# “Precompressed” tiles

See first an overview of what precompressed tiles are and which formats support them.

This feature is primarily exposed in the bfconvert command, using the -precompressed option. When the -precompressed option is used, best results are obtained by:

- using the -noflat option

- not using the -compression option (this can work, but may force tile recompression)

- not using the -tilex or -tiley options (this can work, but may force tile recompression)

There are several advantages to converting using the “precompressed” feature for formats that support it:

- faster tile read and write times, as no tile compression/decompression needs to be performed

- no change in compression quality

- very little change in file sizes between input and output data

There are also a few disadvantages:

- tile sizes cannot be changed during conversion

- compression type cannot be changed during conversion

- input and output format must support same compression type and tile size

- the input format must have precompressed tile reading support (see the list of supported readers)

- the output format must have precompressed tile writing support (see the list of supported writers)

- command-line tools other than bfconvert do not currently make use of the precompressed tile API

For additional discussion of the “precompressed” tiles feature, see:

- https://forum.image.sc/t/vsi-to-dcm-file-conversion/98249/5

- https://forum.image.sc/t/exporting-a-dicom-stack/99400