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There are a few options in the Fog dialog box for configuring fog.
Selecting the Display Fog checkbox displays fog in the drawing area. Deselecting the Display Fog checkbox hides fog.
The left-most fog slider determines where you want the fog to start relative to the camera (your view). Fog can begin right in front of the camera which is zero on the scale or further away from the camera. Move the left-most slider to the right to start fog somewhere beyond the camera.
The right-most slider determines where you want the fog to be at 100% strength (known as zero visibility). Move the right-most slider to the left to establish 100 percent strength closer to the camera (your view). Move the right-most slider all the way to the left to have zero visibility right in front of the camera and extending out to infinity (you wont actually be able to see your model at all and at any distance).
Use Background Color
Selecting the Use background color checkbox uses the currently configured background color as the fog color. Refer to the Background Panel section of the Styles Browser topic for further information on setting the background color.
Deselecting the Use background color and clicking on the color swatch selects a different color to be used as the fog color.