(Sometimes required, as discussed below) The group color space, which is used for the following purposes: • As the color space into which colors are converted when painted into the group • As the blending color space in which objects are composited within the group • As the color space of the group as a whole when it in turn is painted as an object onto its backdrop The group color space may be any device or CIE-based color space that treats its components as independent additive or subtractive values in the range 0.0 to 1.0, subject to the restrictions described in Section 7.2.3, “Blending Color Space.” These restrictions exclude Lab and lightness-chromaticity ICCBased color spaces, as well as the special color spaces Pattern, Indexed, Separation, and DeviceN. Device color spaces are subject to remapping according to the DefaultGray, DefaultRGB, and DefaultCMYK entries in the ColorSpace subdictionary of the current resource dictionary. Ordinarily, the CS entry is allowed only for isolated transparency groups (those for which I, below, is true), and even then it is optional. However, this entry is required in the group attributes dictionary for any transparency group XObject that has no parent group or page from which to inherit — in particular, one that is the value of the G entry in a soft-mask dictionary of subtype Luminosity. In addition, it is always permissible to specify CS in the group attributes dictionary associated with a page object, even if I is false or absent. In the normal case in which the page is imposed directly on the output medium, the page group is effectively isolated regardless of the I value, and the specified CS value is therefore honored. But if the page is in turn used as an element of some other page and if the group is non-isolated, CS is ignored and the color space is inherited from the actual backdrop with which the page is composited. Default value: the color space of the parent group or page into which this transparency group is painted. (The parent’s color space in turn can be either explicitly specified or inherited.)

(Optional) A flag specifying whether the transparency group is isolated. If this flag is true, objects within the group are composited against a fully transparent initial backdrop; if false, they are composited against the group’s backdrop. Default value: false. In the group attributes dictionary for a page, the interpretation of this entry is slightly altered. In the normal case in which the page is imposed directly on the output medium, the page group is effectively isolated and the specified I value is ignored. But if the page is in turn used as an element of some other page, it is treated as if it were a transparency group XObject; the I value is interpreted in the normal way to determine whether the page group is isolated.

(Optional) A flag specifying whether the transparency group is a knockout group. If this flag is false, later objects within the group are composited with earlier ones with which they overlap; if true, they are composited with the group’s initial backdrop and overwrite (“knock out”) any earlier overlapping objects. Default value: false.