With Render Background on, anything visible behind the object, on the page, will be drawn into the rendered image of the object. This will simulate full transparency in the non-rectangular areas of the image.
This is required because HTML only allows for rectangular images. So, if you draw an ellipse, or rotate a rectangle, there will be areas within the image rectangle that are not part of the shape. Even with rectangular images, a piece of the background is drawn to allow the object to be anti-aliased against it.
If you turn Render Background off, one of two things can happen:
If the image format supports transparency (PNG8 GIF), the areas of background will be made transparent
If the image format does not support transparency, the page’s background color (color only - no page background image), will be used to fill the background area.
Generally, using the default of ‘render background’ checked will do the right thing for an object but if, for example, your object uses relative positioning and will move with respect to the objects under them, you will need to use true transparency to prevent to wrong section of background from being drawn into the object’s rendered image. Or, if an object was part a Special Effect and appeared only on mouse-over, you might want it to hide everything behind it and have the background only show the page’s color.
GIF always has transparency turned on so “render background” is not an option if GIF is used.
You may set the Render Background option globally to affect all rendered objects using the Project Target Options dialog box. (For more information, see Project Target Options Dialog Box.) Or, you may set the Render Background option on the Options Tab of the Geometry Editor dialog box. (For more information, see Geometry Editor Dialog Box.)
The figures below show the effects of the Render Background option. The first image shows images juxtaposed on a page with the option turned off. Note how data appears to be missing. The second image shows images juxtaposed with the option enabled.
Do not place transparent objects that have the background rendered into the object’s image over a text object since the appearance of a text object is dependent on the viewer’s system configuration.