Fix comparations for image colorspace literals

[bashkirtsevich]

See issue #131
Python: 3.6.3

[pietermarsman]

Hi @bashkirtsevich, thanks for your contribution. This is a very late response...

We don't have any test that run this code so I am not sure if this breaks any other kind of pdfs. I can imagine that sometimes the colorspace is a list and sometimes it is not. Do you have any idea if that is the case?

If you want this to be merged you should also add a a test. That makes sure that this code can actually be run (which I believe by looking at it) and it will make sure that we don't change it without thinking.

Do you have time to work on this?

[bashkirtsevich]

In my PR I just fix logical mistakes such as is and in.
We using this library with my patch, and its finally work on a lots of PDF documents.
Merge not necessary. May be this PR can helpful as a knowledge base.

[pietermarsman]

I've just checked if any of our current tests is using the ImageWriter, and this is not the case.

But we do have some sample pdf's that contain images. This is the result of evaluating image.colorspace at the start of ImageWriter.export_image().

nonfree/dmca.pdf.

• [/'Indexed', /'DeviceRGB', 255, <PDFObjRef:86>]

nonfree/175.pdf

• [/'DeviceRGB']
• [/'DeviceRGB']

Both pdfs have a list of colorspaces instead of a single value. And thus our current way of checking e.g. image.colorspace is LITERAL_DEVICE_RGB is wrong.

[pietermarsman]

I've checked the code from this PR with nonfree/dmca.pdf and it improves the output! With this PR the code detects that it is a RGB image and should be written to disk as a .bmp. With the old code the image is not recognized as RGB and it is written as a generic .img file that cannot be easily opened.

[pietermarsman]

I would like to have some tests for this code but @bashkirtsevich indicated that he does not have time to work on this.

I've checked the code with pdf's that contain images and that works!

So, I suggest we merge this into develop.

[chengmengli06]

@pietermarsman I have a pdf with some images write as general ".img" files, how could I convert these images to jpg format? I use pdf2txt, the first 3 bytes of the stream are H\x89\xec, the corresponding hex is 4889ec.

[pietermarsman]

pdfminer.six writes to a .img file if it cannot infer the type of the image. It supports jpeg (.jpg), jbig2 (.jb2) and grayscale (.bmp) images. The bytes of an unrecognized images type are written to this extention: '.%d.%dx%d.img' % (image.bits, width, height). From the filename you can infer the number bits, width and height of the image. I don't know how you should open it.

Please share if you find a way to open the images. Perhaps we can add it to pdfminer.six.