diff --git a/common/rfb/PixelFormat.cxx b/common/rfb/PixelFormat.cxx
index 9e762d9..366c0a3 100644
--- a/common/rfb/PixelFormat.cxx
+++ b/common/rfb/PixelFormat.cxx
@@ -32,6 +32,42 @@
 
 using namespace rfb;
 
+rdr::U8 PixelFormat::upconvTable[256*8];
+
+class PixelFormat::Init {
+public:
+  Init();
+};
+
+PixelFormat::Init PixelFormat::_init;
+
+
+PixelFormat::Init::Init()
+{
+  int bits;
+
+  // Bit replication is almost perfect, but not quite. And
+  // a lookup table is still quicker when there is a large
+  // difference between the source and destination depth.
+
+  for (bits = 1;bits <= 8;bits++) {
+    int i, maxVal;
+    rdr::U8 *subTable;
+
+    maxVal = (1 << bits) - 1;
+    subTable = &upconvTable[(bits-1)*256];
+
+    for (i = 0;i <= maxVal;i++)
+      subTable[i] = i * 255 / maxVal;
+
+    // Duplicate the table so that we don't have to care about
+    // the upper bits when doing a lookup
+    for (;i < 256;i += maxVal+1)
+      memcpy(&subTable[i], &subTable[0], maxVal+1);
+  }
+}
+
+
 PixelFormat::PixelFormat(int b, int d, bool e, bool t,
                          int rm, int gm, int bm, int rs, int gs, int bs)
   : bpp(b), depth(d), trueColour(t), bigEndian(e),