# RFC 14: Image Structure Metadata

Author: Frank Warmerdam

Contact: warmerdam@pobox.com

Status: Adopted, amended per RFC 87: Signed int8 data type for raster

## Summary

This RFC attempts to formalize the semantics of the “IMAGE_STRUCTURE” domain of metadata. This metadata domain is used to hold structural information about image organization that would not normally be carried with an image when translated into another format. The IMAGE_STRUCTURE metadata may occur on the GDALDataset or on individual bands, and most items are meaningful in both contexts. When items like NBITS are found on the dataset it is assumed they apply to all bands of that dataset.

## IMAGE_STRUCTURE items

COMPRESSION:: The compression type used for this dataset or band. There is no fixed catalog of compression type names, but where a given format includes a COMPRESSION creation option, the same list of values should be used here as there.

NBITS:: The actual number of bits used for this band, or the bands of this dataset. Normally only present when the number of bits is non-standard for the datatype, such as when a 1 bit TIFF is represented through GDAL as GDT_Byte.

INTERLEAVE:: This only applies on datasets, and the value should be one of PIXEL, LINE or BAND. It can be used as a data access hint.

PIXELTYPE:: This may appear on a GDT_Byte band (or the corresponding dataset) and have the value SIGNEDBYTE to indicate the unsigned byte values between 128 and 255 should be interpreted as being values between -128 and -1 for applications that recognise the SIGNEDBYTE type.

Note

Starting with GDAL 3.7, with the implementation of RFC 87: Signed int8 data type for raster, PIXELTYPE is no longer used, as the Int8 data type is used for signed bytes.

## Compatibility Issues

This RFC has two changes from existing practise that may cause compatibility issues:

1. Traditionally the NBITS metadata appeared in the default metadata domain on datasets, instead of in the IMAGE_STRUCTURE domain.

2. Traditionally the COMPRESSION metadata appeared only on the dataset, never one the band.

I am only aware of one application previously making systematic use of these items, and it will be updated to reflect the new usage as GDAL 1.5.0 is adopted.

## Development

Beyond adopting the definition for the semantics of the IMAGE_STRUCTURE metadata, the following development steps will be taken:

1. The PNG, GTiff, NITF and EHdr drivers will be updated to place NBITS in the IMAGE_STRUCTURE metadata domain.

2. The HFA driver will be updated to return NBITS metadata.

3. The HFA, GTiff, JP2KAK, ECW, JPEG, and PNG drivers will be updated to return INTERLEAVE metadata.

4. The HFA and GTiff drivers will be updated to return PIXELTYPE metadata.

The development will be done by Frank Warmerdam in trunk in time for GDAL/OGR 1.5.0 release. Changes to other drivers that these definitions might be useful for while be done as time permits by interested developers - not necessarily in time for GDAL/OGR 1.5.0.

## Notes

- The gdalinfo utility already reports IMAGE_STRUCTURE metadata when it is available.

- The GTiff, and HFA drivers CreateCopy() methods check the source for NBITS, and PIXELTYPE metadata to create specialized output files types.

- The GTiff, HFA and default CreateCopy() implementations have been reworked to use the new GDALDatasetCopyWholeRaster() function which uses the INTERLEAVE metadata as a clue whether to do interleaved copies if the source dataset is interleaved.