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#1
April 30th, 2004, 10:56 AM
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Reading binary data into 2-D array
I have no problem reading a binary image file into a one dimensional array (here with the number of elements equalling in_pixels).
file = open(in_file, 'rb') data = array.array(in_imgtype) data.fromfile(file,in_pixels) BUT how do I read the same data into a 2-D array (eg in_pixels is arranged in the image as in_rows by in_cols)? Thanks.
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#2
April 30th, 2004, 11:26 AM
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If the number of elements == the number of pixels why worry about using a 2D array? If you know the pixel order (stored in rows or columns) then you can map 2D co-ordinates to a 1D array.
e.g. stored in rows and assume first pixel in at 0,0 say width = 100 say pixel co-orodinate = (x,y) = (10,12) pixel = array1D[x*width+y] = array1D[1012] Grim 
__________________
*** Experimental Python Markup CGI V2 *** Last edited by Grim Archon : April 30th, 2004 at 11:42 AM. Reason: removed 2D code
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#3
April 30th, 2004, 01:49 PM
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Quote:
I suppose I could.... but this becomes more tricky when you want to calculate spatial relationships between elements (e.g. if it is a satellite image and you want to know which element is immediately NW of the element in question...
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#4
April 30th, 2004, 02:14 PM
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Code:
f = file('foo', 'rb')
pixels = []
row = 0
col = 0
for byte in file.read():
if byte == '\n':
row += 1
col = 0
else:
pixels[row][col] = byte
col += 1
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#5
April 30th, 2004, 07:07 PM
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Strike I'm guessing here but I don't think image files have a convenient eol.
 The image file usually has a header which amongst other things will have the dimensions of the image. A bmp for example will have this and the palette used. How about this: Code:
def getpixel(x,y):
return array1D[x*width+y]
or Code:
array2D = []
for n in range(0,len(array1D),width):
array2D.append(array1D[n:n+width])
pixel = array2D[x][y]
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#6
May 1st, 2004, 05:46 AM
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What do you want to do with the data once you have read it in? What is the purpose behind your question? There are a number of alternatives available to you, and the best one will depend on what you want it for.
Here are some alternatives off the top of my head: 1) you cannot create 2D arrays with the array module, but you could create a list of array objects by having a loop that reads in_rows bytes from the file, converts it to an array, and appends it to the list. This is probably closest to the answer that you were seeking. 2) Use the numpy or numarray extensions to create true 2D arrays. See http://www.pfdubois.com/numpy/ and http://www.stsci.edu/resources/soft...rdware/numarray respectively. 3) use PIL (http://www.pythonware.com/products/pil/) to read the file. This will give you the facility to treat it as an image, rather than a collection of bytes. 4) read it into a list of lists using the struct module. This is the slowest option, and I would not recommend it. Dave - The Developers' Coach Last edited by DevCoach : May 1st, 2004 at 05:49 AM.
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#7
May 1st, 2004, 09:57 AM
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I don't really know enough about the internal structure of an image to comment much here but heres attempt - assuming that a line in the image ends with a '\n' char so can be looped over easily line by line.
Code:
#!/usr/bin/env python
array = []
for line in file('image', 'rb'):
array.append(list(line))
Note: Yet again these are untested . If anyone tries this and it doesnt work let me know what's wrong since it came straight from my head and might not actually work. At the very least they should give some idea's.Anyway I'd probably go with the PIL aproch on this one (depending on what it is you want to do with the data) but then i'm a big fan of PIL so  Mark. Last edited by netytan : May 1st, 2004 at 10:12 AM. Reason: Typos
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#8
May 1st, 2004, 11:33 AM
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I was not guessing really
Image files are not organised like text files - they are usually binary data with no EOL. In general the file formats do not have a one to one relationship between bytes/char and pixels at all as either the pixel information is bigger than a basic data unit and/or the format uses compression. Consider jpegs, png and GIF files. So, unless you convert the file from its own structure to something 'flat' and useful using things lke PIL you won't be able to manipulate them by pixel anyway. So this exercise is rather artificial unless the graphics file is basic. grim
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#9
May 2nd, 2004, 10:30 PM
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Hehe my bad
. Though it must be possible to work directly with the pixels an image since if i am remembering corectly there is a section on this in 'The Python Bible' book. I dunno  PIL is the easiest option anyway.PyMagick is another good one but has no docs or docstrings to help you! Mark.
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#10
May 3rd, 2004, 05:20 AM
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FYI:
This link details the structure of a BMP file. http://www.fortunecity.com/skyscrap...4/bmpffrmt.html If would not be difficult to manipulate the file using standard python.
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#11
May 4th, 2004, 03:04 PM
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Hi all - thanks for the info - I should have been more specific in that I want to manipulate "raw" image files - that is, files that do not have a header, but simply contain x bytes = rows * cols * bytes_per_pixel. The reason for reading into an array is that I am doing some calculations which requires the spatial relationships of pixels to be maintained.
Anyways, thanks for all the suggestions - I am reading up on pil - that might do the job. Thanks again. AD
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