--- rxvt-unicode/src/rxvtimg.C	2012/06/09 11:13:14	1.67
+++ rxvt-unicode/src/rxvtimg.C	2012/06/09 11:41:28	1.70
@@ -358,7 +358,8 @@
   Display *dpy = s->display->dpy;
   Picture dst = XRenderCreatePicture (dpy, pm, format, 0, 0);
 
-  while (r | g | b | a)
+  // loop should not be needed for brightness, as only -1..1 makes sense
+  //while (r | g | b | a)
     {
       unsigned short xr, xg, xb, xa;
       XRenderColor mask_c;
@@ -370,6 +371,10 @@
         {
           XRenderColor mask_w = { 65535, 65535, 65535, 65535 };
           XRenderFillRectangle (dpy, PictOpDifference, dst, &mask_w, 0, 0, w, h);
+          mask_c.red   = -mask_c.red; //TODO: verify that doing clamp, assign, and negation does the right thing
+          mask_c.green = -mask_c.green;
+          mask_c.blue  = -mask_c.blue;
+          mask_c.alpha = -mask_c.alpha;
           XRenderFillRectangle (dpy, PictOpAdd, dst, &mask_c, 0, 0, w, h);
           XRenderFillRectangle (dpy, PictOpDifference, dst, &mask_w, 0, 0, w, h);
         }