I while back, I got a bug report for mahotas-imread :
PNG reads in inverted: This image loads in imread 0.3.1 incorrectly. It looks right if I xor it with 255, but I don’t think that’s all that’s wrong.
The first thing I noticed was that this was a 16 bit image. If you’ve been coding for as long as I have, you’ll immediately think: It’s a byte-endiness issue . So, I tested:
and saw the following:
Not good. The I looked at the hint that the original poster provided and it did seem to be true: imshow(~f) worked reasonably well. My working hypothesis was thus that there is a flag whereby the PNG data needs to be interpreted after a bit reversion. I also noticed another thing, though:
max_16bit_value = 2**16-1 imshow(max_16bit_value - f)
Also looks decent.
The TIFF format does allow you to specify whether zero is supposed to be white or black. Maybe PNG has a similar “feature.”
I read through the libpng documentation (which is not great), a bit through its source, and through online descriptions of PNG format. Along the way, I noticed that converting the image to TIFF (with ImageMagick) and loading it with imread also gave the wrong result. Perhaps the TIFF reader had the same bug or ImageMagick .
Eventually, I realised that PNG files are in network order (i.e., in big-endian format) and the code did not convert them to little-endian. Thus, my initial intuition had been right: it was a byte-swapping error!
But in this case, why did imshow(f.byteswap()) result in a mangled image?
I stated to suspect that matplotlib had a bug. I tried to do:
imshow(f.byteswap() / 2.**16)
and it resulted in the correct image being shown.
As it turned out, matplotlib does not do the right thing when given 16 bit files. Thus, I had this figured out. I fixed imread and released version 0.3.2 and closed the bug report.
A single bug is often easy to debug, but when you have multiple bugs interacting; it is much more than twice as hard.
Hairy details: You may want to stop reading now.
Consider the following identities:
255 == 0xff -f == (f ^ 0xff + 1) 2**16 - f = -f + 2**16 == -f (because of overflow)
Thus, it should not be surprising that flipping the bits or subtracting the image resulted int , in two-bit complement, ~f is roughly -f. Not exactly, but similarly enough that, by eye, it is hard to tell apart.
Finally, it all makes sense when you realise that matplotlib assumes that non-8 bit images are floating point and does:
final_image = (input_image * 255) final_image = final_image.astype(np.uint8)
Because what is multiplying by 255? It’s the same as multiplying by -1! Thus, matplotlib would multiply by -1 and then take the low order bits. Thus, it showed a correct image if you pre-multiplied it by -1 (or flipped the bits) and gave it a byteswapped image!
|||People don’t always appreciate how valuable good bug reports are. Seriously, they are a huge help: you are testing the software for me. Unfortunately, either shyness or past bad experiences will often cause people who see something worng to not report it.|
|||I now have over 15 years of experience coding (having had a relative late start [I didn’t care much about computers until I was close to college age], I’ve caught up.) If there is an area where I really feel that my experience shines through is debugging: I’ve seen enough mistakes and errors that my guesses as to what the bug is are more and more accurate (this is true even in code I have not seen).|
|||One of the reasons I started mahotas-imread was that I had not found a single library that could read the formats I wanted without a lot of bugs. So, I trust no one on this. In this case, the paranoia was unwarranted, as we’ll see.|
- imread 0.3.2 (pypi.python.org)