How to change the display resolution of reference design "HDMI Display controller Tutorial for Vivado using Xilinx VIPP"

Hello everyone,
I has successfully implemented reference design "Zedboard HDMI Display Controller Tutorial for Vivado using Xilinx VIPP" on Zedboard with HDMI out at VIDEO_RESOLUTION_1080P.
The reference design supports many resolutions, however when I tried to modify the resolution to 480p, there's nothing showed on the monitor, it displayed "No Signal" only. I edited the resolution here:

   pDemo->hdmio_resolution = VIDEO_RESOLUTION_1080P;

   pDemo->hdmio_width  = 1920;

   pDemo->hdmio_height = 1080;
Could someone help me on how to change the display resolution?
Thank you.