[yt-users] ImageArray.write_png() always rescales ...

[Matthew Turk]

Hi Stuart,

On Mon, Mar 30, 2015 at 3:59 PM, Stuart Levy <salevy at illinois.edu> wrote:
> I'm trying to use the notebook interface to render, view image result, tweak
> parameters, etc. within a single yt session.
>
> I want to disable auto-brightness-scaling of images, so am calling
>
>      ImageArray.write_png()
>
> Although write_png() takes a rescale= parameter, it effectively always
> rescales.
> It passes its array to write_bitmap(), which does rescaling unless the image
> datatype is already numpy.uint8.

You're right.  It does look like write_bitmap accepts a max_val, which
will be used in lieu of computing the max, before converting to uint8.
Would that work for your purposes?

>
> But if you try to do that yourself, then write_png() crashes inside
> matplotlib, because write_bitmap "must be an rgba buffer".  It's using a
> Python C-level routine to do this check, but I think the problem is that the
> array is not C-contiguous,
> since write_bitmap() has called swapaxes(0,1) on it and not copy()ied it
> afterward.
>
> Demo program at:
>
>     http://paste.yt-project.org/show/5502/
>
>
>
> and here:
>
> import yt
> from yt.data_objects.image_array import ImageArray
> import numpy
>
> # 3x4x4 array, ranging from 0 to about 0.5
> v = (numpy.arange(12)/24.) .reshape(3,4,1) * numpy.ones((1,1,4))
> # Write
> ImageArray(v).write_png('grays_rescaled.png', rescale=True)
> # Write, but asking not to rescale.   It rescales anyway since
> yt.data_objects.image_array() calls write_bitmap()
> ImageArray(v).write_png('grays_unscaled.png', rescale=False)
>
> # write_bitmap rescales internally unless the image's datatype is exactly
> "numpy.uint8".
> # Let's try doing that ourselves.
> vv = (v*255).astype(numpy.uint8)
> print vv.flags
> ImageArray(vv).write_png('grays8.png', background=None, rescale=False)
>
> # Crashes! inside matplotlib/src/_png.cpp.
>
> # Why? yt/visualization/image_writer.py's write_bitmap(), if it doesn't
> rescale the image,
> # simply calls swapaxes(0,1) on it and passes the result to matplotlib.
> # But the resulting array is no longer C-contiguous (write_bitmap.flags
> shows False).
>
> # When rescaling *does* happen, write_bitmap() is constructed by
> np.concatenate() which makes a new contiguous array.
>
>
>
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