Specify overview IFD subfile type and IFD order #14
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rouault
reviewed
Nov 29, 2022
| @@ -25,6 +25,10 @@ More formally, the structure of such a file is: | |||
| * Optional: tile content of first overview level | |||
| * Tile content of full resolution image. | |||
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| The IFDs shall be linked by the pointers (the offset of the first IFD in the file header and the offset of the next IFD in each IFD) so that the full resolution image is first, followed by the overviews. | |||
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| Each IFD representing an overview shall have the tag NewSubfileType (254) with a value of 1, indicating the "reduced resolution image" subfile type. | |||
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IFDs representing a mask should have value 4
IFDs representing the overview of a mask should have value 5
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Thanks, I'll study this suggestion and update the pull request.
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In order to recognize overviews as such, GDAL needs overview IFDs to have the NewSubfileType tag equal to 1, and that overview IFDs follow the full resolution image according to the order given by the pointers. This is also the minimal requirement that any COG processing software would need in order to identify overviews as such and to associate them with the full resolution image.
This pull request formalizes the above requirements.
In addition, this pull request changes the word "structure" to "layout" to emphasize the distinction between the layout of data in the file versus the logical order of IFDs given by the pointers.