionModel(lager::cursor<KisPaintingModeOptionData> _optionData,
                               lager::reader<bool> _maskingBrushEnabled)
        : optionData(_optionData)
        , maskingBrushEnabled(_maskingBrushEnabled)

        // in paintingMode cursor we just erase the enum type to be able
        // to make connection to QGroupBox

        , LAGER_QT(paintingMode) {
                .zoom(kiszug::lenses::do_static_cast<enumPaintingMode, int>)

        // effectivePaintingMode depends on both inputs of the model

        , LAGER_QT(effectivePaintingMode) {

        // combine two properties into one state

        , LAGER_QT(paintingModeState) {

    // bakedOptionData() creates a new 'Data' objects that has all
    // the "effective" values baked into it.

    KisPaintingModeOptionData bakedOptionData() const
        KisPaintingModeOptionData data = optionData.get();
        data.paintingMode = static_cast<enumPaintingMode>(effectivePaintingMode());
        return data;

Please pay attention to bakedOptionData() method of the model. The model has one “effective” property that is not directly stored in its Data storage. Therefore, before serializing the model, we should first bake all the “effective” values into the data object and then use this new object for actual writing. Granted copying option’s data objects is cheap and easy now.

‘Widget’ for “painting mode” option

Finally, let’s consider a simplified version of the code in KisPaintingModeOptionWidget:

class KisPaintingModeOptionWidget : public KisPaintOpOption
    KisPaintingModeOptionWidget(lager::cursor<KisPaintingModeOptionData> optionData,
                                lager::reader<bool> maskingBrushEnabled)
        : m_model(optionData, maskingBrushEnabled)
        // for connectControlState()
        using namespace KisWidgetConnectionUtils;

        // Create the main widget

        KisPaintingModeWidget *widget = new KisPaintingModeWidget();

        // Create the button group for mode selection

        QButtonGroup *group = new QButtonGroup(widget);

        // .. skipped ..
        // .. initialize group and add actual buttons to it ...
        // .. skipped ..

        // Connect the group to the model: "paintingModeState" is the
        // "read" property, "paintingMode" is "write" property. We read
        // from "effective" property and write directly into 'data'.

        connectControlState(group, &m_model,

        // connect the changes in the model to the output signal
        // of the configuration page

            std::bind(&KisPaintingModeOptionWidget::emitSettingChanged, this));

    void writeOptionSetting(KisPropertiesConfigurationSP setting) const override
        // write **baked** data!

    void readOptionSetting(const KisPropertiesConfigurationSP setting) override
        KisPaintingModeOptionData data = *m_model.optionData;;

    KisPaintingModeOptionModel m_model;

‘Option’ for “mirror” option

Since painting mode is very simple, it doesn’t have any Option representation. The brush engine uses its Data object directly.

For a good example of an ‘option’ let’s consider KisMirrorOption. This class is used by the brush engine while painting the actual stroke of the canvas. The responsibility of KisMirrorOption is to accept the state of the stylus (in a form of KisPaintInformation object) and calculate MirrorProperties from it.

#include <KisPaintOpOptionUtils.h>
namespace kpou = KisPaintOpOptionUtils;

class KisMirrorOption : public KisCurveOption

    // The public constructor creates a data object from
    // the settings pointer and passes it to a private constructor
    // that initializes all the necessary state

    KisMirrorOption(const KisPropertiesConfiguration *setting)
        : KisMirrorOption(


    // The private constructor initializes all the necessary state
    // from the data and passes it to the base option class.
    // Please note that the data is **not** stored anywhere in the
    // option, it is used only during the initialization

    KisMirrorOption(const KisMirrorOptionData &data)
        : KisCurveOption(data)
        , m_enableHorizontalMirror(data.enableHorizontalMirror)
        , m_enableVerticalMirror(data.enableVerticalMirror)


    MirrorProperties apply(const KisPaintInformation &info) const
        // ...
        // skipped some calculations using:
        //   * m_enableHorizontalMirror
        //   * m_enableVerticalMirror
        //   * KisCurveOption::computeSizeLikeValue(info)
        // ...

        MirrorProperties mirrors;

        mirrors.verticalMirror = ...;
        mirrors.horizontalMirror = ...;
        mirrors.coordinateSystemFlipped = ...;

        return mirrors;

    bool m_enableHorizontalMirror;
    bool m_enableVerticalMirror;

Paint engine porting guide

When porting is it recommended to use KisBrushOp as an reference implementation.

The rough plan for porting an arbitrary painting engine FooOp to lager is the following:

  1. Port the GUI part

  1. Open KisFooOpSettingsWidget class and look at its constructor that creates all the option widgets.

  2. Replace all standard option widgets with the already ported ones. Use KisBrushOpSettingsWidget as a reference of existing widgets.

  3. Test if GUI still works correctly and affects the brush in an expected way

  4. Port all non-standard options to lager and add them to KisFooOpSettingsWidget. Usually, old and new class names map as the following:

  • KisFooBarOptionData usually borrows reading and writing code from KisPressureFooBarOption

  • KisFooBarOptionModel is just written from scratch

  • KisFooBarOptionWidget borrows GUI code from KisPressureFooBarOptionWidget

Use KisScatterOptionData, KisScatterOptionModel and KisScatterOptionWidget as a reference implementation.

  1. Test if GUI still works correctly and affects the brush in an expected way

  1. Port the painting part

  1. Open KisFooOp

  2. Replace all standard KisPressureFooBarOption classes with the already ported ones. Use KisBrushOp as a reference of existing options.

  3. Port all non-standard options to lager: you just need to extract KisPressureFooBarOption::apply() function into a separate class named KisFooBarOption. Use KisScatterOption as a reference implementation.

  4. Test if the brush still reacts to the GUI changes in an expected way

  1. Check if any of the options you ported had KisPressureFooBarOption::lodLimitation() method. If so, port these limitations to your new KisFooBarOptionData and use a special creation function KisPaintOpOptionWidgetUtils::createOptionWidgetWithLodLimitations() to create a widget for it. Use KisSizeOptionData and KisSizeOptionWidget as a reference implementation.

  2. If any new brush option has “effective” values, verify that you have KisFooBarOptionModel::bakedOptionData() method in the model and calls it from KisFooBarOptionWidget::writeOptionSetting() in the widget.

  3. Open KisFooOpSettings and port all the uniform properties to use new data classes. Use KisColorSmudgeOpSettings as a reference implementation.