configs/same70-xplained/twm4nx/defconfig: Switch to 8BPP. That is the negotiated size requested by the VNC client anyway. graphics/vnc/server/vnc_color.c: Fix an error in color conversion.

graphics/vnc/server/Kconfig

@@ -112,8 +112,25 @@ config VNCSERVER_UPDATE_BUFSIZE
 		example, an update buffers of 32 pixels at 8-bits per pixel and
 		32-rows would yield a buffer size of 1024!
 
-		Ideally, this buffer should fit in one network packet to avoid
-		accessive re-assembly of partial TCP packets.
+		There is a very strong interaction with this setting and the network MTU.
+		Ideally, this buffer should fit in one network packet to avoid accessive
+		re-assembly of partial TCP packets.
+
+		REVISIT:  In fact, if the buffer does not fit in one network packet,
+		then there appears to be reliability issues in the connection.  I am
+		not sure why that is; TCP is a stream so it should not matter how
+		many packets are in a transfer.
+
+		Example:  Negotiated pixel depth = 8 BPP, window width = 800 pixels.
+		CONFIG_VNCSERVER_UPDATE_BUFSIZE needs to be the payload size (MSS)
+		of the transfer or 800 bytes.  The MTU is then:
+
+		  MSS = MTU - sizeof(IP Header) - sizeof(VNC FramebufferUpdate Header)
+
+		For IPv4, the IP Header is 20 bytes; 40 bytes for IPv6.  The
+		FramebufferUpdate header is 16 bytes so.  The desired MSS is 800 bytes
+		so MTU = 836 or 856.  For Ethernet, this is a total packet size of 870
+		bytes.
 
 config VNCSERVER_KBDENCODE
 	bool "Encode keyboard input"

graphics/vnc/server/vnc_color.c

@@ -155,7 +155,7 @@ uint8_t vnc_convert_rgb8_332(lfb_color_t rgb)
    *          RRRGGGBB
    */
 
-  return (uint8_t)(((rgb >> 8) & 0x0070)  |
+  return (uint8_t)(((rgb >> 8) & 0x00e0)  |
                    ((rgb >> 6) & 0x001c)  |
                    ((rgb >> 3) & 0x0003));
 }

graphics/vnc/server/vnc_keymap.c

@@ -651,4 +651,4 @@ void vnc_kbdout(FAR void *arg, uint8_t nch, FAR const uint8_t *ch)
   (void)nx_kbdin((NXHANDLE)arg, nch, ch);
 }
 
-#endif /* CONFIG_NX_KBD */
+#endif /* CONFIG_NX_KBD */

graphics/vnc/server/vnc_raw.c

@@ -164,7 +164,8 @@ static size_t vnc_copy16(FAR struct vnc_session_s *session,
 
   /* Source rectangle start address (left/top)*/
 
-  srcleft = (FAR lfb_color_t *)(session->fb + RFB_STRIDE * row + RFB_BYTESPERPIXEL * col);
+  srcleft = (FAR lfb_color_t *)
+    (session->fb + RFB_STRIDE * row + RFB_BYTESPERPIXEL * col);
 
   /* Transfer each row from the source buffer into the update buffer */
 
@@ -235,7 +236,8 @@ static size_t vnc_copy32(FAR struct vnc_session_s *session,
 
   /* Source rectangle start address (left/top)*/
 
-  srcleft = (FAR lfb_color_t *)(session->fb + RFB_STRIDE * row + RFB_BYTESPERPIXEL * col);
+  srcleft = (FAR lfb_color_t *)
+    (session->fb + RFB_STRIDE * row + RFB_BYTESPERPIXEL * col);
 
   /* Transfer each row from the source buffer into the update buffer */