diff --git a/src/kwindoweffects.h b/src/kwindoweffects.h
--- a/src/kwindoweffects.h
+++ b/src/kwindoweffects.h
@@ -127,7 +127,7 @@
  *
  * @param window The window for which to enable the blur effect
  * @param enable Enable the effect if @c true, disable it if @c false
- * @param region The region within the window where the background will be blurred
+ * @param region The region within the window where the background will be blurred, specified in logical pixels
  */
 KWINDOWSYSTEM_EXPORT void enableBlurBehind(WId window, bool enable = true, const QRegion &region = QRegion());
 
@@ -149,7 +149,7 @@
  * @param window The window for which to enable the background contrast effect
  * @param enable Enable the effect if @c true, disable it if @c false
  * @param brightness How to modify the area brightness: from 0 (make it black) to 2 (make it white), 1 leaves it unchanged
- * @param region The region within the window where the background will be modified
+ * @param region The region within the window where the background will be modified, specified in logical pixels
  */
 KWINDOWSYSTEM_EXPORT void enableBackgroundContrast(WId window, bool enable = true, qreal contrast = 1, qreal intensity = 1, qreal saturation = 1, const QRegion &region = QRegion());
 
diff --git a/src/platforms/xcb/kwindoweffects.cpp b/src/platforms/xcb/kwindoweffects.cpp
--- a/src/platforms/xcb/kwindoweffects.cpp
+++ b/src/platforms/xcb/kwindoweffects.cpp
@@ -19,6 +19,7 @@
 #include "kwindoweffects_x11.h"
 
 #include <QVarLengthArray>
+#include <QGuiApplication>
 
 #include "kwindowsystem.h"
 #include <config-kwindowsystem.h>
@@ -260,7 +261,9 @@
         QVector<uint32_t> data;
         data.reserve(rects.count() * 4);
         Q_FOREACH (const QRect &r, rects) {
-            data << r.x() << r.y() << r.width() << r.height();
+            // kwin on X uses device pixels, convert from logical
+            auto dpr = qApp->devicePixelRatio();
+            data << r.x() * dpr << r.y() * dpr << r.width() * dpr << r.height() * dpr;
         }
 
         xcb_change_property(c, XCB_PROP_MODE_REPLACE, window, atom->atom, XCB_ATOM_CARDINAL,
@@ -285,7 +288,8 @@
         QVector<uint32_t> data;
         data.reserve(rects.count() * 4 + 16);
         Q_FOREACH (const QRect &r, rects) {
-            data << r.x() << r.y() << r.width() << r.height();
+            auto dpr = qApp->devicePixelRatio();
+            data << r.x() * dpr << r.y() * dpr << r.width() * dpr << r.height() * dpr;
         }
 
         QMatrix4x4 satMatrix; //saturation