On 11/13/18 4:10 PM, Howard, Tim G (DEC) via R-sig-Geo wrote: > Dear list, > > I am exploring the different options for reading parts of large imagery > object in stars, as discussed here: > > https://r-spatial.github.io/stars/articles/proxy.html > > My ultimate goal is to read into RAM only a clipped portion of a large raster > (well, actually a raster stack, but taking baby steps here). > > My immediate question: the `RasterIO` option of read_stars defines cell > offsets and cell counts (*Size). Is there a straightforward way to calculate > these values given extent information? > > Reproducible example (mostly taken from here: > https://www.r-spatial.org/r/2018/03/22/stars2.html): > > library(stars) > tif <- system.file("tif/L7_ETMs.tif", package = "stars") > x <- read_stars(tif) # read entire tif into ram > x <- x[,,,1] #get just one layer for now > # calculate a circular polygon at the center of the raster > pol <- x %>% st_bbox() %>% st_as_sfc() %>% st_centroid() %>% st_buffer(500) > plot(x) > # interestingly, I don't think the circle is in the right place when plotted > plot(st_geometry(pol), add = TRUE, border = "red") > # this is what I'd like to be able to restrict to what is read in memory: > plot(x[pol]) > > ## read only portion of tif using proxy object > x <- read_stars(tif, proxy = TRUE) > x <- x[,,,1] > y <- st_as_stars(x[pol]) > plot(y) # this is cropped to the extent (but not the circle - let's not worry > about that right now) > > Question: can I do the equivalent with the RasterIO options in stars? Said > another way, instead of setting up the proxy, can I map my extent object (or > bounding box) directly to the cell count values needed for RasterIO?
stars can do the math, and so can you; it is explained here: https://r-spatial.github.io/stars/articles/data_model.html stars uses some functions directly from GDAL which it doesn't expose to the user, but there is no magic going on here. > > > Thanks in advance for any tips. > Tim > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > -- Edzer Pebesma Institute for Geoinformatics Heisenbergstrasse 2, 48151 Muenster, Germany Phone: +49 251 8333081
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