Building a Custom Timeline Editor in C++

Building a Custom Timeline Editor in C++ requires a combination of good UI design, data management, and efficient handling of user input and animations. Such an editor is often used in game engines, video editing software, animation tools, or other multimedia applications. Here’s a comprehensive guide to help you build a custom timeline editor from scratch in C++.

1. Understanding the Core Features

A timeline editor typically has several key features:

• Track Representation: Different types of media (audio, video, animations) are displayed as tracks in a timeline.

• Keyframe Management: Users can place and adjust keyframes, which represent specific states of an object at certain points in time.

• Time Scrubbing: Allow the user to scrub (move the playhead) across time to preview changes or adjust keyframes.

• Zoom and Scroll: Let users zoom in/out to adjust the granularity of time and scroll to navigate through large timelines.

• Playback Controls: Implement basic play, pause, and stop controls for previewing animations.

2. Choosing a GUI Framework

Since a timeline editor requires an interactive graphical user interface (GUI), you need to select a GUI framework. Here are a few options commonly used in C++:

• Qt: A very popular, well-documented framework for building cross-platform GUIs. It provides powerful tools for handling 2D graphics, events, and animations.

• SFML (Simple and Fast Multimedia Library): A multimedia library for creating interactive applications with graphics and audio, suitable for simple UI development.

• ImGui: A lightweight and easy-to-use graphical user interface library, great for prototyping. It’s popular in game development and can be used to quickly build custom editors.

For this tutorial, let’s assume you’re using Qt as the GUI framework because of its extensive support for UI design.

3. Basic Data Structure for Timeline

Your timeline editor needs a clear structure to store data such as keyframes, tracks, and the timeline’s metadata. A good starting point is to create classes for each component.

Keyframe Class

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class Keyframe {
public:
    float time;     // Time at which this keyframe occurs
    float value;    // The value of the object at this time (e.g., position, scale, etc.)

    Keyframe(float t, float v) : time(t), value(v) {}
};

Track Class

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class Track {
public:
    std::string name;
    std::vector<Keyframe> keyframes;

    void addKeyframe(float time, float value) {
        keyframes.push_back(Keyframe(time, value));
    }

    void removeKeyframe(float time) {
        keyframes.erase(std::remove_if(keyframes.begin(), keyframes.end(), [time](const Keyframe& k) { return k.time == time; }), keyframes.end());
    }
};

Timeline Class

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class Timeline {
public:
    std::vector<Track> tracks;
    float currentTime; // Current time of the playhead

    void addTrack(const Track& track) {
        tracks.push_back(track);
    }

    void removeTrack(const std::string& trackName) {
        tracks.erase(std::remove_if(tracks.begin(), tracks.end(), [&trackName](const Track& t) { return t.name == trackName; }), tracks.end());
    }

    void update(float deltaTime) {
        currentTime += deltaTime;
    }
};

4. Building the Timeline UI in Qt

With Qt, you can create custom widgets to represent the timeline visually. The timeline will be a custom QWidget, and you can draw tracks, keyframes, and other elements on it using the QPainter class.

Setting up the Qt project

First, make sure to include the necessary Qt modules in your .pro file:

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QT += core gui widgets

Then, create the TimelineEditorWidget class that will display the timeline.

Drawing the Timeline

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#include <QWidget>
#include <QPainter>
#include <QMouseEvent>

class TimelineEditorWidget : public QWidget {
    Q_OBJECT

public:
    TimelineEditorWidget(QWidget* parent = nullptr)
        : QWidget(parent), currentTime(0.0f) {
        setFixedSize(800, 200); // Set a fixed size for the widget
    }

protected:
    void paintEvent(QPaintEvent* event) override {
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);
        painter.setPen(QPen(Qt::black));

        // Draw timeline background
        painter.fillRect(0, 0, width(), height(), Qt::lightGray);

        // Draw time axis
        int numTicks = 10;
        int tickInterval = width() / numTicks;
        for (int i = 0; i <= numTicks; ++i) {
            int xPos = i * tickInterval;
            painter.drawLine(xPos, height() - 10, xPos, height() - 5);
            painter.drawText(xPos - 10, height() - 15, QString::number(i * 10));
        }

        // Draw keyframes
        for (const auto& track : tracks) {
            for (const auto& keyframe : track.keyframes) {
                int xPos = static_cast<int>(keyframe.time * (width() / 100.0f)); // Scale keyframe time to fit the widget width
                painter.setBrush(QBrush(Qt::blue));
                painter.drawEllipse(xPos - 5, height() / 2 - 5, 10, 10);
            }
        }

        // Draw playhead
        int playheadX = static_cast<int>(currentTime * (width() / 100.0f));
        painter.setPen(QPen(Qt::red));
        painter.drawLine(playheadX, 0, playheadX, height());
    }

    void mousePressEvent(QMouseEvent* event) override {
        int mouseX = event->x();
        currentTime = mouseX * 100.0f / width(); // Calculate time based on mouse position
        update();
    }

private:
    std::vector<Track> tracks;
    float currentTime; // Playhead position

    void addTrack(const Track& track) {
        tracks.push_back(track);
    }

    void addKeyframeToTrack(const std::string& trackName, float time, float value) {
        auto track = std::find_if(tracks.begin(), tracks.end(), [&trackName](const Track& t) {
            return t.name == trackName;
        });

        if (track != tracks.end()) {
            track->addKeyframe(time, value);
        }
    }
};

5. Handling Playback Controls

You’ll want to provide controls like play, pause, and stop. These can be added via buttons in the UI.

Playback Controls in Qt

You can create a basic control panel using QPushButton widgets for controlling the timeline.

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#include <QPushButton>
#include <QVBoxLayout>

class TimelineWindow : public QWidget {
    Q_OBJECT

public:
    TimelineWindow(QWidget* parent = nullptr)
        : QWidget(parent), timer(new QTimer(this)) {
        QVBoxLayout* layout = new QVBoxLayout(this);

        timelineWidget = new TimelineEditorWidget();
        layout->addWidget(timelineWidget);

        QPushButton* playButton = new QPushButton("Play", this);
        layout->addWidget(playButton);

        connect(playButton, &QPushButton::clicked, this, &TimelineWindow::onPlayClicked);

        timer->setInterval(33); // ~30 FPS
        connect(timer, &QTimer::timeout, this, &TimelineWindow::onUpdate);

        setLayout(layout);
    }

private slots:
    void onPlayClicked() {
        if (timer->isActive()) {
            timer->stop();
        } else {
            timer->start();
        }
    }

    void onUpdate() {
        timelineWidget->update(0.033f); // Update the timeline by 1/30th of a second
        timelineWidget->update();
    }

private:
    TimelineEditorWidget* timelineWidget;
    QTimer* timer;
};

6. Adding User Interaction

Allow users to add or remove keyframes with mouse clicks and implement the ability to drag keyframes to new positions.

• Add Keyframe: When the user clicks on the timeline, you can create a new keyframe at that position.

• Drag Keyframe: Implement mouse drag functionality to move keyframes.

This can be done by storing the keyframe’s position and tracking mouse movement to update its location.

7. Conclusion

The above guide outlines the basic structure for a custom timeline editor in C++. It involves building classes to manage the timeline data (tracks, keyframes), designing the user interface using Qt, and implementing core features like playback controls and interactive keyframe manipulation.

By building out these basic functionalities, you’ll be able to extend the editor further to include more advanced features like zooming, snapping, and even integrating audio or video playback.