I have an object with an hdr for the background. What do I do in Cycles to render the background with my object?
How is this different from an ordinary render in Cycles?
The hdr is a big one, 350MB.
There are a few pieces to make this work.
In the world section, set the surface to Background and the Color to use Environment Texture. Then select the HDRI image you want to use.
To view the image in the 3d viewport enable Display> World Background.
Usually HDRi images use an Equirectangular Projection, that is mapped as a big sphere that completely surrounds the scene.
Note that the Background can only be displayed in perspective view, it will not be on orthogonal view.
The HDRi not only becomes the background for the scene, but is also used to light the scene. The intensity of the light can be controlled with the Strength value on the world settings.
To render the HDRi in the background of the scene disable>Film>Transparent.
If you render, you will see the HDR image as the background of your image.
If you want to use the HDRi to light the scene, but want a transparent background to be composited later with the background, or want to blur the HDR or maybe replace it all together, then enable film tranparent and enable the Environment output for the render layers, and use it as a different element in the compositor.
To change the placement for the background
In the World section of the Material nodes add a Texture Coordinate node and a Mapping node to control the placement of the Environment texture. By changing the rotation on the mapping node you can place the HDR where you need it.