For build an environment,I used to create a geometry sphere as background then assign a material with a bitmap as diffuse color to it.

If build a sky system without create a sphere just like vray sun and vray sky or assign a backgrount map to image plane of camera in maya,how to do it ?

Houdini doesn't have a sky system (yet), and to be honest I'm not sure if that's terribly high on the priority list.

What it does have however is an environment light that you can assign a texture to - this texture can be a panoramic sky view. You can create an environment light from the shelf tab called “lights and cameras”. When it's created, select the light's node, and in the parameter pane, navigate to the “light” tab. In the bottom of the parameter pane there another tab called “area light options”. In there you can activate both “use full sphere” and “use area map”. Try the example HDRIs that are shipped with Houdini.

cheers,
Abdelkareem

There is also nice, tiny code for procedural generation of sky environment maps by Wolfwood, you can find it here:
http://www.shadeops.com/skycolor/ [shadeops.com]
http://forums.odforce.net/index.php?showtopic=6904&hl= [forums.odforce.net]

thanks.

about wolfwood's sky color,i don't know how to control the sun's positon to change the color of sky.

one more thing,i can't understand the theory about Chromaticity,can u recommend a place where i can't find some basic knowledge about Chromaticity?

sky color