diff --git a/effects/CMakeLists.txt b/effects/CMakeLists.txt
index b187624b03..7fe559e275 100644
--- a/effects/CMakeLists.txt
+++ b/effects/CMakeLists.txt
@@ -113,13 +113,15 @@ if(OPENGL_FOUND)
         DESTINATION  ${DATA_INSTALL_DIR}/kwin )
 endif(OPENGL_FOUND)
 
-# showfps - need both xrender and opengl
+# showfps, showpaint - need both xrender and opengl
 if( OPENGL_FOUND AND X11_Xrender_FOUND )
     SET(kwin4_effect_builtins_sources ${kwin4_effect_builtins_sources}
         showfps.cpp
+        showpaint.cpp
         )
     install( FILES
         showfps.desktop
+        showpaint.desktop
         DESTINATION ${SERVICES_INSTALL_DIR}/kwin )
 endif( OPENGL_FOUND AND X11_Xrender_FOUND )
 
diff --git a/effects/showpaint.cpp b/effects/showpaint.cpp
new file mode 100644
index 0000000000..043e530641
--- /dev/null
+++ b/effects/showpaint.cpp
@@ -0,0 +1,101 @@
+/*****************************************************************
+ KWin - the KDE window manager
+ This file is part of the KDE project.
+
+Copyright (C) 2007 Lubos Lunak <l.lunak@kde.org>
+
+You can Freely distribute this program under the GNU General Public
+License. See the file "COPYING" for the exact licensing terms.
+******************************************************************/
+
+#include "showpaint.h"
+
+#include <config-X11.h>
+
+#ifdef HAVE_OPENGL
+#include <GL/gl.h>
+#endif
+#ifdef HAVE_XRENDER
+#include <X11/Xlib.h>
+#include <X11/extensions/Xrender.h>
+#endif
+
+#include <math.h>
+
+#include <qcolor.h>
+
+namespace KWin
+{
+
+KWIN_EFFECT( showpaint, ShowPaintEffect )
+
+static QColor colors[] = { Qt::red, Qt::green, Qt::blue, Qt::cyan, Qt::magenta,
+    Qt::yellow, Qt::gray };
+
+ShowPaintEffect::ShowPaintEffect()
+    : color_index( 0 )
+    {
+    }
+
+void ShowPaintEffect::paintScreen( int mask, QRegion region, ScreenPaintData& data )
+    {
+    painted = QRegion();
+    effects->paintScreen( mask, region, data );
+#ifdef HAVE_OPENGL
+    if( effects->compositingType() == OpenGLCompositing)
+        paintGL();
+#endif
+#ifdef HAVE_XRENDER
+    if( effects->compositingType() == XRenderCompositing)
+        paintXrender();
+#endif
+    if( ++color_index == sizeof( colors ) / sizeof( colors[ 0 ] ))
+        color_index = 0;
+    }
+
+void ShowPaintEffect::paintWindow( EffectWindow* w, int mask, QRegion region, WindowPaintData& data )
+    {
+    painted |= region;
+    effects->paintWindow( w, mask, region, data );
+    }
+
+// TODO I think we need some kind of generic paintRect()
+void ShowPaintEffect::paintGL()
+    {
+#ifdef HAVE_OPENGL
+    glPushAttrib( GL_CURRENT_BIT | GL_ENABLE_BIT );
+    glEnable( GL_BLEND );
+    glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );
+    float alpha = 0.2;
+    const QColor& color = colors[ color_index ];
+    glColor4f( color.red() / 255., color.green() / 255., color.blue() / 255., alpha );
+    glBegin( GL_QUADS );
+    foreach( QRect r, painted.rects())
+        {
+        glVertex2i( r.x(), r.y());
+        glVertex2i( r.x() + r.width(), r.y());
+        glVertex2i( r.x() + r.width(), r.y() + r.height());
+        glVertex2i( r.x(), r.y() + r.height());
+        }
+    glEnd();
+    glPopAttrib();
+#endif
+    }
+
+void ShowPaintEffect::paintXrender()
+    {
+#ifdef HAVE_XRENDER
+    XRenderColor col;
+    int alpha = 0.2;
+    const QColor& color = colors[ color_index ];
+    col.alpha = int( alpha * 0xffff );
+    col.red = int( alpha * 0xffff * color.red() / 255 );
+    col.green = int( alpha * 0xffff * color.green() / 255 );
+    col.blue= int( alpha * 0xffff * color.blue() / 255 );
+    foreach( QRect r, painted.rects())
+        XRenderFillRectangle( display(), PictOpOver, effects->xrenderBufferPicture(),
+            &col, r.x(), r.y(), r.width(), r.height());
+#endif
+    }
+
+} // namespace
diff --git a/effects/showpaint.desktop b/effects/showpaint.desktop
new file mode 100644
index 0000000000..eb01923f9d
--- /dev/null
+++ b/effects/showpaint.desktop
@@ -0,0 +1,16 @@
+[Desktop Entry]
+Encoding=UTF-8
+Name=Show Paint
+Comment=Shows areas painted by KWin
+
+Type=Service
+ServiceTypes=KWin/Effect
+X-KDE-PluginInfo-Author=Luboš Luňák
+X-KDE-PluginInfo-Email=l.lunak@kde.org
+X-KDE-PluginInfo-Name=kwin4_effect_showpaint
+X-KDE-PluginInfo-Version=0.1.0
+X-KDE-PluginInfo-Category=Misc
+X-KDE-PluginInfo-Depends=
+X-KDE-PluginInfo-License=GPL
+X-KDE-PluginInfo-EnabledByDefault=false
+X-KDE-Library=kwin4_effect_builtins
diff --git a/effects/showpaint.h b/effects/showpaint.h
new file mode 100644
index 0000000000..4c821d49a1
--- /dev/null
+++ b/effects/showpaint.h
@@ -0,0 +1,35 @@
+/*****************************************************************
+ KWin - the KDE window manager
+ This file is part of the KDE project.
+
+Copyright (C) 2007 Lubos Lunak <l.lunak@kde.org>
+
+You can Freely distribute this program under the GNU General Public
+License. See the file "COPYING" for the exact licensing terms.
+******************************************************************/
+
+#ifndef KWIN_SHOWPAINT_H
+#define KWIN_SHOWPAINT_H
+
+#include <kwineffects.h>
+
+namespace KWin
+{
+
+class ShowPaintEffect
+    : public Effect
+    {
+    public:
+        ShowPaintEffect();
+        virtual void paintScreen( int mask, QRegion region, ScreenPaintData& data );
+        virtual void paintWindow( EffectWindow* w, int mask, QRegion region, WindowPaintData& data );
+    private:
+        void paintGL();
+        void paintXrender();
+        QRegion painted; // what's painted in one pass
+        int color_index;
+    };
+
+} // namespace
+
+#endif