[R-sig-Geo] Error when combining raster and foreach packages

[Loïc Dutrieux]

Hi Lukas,

What does your function do? Can you use the built-in parallel 
functionalities of the raster package instead? See ?beginCluster

Cheers,
Loïc

On 26/08/2016 12:23, Lukas Lehnert via R-sig-Geo wrote:
> Dear list members,
>
> I tried to process a large GeoTIFF file in a blockwise manner (following the
> vignette "Writing functions for large raster files").
>
> I then combined the approach with a "foreeach"-loop to parallelize the
> calculation on the raster data:
>
> library(raster)
> library(doMC)
> library(foreach)
> registerDoMC(3)
>
> ra <- raster("dem.tif")
> tr <- blockSize(ra)
> outfile <- "test_2.tif"
> res <- writeStart(raster(ra), outfile, overwrite = TRUE)
> for (i in 1:tr$n)
> {
>   v <- getValuesBlock(ra, row=tr$row[i], nrows=tr$nrows[i])
>
>   v2 <- foreach(i=1:length(v), .combine = 'c') %dopar% {
>      i*(-1)
>   }
>   res <- writeValues(res, v2, tr$row[i])
>   cat(round(i/tr$n*100, 2), "%\n")
> }
> res <- writeStop(res)
>
> However, I get errors and warnings like the following ones:
> ERROR 1: /tmp/Rtmpz6qEOE/xortest_2.tif:Failed to allocate memory for to read
> TIFF directory (0 elements of 12 bytes each)
> ERROR 1: TIFFReadDirectory:Failed to read directory at offset 4639232
> Warning 1: TIFFFetchNormalTag:ASCII value for tag "GeoASCIIParams" contains
> null byte in value; value incorrectly truncated during reading due to
> implementation limitations
>
> The errors are different each time I restart the process. If I do not use
> foreach but a common for-loop, the errors do not occur. I tried several raster
> files (large ones and small ones) and two different computers, but the problem
> remains.
>
> Does somebody has any idea, how to solve this problem?
>
> Thank you in advance
>
> Lukas
>
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