Histogram for display?

to

How to compute histogram for the complete display area of an imagelayer on the fly.
ReaderWriterGDAL has the display buffer but for the extent of the tile size.
Not for the complete display extent.
I would like to know how to compute the extents of display area of
an image layer?

How to get GeoExtent of display area at run time while PAN/ZoomIn/ZoomOut?

Sorry -- there is no API to extract this information.

My ultimate goal is to compute histogram for the display area.
Since there is no direct API, I would like to implement indirectly,

(through ReaderWriterGDAL.cpp/createImage())

1) Store all the Tilekey information i.e., off_x, off_y, etc., for given LOD
2) and When want to read display area data into buffer, I would use all the Tilekey offsets (for given LOD) to
fetch from rasterIO.

Is it make sense?
One problem would be that even the Tilekeys beyond display boundary will also get included.
Let me know if there is a way to remove out of bound tiles?