Re: [Matplotlib-users] imread() and PNGs

> Tobias,
>
> I would like to apply your patch, but the test in
> examples/tests/pngsuite fails. If you can submit a new patch where this
> test passes, and, even better, if a small example 12-bit PNG of yours is
> added to the test, I will apply it.
>
> Apart from that, I would echo Eric's thanks for the patch and  
> explanation.
>
> -Andrew

Hi Andrew and Eric,
Thanks for the responses. I was unaware of the png test suite. I have  
attached a new diff that passes this test correctly. It originally failed  
because I was not handling greyscale images with an alpha channel, but it  
also brought to light several other issues with my code that I have fixed.  
I have changed the structure of the code significantly - the if/else  
struct has gone and a single loop returns the different image matrices.  
Although this is more concise, it no longer informs the user if it hits an  
unsupported image type.

Unfortunately I do not have a small test image available, all of ours are  
a minimum of 512x620. However the pngsuite page,  
http://libpng.org/pub/png/pngsuite.html, does have a set of suitable  
images labelled cs*n*.png. These have thrown up an interesting issue - to  
what maximum value should n-bit images be scaled when n is between 8 and  
16? The png spec and test images suggest it should be (2^n - 1). This  
means that higher bit depths give higher precision over the same intensity  
range and the same maximum value. However for my particular camera and  
software this would be wrong, as the CCD has a fixed 12-bit dynamic range  
and the lower png bit depths are only used to save file space. Hence at  
the moment I have set my software to scale to (2^16 - 1) for 8 < n < 16,  
but it follows the png spec for n < 8, so there are two contradictory  
behaviours and I am unsure which is the best approach. Personally I would  
prefer matplotlib to return raw integer values, not floats scaled between  
0 and 1 and then I can apply the scaling myself, but I am aware that this  
is not particularly user friendly for anyone else. imshow() seems to  
handle integer values fine and correctly scales for display, provided that  
no alpha channel is present.

Should I post another message to the developer list about this to see what  
people think? I'd very much like to discuss this with someone who has a  
lot more experience of pngs than me.

Thanks,
Tobias