Question

Does anyone know a way get the pixel data from a PythonMagick.Image instance without having to write it to disk first?

For instance, I can read in an image using:

import PythonMagick
im = PythonMagick.Image('image.jp2')

I would now like to be able to get the uncompressed image data so that I can use it in something else like NumPy or matplotlib, but I can’t seem to find any way to do this. I would just use matplotlib or PIL directly but the image format I’m reading in is JPEG 2000 which is only supported by PythonMagick as far as I know.

Any suggestions?

Answer 1

Disclaimer: I don’t have PythonMagick built where I am right now and am no expert, so (1) any or all of the following may be wrong, (2) it will certainly be less specific than you’d like, and (3) if someone else knows better I hope they won’t be put off by seeing an answer already here. Anyway:

From a quick look at the code, it looks as if you can read pixel values one by one using the pixelColor method on the Image class. This returns a PythonMagick.Color value, from which you can extract R,G,B components. The underlying C++ library supports reading out lots of pixels at a time using Image::writePixels, which is also present in PythonMagick.Image; but I think the proper use of that method depends on other things that aren’t implemented in PythonMagick. That’s a pity, because I bet it would have been much much more efficient than reading one pixel at a time.

Alternatively and probably better, it looks as if you can write the contents of the image to a PythonMagick.Blob object in memory, which basically does the same as writing to a file only without the file :-). You can choose what format it should write in, just as you do when writing to a file. There seems to be something called get_blob_data for extracting the contents of a Blob. Something like this:

im = PythonMagick.Image('image.jp2')
blob = PythonMagick.Blob()
im.write(blob, "png")
data = PythonMagick.get_blob_data(blob)

The resulting data is, I think, a Python string whose bytes are the binary representation of the image. (I’m assuming you’re using Python 2.x, where the string type is 8-bit. I don’t know whether PythonMagick works with 3.x.) I think there are some formats that are basically raw pixel data; try “RGB”. You can then extract the contents via lots of struct.unpack or whatever.