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Adjusts or corrects the intensity of an image for a particular output device, such as a monitor or scanner. The default value is 1 (no correction).
Optional string. Represents a unit of measure. The default is NUM. For details about NUM, see https://msdn.microsoft.com/en-us/library/aa218417(v=office.10) Units of measure.
Optional string. Represents the element's formula. This attribute can contain the string "someFormula" if the formula exists locally, "No Formula" if the formula is locally deleted or blocked, or "Inh" if the formula is inherited.
If the attribute is not present, the element's formula is a simple constant, for example, <element> 5 </element>.
Optional string. Indicates that the formula evaluates to an error. The value of Err is the current value (an error message string); the value of the element is the last valid value.
The Gamma element is relevant only when contained in a https://msdn.microsoft.com/en-us/library/aa203207(v=office.10) Shape or https://msdn.microsoft.com/en-us/library/aa203205(v=office.10) StyleSheet element. It is ignored when contained in a https://msdn.microsoft.com/en-us/library/aa191981(v=office.10) DocumentSheet or https://msdn.microsoft.com/en-us/library/aa203159(v=office.10) PageSheet element.
https://msdn.microsoft.com/en-us/library/aa166010(v=office.10) Gamma cell (Image Properties section)