When you select the Background item in the HDRI Editor Pin List you have four options for your HDRI Editor Background: Color, Gradient, Sun & Sky and Image.
Resolution
The Image background is automatically set to the resolution of the selected image. But for the other 3 types you will have to set the Resolution.
Overall a smaller resolution gives better performance, but a larger resolution gives better shadow and reflection quality. Normally it the default value will suffice. But if you are operating with highly reflective materials and half/square pins or pins with little/no falloff, you may need to increase the resolution.
Color
Color
Select a solid color as background for your environment. This should not be confused with the setting of a solid background color (can be set in Environment settings or Photographic Image Style) as it will affect the lighting in the scene.
Brightness
Adjust the brightness of the background color.
Gradient
Color bar
Here you can set when and what colors will fade from one to the the next. Click on the dropper circle to select a color stop, then use the color swatch to choose a color.
The little triangles shows where the 2 colors are mixed 50-50 and they can be dragged to control the course of the blend.
Add Color Stop
If you would like to fade between more than 2 colors, click on the Add stop button below the color bar to create another color swatch. click and drag the color pins and the gradient apex slider to your desired animation effect.
Delete Stop
If you want to remove a color stop, just select it and click delete.
Stop Brightness
Select a color stop and adjust the brightness.
Latitude
Select a color stop and adjust the position of the color on the gradient. This can also be achieved by dragging the color stops on the color bar.
Brightness
Adjust the brightness of the whole gradient background.
Saturation
Adjust the saturation of the whole gradient background.
Sun & Sky
Resolution
Here, set the resolution of the sun and sky that will be generated. A smaller resolution has better performance, but a larger resolution has better shadow and reflection quality.
Location
Here, you can choose a preset city that is closest to where the scene is taking place for accurate portrayal of the sun and seasons at location.
Coordinates
Another option is to choose “Custom Location” and enter the geo coordinates of a location.
Date
Use this to set the date to the day the scene will be taking place for accurate portrayal of color temperatures of the seasons.
Custom Sun Position
With this toggled, you will be able to set the exact position of the sun, instead of relying on date, location and time of day.
Time
Use this to set the time the scene takes place for correct placement of the sun.
Turbidity
Increase this setting to add more haze to the sky. This will tint the sky with a warm tone and filter the sunlight that is cast in the scene.
Sun Size
Adjust the size of the sun.
Ground Color
Set the color of the ground on the background.
Color
Brightness
Adjust the brightness of the Sun & Sky background.
Contrast
Adjust the contrast of the environment
Saturation
Adjust the saturation of the environment
Hue
Adjust the hue of the environment
Colorize
Add a color that the environment will be blended with.
Blur
Adjust the blur of the environment background, increase to soften contours.
Transform
Tilt
This will tilt the vertical axis of the environment.
Rotation
Rotate the environment background without affecting any pins in the environment. If you want to rotate the entire environment you can do it in the Environment settings.
Image
Resolution
This is set by the image size. If you experience bad reflections with the environment you may need to use an image with a higher resolution.
Image
Select an image file for the environment background.
browse for another image file
Refresh the image file
Remove the image file – this will change the environment type to Color.
Brightness
Adjust the brightness of the Sun & Sky background.
Contrast
Adjust the contrast of the environment
Saturation
Adjust the saturation of the environment
Hue
Adjust the hue of the environment
Colorize
Add a color that the environment will be blended with.
Blur
Adjust the blur of the environment background, increase to soften contours.
Transform
Tilt
This will tilt the vertical axis of the environment.
Rotation
Rotate the environment background without affecting any pins in the environment. If you want to rotate the entire environment you can do it in the Environment settings.