Added support for drawing debug information

Press on the example app to toggle the debug information.

box2d.pro

@@ -16,9 +16,11 @@ INSTALLS += target qmldir
 
 SOURCES += box2dplugin.cpp \
     box2dworld.cpp \
-    box2dbody.cpp
+    box2dbody.cpp \
+    box2ddebugdraw.cpp
 
 HEADERS += \
     box2dplugin.h \
     box2dworld.h \
-    box2dbody.h
+    box2dbody.h \
+    box2ddebugdraw.h

box2dbody.cpp

@@ -25,8 +25,6 @@
 #include <Box2D.h>
 #include <cmath>
 
-static const float scaleRatio = 32.0f; // 32 pixels in one meter
-
 Box2DBody::Box2DBody(QDeclarativeItem *parent) :
     QDeclarativeItem(parent),
     mBody(0),

box2ddebugdraw.cpp

@@ -0,0 +1,185 @@
+/*
+ * Box2D QML plugin
+ * Copyright (C) 2010 Nokia Corporation
+ *
+ * This file is part of the Box2D QML plugin.
+ *
+ * This library is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or (at
+ * your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library;  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "box2ddebugdraw.h"
+
+#include "box2dworld.h"
+
+#include <Box2D.h>
+
+#include <QPainter>
+
+class DebugDraw : public b2DebugDraw
+{
+public:
+    DebugDraw(QPainter *painter, Box2DWorld *world);
+
+    void draw();
+
+    void DrawPolygon(const b2Vec2 *vertices, int32 vertexCount,
+                     const b2Color &color);
+    void DrawSolidPolygon(const b2Vec2 *vertices, int32 vertexCount,
+                          const b2Color &color);
+    void DrawCircle(const b2Vec2 &center, float32 radius,
+                    const b2Color &color);
+    void DrawSolidCircle(const b2Vec2 &center, float32 radius,
+                         const b2Vec2 &axis, const b2Color &color);
+    void DrawSegment(const b2Vec2 &p1, const b2Vec2 &p2,
+                     const b2Color &color);
+    void DrawTransform(const b2Transform &xf);
+
+private:
+    QPainter *mP;
+    b2World *mWorld;
+};
+
+DebugDraw::DebugDraw(QPainter *painter, Box2DWorld *world)
+    : mP(painter)
+    , mWorld(world->world())
+{
+    SetFlags(e_shapeBit |
+             e_jointBit |
+             e_aabbBit |
+             e_pairBit |
+             e_centerOfMassBit);
+}
+
+void DebugDraw::draw()
+{
+    mWorld->SetDebugDraw(this);
+    mWorld->DrawDebugData();
+    mWorld->SetDebugDraw(0);
+}
+
+static QPointF toQPointF(const b2Vec2 &vec)
+{
+    return QPointF(vec.x * scaleRatio,
+                   -vec.y * scaleRatio);
+}
+
+static QColor toQColor(const b2Color &color)
+{
+    return QColor(color.r * 255,
+                  color.g * 255,
+                  color.b * 255);
+}
+
+static QPolygonF toQPolygonF(const b2Vec2 *vertices, int32 vertexCount)
+{
+    QPolygonF polygon;
+    polygon.reserve(vertexCount);
+
+    for (int i = 0; i < vertexCount; ++i)
+        polygon.append(toQPointF(vertices[i]));
+
+    return polygon;
+}
+
+void DebugDraw::DrawPolygon(const b2Vec2 *vertices, int32 vertexCount,
+                            const b2Color &color)
+{
+    mP->setPen(toQColor(color));
+    mP->setBrush(Qt::NoBrush);
+    mP->drawPolygon(toQPolygonF(vertices, vertexCount));
+}
+
+void DebugDraw::DrawSolidPolygon(const b2Vec2 *vertices, int32 vertexCount,
+                                 const b2Color &color)
+{
+    mP->setPen(Qt::NoPen);
+    mP->setBrush(toQColor(color));
+    mP->drawPolygon(toQPolygonF(vertices, vertexCount));
+}
+
+void DebugDraw::DrawCircle(const b2Vec2 &center, float32 radius,
+                           const b2Color &color)
+{
+    mP->setPen(toQColor(color));
+    mP->setBrush(Qt::NoBrush);
+    mP->drawEllipse(toQPointF(center),
+                    radius * scaleRatio,
+                    radius * scaleRatio);
+}
+
+void DebugDraw::DrawSolidCircle(const b2Vec2 &center, float32 radius,
+                                const b2Vec2 &axis, const b2Color &color)
+{
+    Q_UNUSED(axis)
+
+    mP->setPen(Qt::NoPen);
+    mP->setBrush(toQColor(color));
+    mP->drawEllipse(toQPointF(center),
+                   radius * scaleRatio,
+                   radius * scaleRatio);
+}
+
+void DebugDraw::DrawSegment(const b2Vec2 &p1, const b2Vec2 &p2,
+                            const b2Color &color)
+{
+    mP->setPen(toQColor(color));
+    mP->drawLine(toQPointF(p1), toQPointF(p2));
+}
+
+void DebugDraw::DrawTransform(const b2Transform &xf)
+{
+    Q_UNUSED(xf)
+    // TODO: Not sure how to draw transforms
+}
+
+
+Box2DDebugDraw::Box2DDebugDraw(QDeclarativeItem *parent) :
+    QDeclarativeItem(parent),
+    mWorld(0)
+{
+    setFlag(QGraphicsItem::ItemHasNoContents, false);
+}
+
+void Box2DDebugDraw::setWorld(Box2DWorld *world)
+{
+    if (mWorld == world)
+        return;
+
+    if (mWorld)
+        mWorld->disconnect(this);
+
+    mWorld = world;
+
+    if (mWorld)
+        connect(mWorld, SIGNAL(stepped()), SLOT(onWorldStepped()));
+}
+
+void Box2DDebugDraw::paint(QPainter *p, const QStyleOptionGraphicsItem *,
+                           QWidget *)
+{
+    if (!mWorld)
+        return;
+
+    // Darken the view to make the debug draw stand out more
+    p->fillRect(0, 0, width(), height(), QColor(0, 0, 0, 128));
+
+    DebugDraw debugDraw(p, mWorld);
+    debugDraw.draw();
+}
+
+void Box2DDebugDraw::onWorldStepped()
+{
+    if (isVisible() && opacity() > 0)
+        update();
+}

box2ddebugdraw.h

@@ -0,0 +1,49 @@
+/*
+ * Box2D QML plugin
+ * Copyright (C) 2010 Nokia Corporation
+ *
+ * This file is part of the Box2D QML plugin.
+ *
+ * This library is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 2.1 of the License, or (at
+ * your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library;  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef BOX2DDEBUGDRAW_H
+#define BOX2DDEBUGDRAW_H
+
+#include <QDeclarativeItem>
+
+class Box2DWorld;
+
+class Box2DDebugDraw : public QDeclarativeItem
+{
+    Q_OBJECT
+
+    Q_PROPERTY(Box2DWorld *world READ world WRITE setWorld)
+
+public:
+    explicit Box2DDebugDraw(QDeclarativeItem *parent = 0);
+
+    Box2DWorld *world() const { return mWorld; }
+    void setWorld(Box2DWorld *world);
+
+    void paint(QPainter *, const QStyleOptionGraphicsItem *, QWidget *);
+
+private slots:
+    void onWorldStepped();
+
+private:
+    Box2DWorld *mWorld;
+};
+
+#endif // BOX2DDEBUGDRAW_H

box2dplugin.cpp

@@ -22,6 +22,7 @@
 
 #include "box2dworld.h"
 #include "box2dbody.h"
+#include "box2ddebugdraw.h"
 
 Box2DPlugin::Box2DPlugin(QObject *parent) :
     QDeclarativeExtensionPlugin(parent)
@@ -32,6 +33,7 @@ void Box2DPlugin::registerTypes(const char *uri)
 {
     qmlRegisterType<Box2DWorld>(uri, 2, 0, "World");
     qmlRegisterType<Box2DBody>(uri, 2, 0, "Body");
+    qmlRegisterType<Box2DDebugDraw>(uri, 2, 0, "DebugDraw");
 }
 
 Q_EXPORT_PLUGIN2(Box2DPlugin, Box2DPlugin)

box2dworld.cpp

@@ -97,6 +97,7 @@ void Box2DWorld::timerEvent(QTimerEvent *event)
         mWorld->ClearForces();
         foreach (Box2DBody *body, mBodies)
             body->synchronize();
+        emit stepped();
     }
     QDeclarativeItem::timerEvent(event);
 }

box2dworld.h

@@ -28,6 +28,10 @@ class Box2DBody;
 
 class b2World;
 
+// TODO: Maybe turn this into a property of the world, though it can't be
+// changed dynamically.
+static const float scaleRatio = 32.0f; // 32 pixels in one meter
+
 /**
  * Wrapper class around a Box2D world.
  */
@@ -86,8 +90,11 @@ class Box2DWorld : public QDeclarativeItem
     void registerBody(Box2DBody *body);
     void unregisterBody(Box2DBody *body);
 
+    b2World *world() const { return mWorld; }
+
 signals:
     void gravityChanged();
+    void stepped();
 
 protected:
     void timerEvent(QTimerEvent *);

examples/boxes/boxes.qml

@@ -46,6 +46,19 @@ Image {
             anchors { left: parent.right; bottom: ground.top; top: ceiling.bottom }
             bodyType: Body.Static
         }
+
+        DebugDraw {
+            id: debugDraw
+            world: world
+            anchors.fill: world
+            opacity: 0.75
+            visible: false
+        }
+        MouseArea {
+            id: debugMouseArea
+            anchors.fill: world
+            onPressed: debugDraw.visible = !debugDraw.visible
+        }
     }
 
     Accelerometer  {