Generally, we recommend using the direct/reflect/refract environment overrides in Render Setup > Scene > Scene Environment.
See: How to set up HDRI environment?
But sometimes you may need to use a different approach - for example if you are not using a single flat or spherical background image, but a set of images that you want to be visible outside of each window, or if you want to tweak the UV mapping of your background image. To get these kinds of results, after creating the background geometry, you need to assign a special material to it:
Create a Corona Light Material with "Emit light" option turned off in its settings and load the background image as the Texmap - this method is easy and fast to set up. You can additionally toggle direct, reflection, refraction, alpha channel and masking element visibility for such materials in their settings.
1. Create a Corona Rayswitch material (let's call it A).
2. Create a Corona Legacy Material (let's call it B). We are using the Corona Legacy Material because we need to keep it non-physical. The Corona Physical Material wouldn't be a good choice here.
3. In the Corona Legacy Material (B), disable the diffuse component (set its diffuse level to 0 or color to pure black).
4. In the Corona Legacy Material's (B) self-illumination slot, put the image you want to use as the background - use self illumination multiplier to set the brightness of the background image.
5. Put the Corona Legacy Material (B) into the Corona Rayswitch material's (A) "reflection", "refraction" and "directly visible" slots.
6. Leave the "Global illumination" slot empty! This way the material won't be affecting GI or casting shadows.
7. Assign the Rayswitch (A) material to your background object.
An example setup involving the Corona Rayswitch Material. Note that the Global Illumination slot is empty. For details, see the Examples section below.
This way you will create a material that is bright when seen directly and visible in reflections and refractions, but does not affect your scene by casting shadows or generating GI. To change the appearance of the material when seen in reflections or refractions, simply alter the materials used in the "reflection" or "refraction" Rayswitch material's slots.
Regardless of the workflow of your choice, to load the background texture you can also use the Corona Tonemap Control map to make it unaffected by color mapping (EV value, highlight compression, etc).
Examples
1. Corona Sun + Corona Sky - no background image, no geometry capping the window holes - Corona sun and sky generate lighting and are directly visible.
2. Corona Light Material assigned a plane placed behind the window. "Emit light" disabled. Lighting is still generated by the Corona sun and sky, but the background image is visible directly, in reflections and in refractions. The image is in "screen" mapping mode.
Material setup:
3. Environment overrides in render settings. Lighting is still generated by the Corona sun and sky, but the background image is visible directly, in reflections and in refractions. The image is in "screen" mapping mode.
Render setup:
4. Corona RaySwitch material. Lighting is still generated by Corona the sun and sky, but the background image is visible directly, in reflections and in refractions. The image is in "screen" mapping mode.
Material setup:
5. Corona RaySwitch material. A slightly darker material was used for reflections and refractions. Lighting is still generated by the Corona sun and sky, the brighter background image is visible directly, and the darker image is visible in reflections and in refractions.
Material setup - self-illumination was decreased in the material used for reflections and refractions: