How can sand dunes be represented in cityengine ?
Or maybe you will need a good terrain and texture(satellite imagery) like https://www.arcgis.com/home/item.html?id=34077131bff7470e892951862df655fd
Where do I get clear terrain and how do I apply it to my city?
Like L R suggested creating DEM is good way to start. There is plethora of applications for height map generation so something like Height Map Editor may be good for one off job.
Are you using CE 2017?
There´s a option called Get Map Data, then it´s possible to import a good terrain+texture from ArcGIS Online.
Another option SRTM it´´s a Free Digital Elevation Model(DEM) provided by NASA.
how can I make video from CE PLz?
The easiest solution would be to just draw the height layer with an image editor (like Photoshop). Create a greyscale image, paint it black and draw with a soft white brush on a layer on top of it. The brighter, the taller the terrain.
i can't understand can you explain, or any video explain this step plz?
There isn't much to explain. Make a new image, go Image > Mode > Greyscale. Then paint whatever you want with a white brush, save the image and open it in CE: