[yt-users] Why the two figures have different resolutions?

Tue May 16 05:39:54 PDT 2017

On Tue, May 16, 2017 at 6:38 AM bazhaoyu_sjtu <hudan_bazhaoyu at sjtu.edu.cn>
wrote:

> Hi Nathan,
>
> Thank you for your prompt reply!
>
> I also have two questions:
>
> 1.  When I tried your new script, the bug are presented below:
>
> -------------------------------------------------------------------------------------------------
> pz.set_size_inches(10)
> AttributeError: 'ProjectionPlot' object has no attribute 'set_size_inches'
>

Oops, that should have been pz.set_figure_size(10). I thought I fixed that
it I guess I forgot to update the email. Sorry for the confusion.

>
> -------------------------------------------------------------------------------------------------
>
> 2.  For your two images, I also found that figure's resolution of the
> 'off-axis projection' is worse than the 'regular projection', for example,
> the 'off-axis projection' figure has more obvious grid point.
> Is there any difference between two projection ways? Because I supposed it
> should be same.
>

They are generated using very different algorithms. The axis-aligned
projection is calculated based on an adaptive quadtree while the off-axis
projection is calculated using a ray-casting volume renderer.

I agree with you that in principle they should come out with
identical-looking results for the script you provided. It's possible that
you're data is exposing a corner case that somehow makes the off-axis
projection appear blockier. It's hard to say without more info. Please see
the last paragraph of my last message about this, if you share an example
dataset that triggers this behavior or share the images you are seeing it
will make it easier to speak intelligently about what you are seeing.

>
> Thank you again!
>
> Regards,
> Dan
>
>
>
>
>
>
> On 05/15/2017 09:56 PM, Nathan Goldbaum wrote:
>
> Hi Dan,
>
> I adapted your script to use a public dataset from yt-project.org/data. I
> also don't have a definition for a surface_brightness field in my copy of
> yt, so I guess you defined that elsewhere? I used 'density'. Here's the
> script I ended up coming up with:
>
> http://paste.yt-project.org/show/7142/
>
> (note that you can upload scripts with the "yt pastebin" command line
> tool, e.g. "yt pastebin my_script.py")
>
> And indeed, in that case I get two images that aren't the desired high
> resolution you're looking for (although both images have the same
> resolution). The issue is that yt is saving the figure to disk through
> matplotlib. To control the image size in pixels through matplotlib you need
> to know the figure size in inches and the dpi (e.g. the number of pixels
> per inch of figure). Here's a revised version of the script above which
> saves to disk two images with the desired 3200x3200 resolution:
>
> http://paste.yt-project.org/show/7145/
>
> It also hides the axes and colorbar, since that makes it easier to guess
> ahead of time what the final image resolution will be. Here are the two
> images:
>
> regular projection: http://i.imgur.com/ialfYC4.png
> off-axis projection: http://i.imgur.com/PcvPZ9f.png
>
> You can share images you make using yt using the "yt upload_image"
> command, which will upload an image stored locally to imgur. We usually
> suggest using that to share images or alternatively something like a
> Jupyter notebook on gist.github.com to avoid clogging people's e-mail
> with high resolution images.
>
> Finally, if you don't want to mess with matplotlib and just want the
> underlying images, you can get those via the frb attribute of the plot
> object. Something like:
>
>     plot = yt.ProjectionPlot(ds, 'z', ('gas', 'density'))
>     plot.set_buff_size((3200, 3200))
>
>     # image will be a 3200x3200 numpy array
>     image = plot.frb['gas', 'density']
>
> I hope that helps clear things up. If you are still having issues it would
> help if you could share the exact yt version you are using as well as a
> runnable test script that demonstrates the issue. If you can't trigger the
> issue using one of the public SPH datasets on yt-project.org/data, you
> can share one of your datasets using the yt curldrop:
> <https://docs.hub.yt/services.html#curldrop>
> https://docs.hub.yt/services.html#curldrop
>
> -Nathan
>
> On Mon, May 15, 2017 at 4:40 AM, bazhaoyu_sjtu <hudan_bazhaoyu at sjtu.edu.cn
> > wrote:
>
>> Hi yt-users:
>>
>> I want to ask you a question about the figures' resolution, I handled
>> Gadget file -- hdf5.
>>
>> I used the 'ProjectionPlot' to plot the surface brightness map which is
>> integrated along the 'z' axis, please see the figure 1; And I plot another
>> surface brightness map by 'OffAxisProjectionPlot' with the normal_vector is
>> [0, 0, 1], please see the figure 2.
>>
>> # I can't upload attachments ??? #
>>
>> Comparing two figures, we can find the structures of them is same, but
>> the resolution is different! The 'OffAxisProjectionPlot' is obviously worse
>> than 'ProjectionPlot', I don't know why!
>>
>> Below are the process I created two figures:
>>
>> ======================================================================
>> import yt
>> import numpy as np
>> import yt.units as units
>> import pylab
>>
>> fname = "snap_206.hdf5"
>> unit_base = {'UnitLength_in_cm' : 3.08568e+21,
>> 'UnitMass_in_g' : 1.989e+43,
>> 'UnitVelocity_in_cm_per_s' : 100000}
>> bbox_lim = 1e5 # kpc
>> bbox = [[-bbox_lim,bbox_lim],
>> [-bbox_lim,bbox_lim],
>> [-bbox_lim,bbox_lim]]
>> ds = yt.load(fname,unit_base=unit_base,bounding_box=bbox)
>> ds.index
>> ad= ds.all_data()
>>
>>
>> ## For 'ProjectionPlot' ##
>> pz = yt.ProjectionPlot(ds, 'z', ('gas', 'surface_brightness'), 'max',
>> width=(0.4,'Mpc'))
>>
>>
>> ## For 'OffAxisProjectionPlot' ##
>> L = [0,0,1]
>> north_vector = [0,1,0]
>> pz = yt.OffAxisProjectionPlot(ds, L, ('gas', 'surface_brightness'),
>> 'max', north_vector = north_vector, width=(0.4,'Mpc'))
>>
>>
>> pz.set_buff_size((3200,3200))
>> pz.save()
>> =========================================================================
>>
>> What can i do to fix this difference of resolution between two plot ways?
>> Thanks a lot!
>>
>> Regards,
>> Dan
>>
>>
>> ==================================
>> Department of Physics, Shanghai Jiao Tong University
>> 800 Dongchuan Road, Minhang, Shanghai 200240, PRC
>> E-mail : hudan_bazhaoyu at 163.com
>> hudan_bazhaoyu at sjtu.edu.cn
>> ==================================
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