added viewer instruction.

README.md

@@ -32,6 +32,7 @@ Optional :
 * generate Makefiles :
     cd OpenSubdiv
     mkdir build
+    cd build
     cmake ..
 
 * build the project :
@@ -49,6 +50,22 @@ Optional :
 
 </code></pre>
 
+### Standalone viewer ###
+
+<pre><code>
+* To run viewer:
+    bin/viewer
+* Usage:
+    Left mouse button drag   : orbit camera
+    Middle mouse button drag : dolly camera
+    Right mouse button       : show popup menu
+    n, p                     : next/prev model
+    1, 2, 3, 4, 5, 6, 7      : specify subdivision level
+    w                        : switch display mode
+    q                        : quit
+</code></pre>
+
+
 ## Why fast subdivision? ##
 
 Subdivision surfaces are commonly used for final rendering of character shapes for a smooth and controllable limit surfaces. However, subdivision surfaces in interactive apps are typically drawn as their polygonal control hulls because of performance.  The polygonal control hull is an approximation that is offset from the true limit surface,  Looking at an approximation in the interactive app makes it difficult to see exact contact, like fingers touching a potion bottle or hands touching a cheek.  It also makes it difficult to see poke throughs in cloth simulation if the skin and cloth are both approximations.  This problem is particularly bad when one character is much larger than another and unequal subdiv face sizes cause approximations errors to be magnified.