[R-sig-Geo] stars::RasterIO using extent info?

[Edzer Pebesma]

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
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics
Heisenbergstrasse 2, 48151 Muenster, Germany
Phone: +49 251 8333081

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