accelerate imread on large images

mat
2018 4 26
2 个回答
更新时间：2018 4 30
17 次查看（30 天）

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hi
is there a way to accelerate imread on large (> 2GB) images?
thanks a lot,
mat

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KALYAN ACHARJYA
KALYAN ACHARJYA 2018-4-28
Do you want to call multiple images?
Ameer Hamza
Ameer Hamza 2018-4-28
Is 2GB size of a single image, or several images together make up 2GB?
mat
mat 2018-4-29
hello Ameer,
just 1 image.
thanks again,
mat

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回答（2 个）

Walter Roberson
Walter Roberson 2018-4-29

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Not in general.
If the images are TIFF images, then you could get more flexibility about how you read by using the Tiff() class. This will not necessarily reduce total time to read an image, but if it turned out that you only needed part of the image then it would permit you to read it in pieces if it was stored the right way.
If you are using blockproc() then there is an option to process a file instead of a matrix. blockproc() will take advantage of any facilities offered by the type of image to read only a section at a time. In practice this probably means special handling for TIFF but not for much else -- though in theory it might also be able to handle some JPEG images in sections.

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mat
mat 2018-4-29
thanks walter. i am reading tiff files and indeed am aware of the Tiff() class option for strip reading. unfortunately, i need the full content of the files and a strip by strip reading would only slow the process down. thanks again. mat
Walter Roberson
Walter Roberson 2018-4-29
Probably the best speedup would be to use an SSD.
If you have the Parallel Computing toolbox, you could experiment with having different workers read different sections of the file and then merge them together. That would only really be useful for the situation in which the decompression was significantly slower than the reading from disk, which I doubt to be the case in practice. In the more typical situation where the controller bandwidth is the limiting factor, then adding more readers just slows everything down due to contention for access to the scarce resource (disk bandwidth.)
mat
mat 2018-4-30
hi walter, unfortunately i don't have the said toolbox but i really appreciate all your comments !! thanks !
Walter Roberson
Walter Roberson 2018-4-30
You can get an estimate for how long the decoding takes by using timeit() on imread and subtract off timeit of fileread() of the tiff. fileread is not necessarily the fastest possible way to read but it is relatively low overhead.
Just be sure to repeat the tests to account for the fact that the OS might be reading the file into cache, so the first access might be slow.

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mat
mat 2018-4-30

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timed both on a 4.8gb rgb tiff file. imread - 75sec fileread - 0.3sec !! - resulting in a much smaller size vector ofcourse a lot of decoding and formatting going on..
if only i could efficiently reconstruct the tiff file from the this fileread character vector..

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Walter Roberson
Walter Roberson 2018-4-30
Perhaps you could try something like https://blog.idrsolutions.com/2015/08/how-to-read-tiff-images-in-java/ to see how the performance goes?
mat
mat 2018-4-30
good point, i shall try this. tx.

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