diff --git a/libkwineffects/kwingltexture.cpp b/libkwineffects/kwingltexture.cpp
index ab38883d88..06c2ed8246 100644
--- a/libkwineffects/kwingltexture.cpp
+++ b/libkwineffects/kwingltexture.cpp
@@ -47,6 +47,7 @@ bool GLTexturePrivate::s_supportsARGB32 = false;
 bool GLTexturePrivate::s_supportsUnpack = false;
 bool GLTexturePrivate::s_supportsTextureStorage = false;
 bool GLTexturePrivate::s_supportsTextureSwizzle = false;
+bool GLTexturePrivate::s_supportsTextureFormatRG = false;
 uint GLTexturePrivate::s_textureObjectCounter = 0;
 uint GLTexturePrivate::s_fbo = 0;
 
@@ -290,12 +291,16 @@ void GLTexturePrivate::initStatic()
             hasGLExtension("GL_ARB_framebuffer_object") || hasGLExtension(QByteArrayLiteral("GL_EXT_framebuffer_object"));
         s_supportsTextureStorage = hasGLVersion(4, 2) || hasGLExtension(QByteArrayLiteral("GL_ARB_texture_storage"));
         s_supportsTextureSwizzle = hasGLVersion(3, 3) || hasGLExtension(QByteArrayLiteral("GL_ARB_texture_swizzle"));
+        // see https://www.opengl.org/registry/specs/ARB/texture_rg.txt
+        s_supportsTextureFormatRG = hasGLVersion(3, 0) || hasGLExtension(QByteArrayLiteral("GL_ARB_texture_rg"));
         s_supportsARGB32 = true;
         s_supportsUnpack = true;
     } else {
         s_supportsFramebufferObjects = true;
         s_supportsTextureStorage = hasGLVersion(3, 0) || hasGLExtension(QByteArrayLiteral("GL_EXT_texture_storage"));
         s_supportsTextureSwizzle = hasGLVersion(3, 0);
+        // see https://www.khronos.org/registry/gles/extensions/EXT/EXT_texture_rg.txt
+        s_supportsTextureFormatRG = hasGLVersion(3, 0) || hasGLExtension(QByteArrayLiteral("GL_EXT_texture_rg"));
 
         // QImage::Format_ARGB32_Premultiplied is a packed-pixel format, so it's only
         // equivalent to GL_BGRA/GL_UNSIGNED_BYTE on little-endian systems.
@@ -653,4 +658,9 @@ bool GLTexture::supportsSwizzle()
     return GLTexturePrivate::s_supportsTextureSwizzle;
 }
 
+bool GLTexture::supportsFormatRG()
+{
+    return GLTexturePrivate::s_supportsTextureFormatRG;
+}
+
 } // namespace KWin
diff --git a/libkwineffects/kwingltexture.h b/libkwineffects/kwingltexture.h
index 250a273f71..3cd8f1996b 100644
--- a/libkwineffects/kwingltexture.h
+++ b/libkwineffects/kwingltexture.h
@@ -127,6 +127,15 @@ public:
      */
     static bool supportsSwizzle();
 
+    /**
+     * Returns @c true if texture formats R* are supported, and @c false otherwise.
+     *
+     * This requires OpenGL 3.0, GL_ARB_texture_rg or OpenGL ES 3.0 or GL_EXT_texture_rg.
+     *
+     * @since 5.2.1
+     **/
+    static bool supportsFormatRG();
+
 protected:
     QExplicitlySharedDataPointer<GLTexturePrivate> d_ptr;
     GLTexture(GLTexturePrivate& dd);
diff --git a/libkwineffects/kwingltexture_p.h b/libkwineffects/kwingltexture_p.h
index 656f2944fa..440d2ce942 100644
--- a/libkwineffects/kwingltexture_p.h
+++ b/libkwineffects/kwingltexture_p.h
@@ -74,6 +74,7 @@ public:
     static bool s_supportsUnpack;
     static bool s_supportsTextureStorage;
     static bool s_supportsTextureSwizzle;
+    static bool s_supportsTextureFormatRG;
     static GLuint s_fbo;
     static uint s_textureObjectCounter;
 private:
diff --git a/scene_opengl.cpp b/scene_opengl.cpp
index 7584dd5c54..cb0f469e0a 100644
--- a/scene_opengl.cpp
+++ b/scene_opengl.cpp
@@ -2252,7 +2252,7 @@ bool SceneOpenGLShadow::prepareBackend()
     p.end();
 
     // Check if the image is alpha-only in practice, and if so convert it to an 8-bpp format
-    if (!GLPlatform::instance()->isGLES() && GLTexture::supportsSwizzle()) {
+    if (!GLPlatform::instance()->isGLES() && GLTexture::supportsSwizzle() && GLTexture::supportsFormatRG()) {
         QImage alphaImage(image.size(), QImage::Format_Indexed8); // Change to Format_Alpha8 w/ Qt 5.5
         bool alphaOnly = true;