read.*: How to read from a URL?

On Wed, 10 Dec 2008, hadley wickham wrote:

Hi Michael,

In general, I think you should be able to do:

gimage <- read.jpeg(url(gimageloc))

Note that would not be really correct: it would need to be

gimage <- read.jpeg(con <- url(gimageloc))
close(con)

since it otherwise leaks a connection (which would eventually be closed).

However, from ?read.jpeg

Arguments:

filename: filename of JPEG image

so it does not accept a connection (and the source code wll confirm that). 
In fact virtually all functions that accept a 'file name or connection' 
will work with URLs, as file() accepts URLs as well as file names (see 
?file).

The issue is that writers of third-party readers should be encouraged to 
support connections (which have been around for ca 7 years in R).
It is ammazing how people take such innovations for granted.

or alternatively use the EBImage from bioconductor which will read
from a url automatically (it also opens a much wider range of file
types)

library(EBImage)
img <- readImage(gimageloc, TrueColor)

Hadley


On Wed, Dec 10, 2008 at 1:17 PM, Michael Friendly <friendly at yorku.ca> wrote:

The question is how to use a URL in place of a file= argument for
read.*.functions that do
not support this internally.

e.g., utils::read.table() and her family all support a file= argument that
can take a URL
equally well as a local file.  So, if I have a file on the web, I can
equally well do

langren <- read.csv("langrens.csv", header=TRUE)
langren <-
read.csv("http://euclid.psych.yorku.ca/SCS/Gallery/Private/langrens.csv",
header=TRUE)

where the latter is more convenient for posts to this list or distributed
examples.
rimage::read.jpeg() doesn't support URLs, and the only way I've found is to
download the
image file from a URL to a temp file, in several steps.
This is probably a more general problem than just read.jpeg,
so maybe there is a general idiom for this case, or better-- other read.*
functions could
be encouraged to support URLs.

library(rimage)
# local file: OK
gimage <-
read.jpeg("C:/Documents/milestone/images/vanLangren/google-toledo-rome3.jpg")

gimageloc <-
"http://euclid.psych.yorku.ca/SCS/Gallery/images/Private/Langren/google-toledo-rome3.jpg"
dest <- paste(tempfile(),'.jpg', sep='')
download.file(gimageloc, dest, mode="wb")

trying URL
'http://euclid.psych.yorku.ca/SCS/Gallery/images/Private/Langren/google-toledo-rome3.jpg'
Content type 'image/jpeg' length 35349 bytes (34 Kb)
opened URL
downloaded 34 Kb

dest

[1] "C:\\DOCUME~1\\default\\LOCALS~1\\Temp\\Rtmp9nNTdV\\file5f906952.jpg"

#  Is there something simpler??
gimage <- read.jpeg(dest)
#  I thought file() might work, but evidently not.
gimage <- read.jpeg(file(gimageloc))

Error in read.jpeg(file(gimageloc)) : Can't open file.
--
Michael Friendly     Email: friendly AT yorku DOT ca Professor, Psychology
Dept.
York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street    http://www.math.yorku.ca/SCS/friendly.html
Toronto, ONT  M3J 1P3 CANADA

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