问题
What does this ExtGState object do to an image:
<< /Type /ExtGState /TR /Identity >>
I have the PDF spec, but it is not clear at all. Apparently this maps the identity function (identity of what? the identity matrix?) from [0.0 1.0] to [0.0 1.0] (the same), which makes no sense.
Could someone please explain to me what this does?
回答1:
It specifies a function that should be used for processing colors or color components (e.g. for gamma correction). Specifying /Identity should actually be the same as not specifying it at all (it's an optional field), so I'd suspect that the software that produced the PDF just always writes a default ExtGState object for simplicity's sake.
See the chapter on Transfer Functions in the PDF spec for more information.
来源：https://stackoverflow.com/questions/9388326/pdf-spec-extgstate