matlab regional grid map

I have a global 0.25*0.25 metadata and want to get the gird map in China. Using the following code, I get the figure. How can I mask the "NaN" with White color and pick put the China as a region. R = georasterref('RasterSize', size(world), 'Latlim', [-90.000 90.000], 'Lonlim', [0 360]); geoshow(world, R, 'DisplayType', 'texturemap') caxis([0 2500]); colorbar; load coast geoshow(lat,long)