How does ENVI use band interleave as described in the GeoTIFF file format?

GeoTIFF files are a popular and common format for data files in remote sensing. Neither TIFF nor GeoTIFF explicitly support band interleave by line (BIL) format. However, band sequential (BSQ), band interleave by pixel (BIP), and BIL format data are all frequently distributed as TIFF files and are read into ENVI. How does this work?
 
By definition, the TIFF format, and thus the GeoTIFF format, don't explicitly support BSQ, BIL, or BIP as a specific interleave. That is, there is no TIFF tag which defines the interleave to be BSQ, BIL, or BIP. However, there is a TIFF tag called PLANARCONFIG which is similar to an interleave tag.

The origin of this situation is in the format's original uses, when TIFF was primarily used for 1 band (gray scale) and 3 band (RGB) files. A PLANARCONFIG value of 1 meant that the file was either a single band (gray scale) image file, or a 3 band (RGB) image file which was interleaved by pixel (BIP). A PLANARCONFIG of 2 meant that the file was a 3 band (RGB) file which was interleaved by band (BSQ).

Because those are the only two values allowed for PLANARCONFIG, the traditional thinking has been that the TIFF format only supports a BSQ or BIP interleave. With the rise in popularity of multi and hyperspectral imagery, this situation could have severely limited the functionality of TIFF. However, there are additional tags which can be used to define a BIL formatted file.

If PLANARCONFIG is set to 2, then the tag ROWS_PER_STRIP can be set equal to 1, as opposed to the number of lines for a BSQ file. Then, it is possible to use the STRIP_OFFSET tag (one location for each strip in the file) to point to a set of bands interleaved by line. The result is a TIFF file stored with a BIL interleave.

Because ENVI recognizes the PLANARCONFIG tag, along with the STRIP_OFFSET and ROWS_PER_STRIP tags, if there is a GeoTIFF file that is BIL and properly explained via these tags, then ENVI will have no trouble reading this file correctly.