How to query overview compression?

[wiesehahn]

I am afraid this is more question related to gdal than gdalraster, feel free to delete if it doesnt fit here.

I am wondering if it is possible to query the compression type of overviews of GeoTIFF/COG images?
By default it should be the same as the orginal image data but we can also use different settings.

library(gdalraster)

infile <- "/vsicurl/https://dgm.s3.eu-de.cloud-object-storage.appdomain.cloud/325735712/2016-04-04/dgm1_32_573_5712_1_ni_2016.tif"

ds <- new(GDALRaster, infile)
ds$getOverviewCount(band = 1)

outfile <- tempfile(fileext = ".tif")

temp_raster <- createCopy(
  src_filename = infile,
  dst_filename = outfile,
  format = "COG",
  options = c("COMPRESS=LZW", "OVERVIEWS=IGNORE_EXISTING", "OVERVIEW_COMPRESS=DEFLATE")
)

ds <- new(GDALRaster, outfile)
ds$getOverviewCount(band = 1)
ds$getMetadataItem(band = 0, mdi_name = "COMPRESSION", domain = "IMAGE_STRUCTURE")

Is there a way we could see that its DEFLATE in this case?

[ctoney]

Yes, GDAL has a dataset open option:

An option exists for all drivers, OVERVIEW_LEVEL=level, to select a particular overview level of a dataset. The level index starts at 0. The level number can be suffixed by "only" to specify that only this overview level must be visible, and not sub-levels.

Also:

Starting with GDAL 3.3, OVERVIEW_LEVEL=NONE is supported to indicate that no overviews should be exposed.

You'll need to use the constructor for GDALRaster that includes open_options, i.e., all arguments are required up to that one (see Constructors under Usage at https://usdaforestservice.github.io/gdalraster/reference/GDALRaster-class.html). With your example above:

library(gdalraster)
#> GDAL 3.9.3 (released 2024-10-07), GEOS 3.12.2, PROJ 9.4.1

infile <- "/vsicurl/https://dgm.s3.eu-de.cloud-object-storage.appdomain.cloud/325735712/2016-04-04/dgm1_32_573_5712_1_ni_2016.tif"

ds <- new(GDALRaster, infile)
ds$getOverviewCount(band = 1)
#> [1] 1

ds$close()

outfile <- tempfile(fileext = ".tif")

temp_raster <- createCopy(
  src_filename = infile,
  dst_filename = outfile,
  format = "COG",
  options = c("COMPRESS=LZW", "OVERVIEWS=IGNORE_EXISTING", "OVERVIEW_COMPRESS=DEFLATE")
)
#> 0...10...20...30...40...50...60...70...80...90...100 - done.

ds <- new(GDALRaster, outfile)
ds$getOverviewCount(band = 1)
#> [1] 1
ds$getMetadataItem(band = 0, mdi_name = "COMPRESSION", domain = "IMAGE_STRUCTURE")
#> [1] "LZW"

# use the OVERVIEW_LEVEL open option to open an overview as a dataset, level index starts at 0
ds_ovr <- new(GDALRaster, outfile, read_only = TRUE, open_options = "OVERVIEW_LEVEL=0")
ds_ovr$getMetadataItem(band = 0, mdi_name = "COMPRESSION", domain = "IMAGE_STRUCTURE")
#> [1] "DEFLATE"

ds_ovr$close()
ds$close()

^{Created on 2025-03-18 with reprex v2.1.1}

[wiesehahn]

wow, awesome! thanks for the quick answer, wasnt aware what open_options does. So much to learn...

[ctoney]

In this case it was a "general" dataset open option that applies to all formats. Usually though, dataset open options are format-specific. So when working with a certain format it may worth checking the driver documentation page to see what options are available, e.g.,

https://gdal.org/en/stable/drivers/raster/gtiff.html#open-options

The OVERVIEW_LEVEL open option is more hidden. It is in the API documentation at:
https://gdal.org/en/stable/api/raster_c_api.html#_CPPv410GDALOpenExPKcjPPCKcPPCKcPPCKc

down under Parameters: papszOpenOptions.

I'm not sure if it appears elsewhere in the GDAL docs. It may be worth adding to the gdalraster documentation, and adding it somewhere in the GDAL docs if confirmed that it's not covered other than in the API.