[yt-users] How to re-process a scene?

Scott Feister sfeister at gmail.com
Fri Sep 16 15:13:39 PDT 2016


Hi Nathan,

Thank you so much for this explanation. It gets me off the ground running!
I appreciate the work you've done to make the volume rendering so amazing;
these are small final touches. I'll be happy to open a bug report.

Scott


Scott Feister, Ph.D.
Postdoctoral Researcher, Flash Center for Computational Science
University of Chicago, Department of Astronomy and Astrophysics

On Thu, Sep 15, 2016 at 8:57 PM, Nathan Goldbaum <nathan12343 at gmail.com>
wrote:

> Hi Scott,
>
> So this is really two issues.
>
> The first, is that the camera.zoom() function doesn't really zoom in on
> the image, instead it decreases the "width" the volume rendering region by
> the zoom factor. This makes a lot of sense for plane-parallel volume
> renderings (the default), but not so much for perspective lenses as you
> saw. For a perspective lens you should instead reposition the camera to be
> closer to the focus to get the same effect.
>
> Next, you also saw that manipulating the TransferFunctionHelper object
> (source.tfh) after you've already done a volume rendering doesn't update
> the transfer function. That's because the TransferFunctionHelper object is
> only used to generate the transfer function (source.transfer_function) if
> it isn't set yet. If it's already set, it reuses it. So in your example the
> first time you called save(), the VolumeSource saw that no one had manually
> created a transfer function, and used the TransferFunctionHelper object to
> build one. Then, when you asked it for the second rendering, it just reused
> the same one because right now the VolumeSource doesn't track if the
> TransferFunctionHelper has been updated. To do what you mean, you need to
> manually set the transfer function to be the one generated by the
> TransferFunctionHelper:
>
> source.tfh.build_transfer_function()
> source.tfh.setup_default()
> source.transfer_function = source.tfh.tf
>
> After manipulating source.tfh.
>
> Using as an example the script from the docs, here's how to make the
> second image come out as you would expect for the perspective lens:
>
> http://paste.yt-project.org/show/6823/
>
> Which makes these two images:
>
> http://i.imgur.com/gwUTWz1.png (rendering.png)
> http://i.imgur.com/M1lSz0N.png (rendering2.png)
>
> I think we could probably do a better job of detecting that the
> TransferFunctionHelper has been manipulated and avoid this confusion, if
> you'd like I invite you to open an issue about this on our issue tracker.
> One might also argue that the zoom function should adjust the camera
> position for perspective lenses.
>
> Sorry the volume renderer isn't totally intuitive for this use case. I did
> some work before the yt 3.3.1 release to improve things, but it's still
> definitely not perfect. I think there's a lot of power there but it also
> really needs some love from someone who is willing to think about corner
> cases and interactive workflows.
>
> -Nathan
>
> On Thu, Sep 15, 2016 at 7:38 PM, Scott Feister <sfeister at gmail.com> wrote:
>
>> Hi yt users,
>>
>> Hi there! I'm just starting to use yt to make volume renderings of FLASH
>> simulations. While working through examples, I hit a roadblock, and I've
>> had some difficulty finding good documentation on the what the
>> "Scene.save()" function actually does; specifically, why it does different
>> things on the first and second call. The first example in the user tutorial
>> page for 3D volume rendering (http://yt-project.org/doc/vis
>> ualizing/volume_rendering.html) looks something like this:
>>
>> ...
>> *sc = yt.create_scene(ds, lens_type='perspective')*
>> ...
>> *source = sc[0]*
>> *source.tfh.set_bounds((3e-31, 5e-27))*
>> *...*
>>
>> *sc.save('rendering.png', sigma_clip=6.0)*
>>
>> And, voila, volume rendering saved to png.
>>
>> However, if I naively continue the script to re-render with new settings:
>> *source.tfh.set_bounds((3e-35, 5e-27))*
>> *sc.camera.zoom(2.0)*
>>
>> *sc.save('rendering2.png', sigma_clip=6.0)*
>>
>> I find that none of my new settings are reflected in "rendering2.png" --
>> it's just a duplicate of "rendering.png"! But if I start again from scratch
>> with a new scene, the settings take hold. This leaves me (a new user)
>> scratching my head.
>>
>> So here is my question: Once you've created and saved a scene once, how
>> do you change scene settings like colormap and camera angle?
>>
>> Best,
>>
>> Scott
>>
>>
>> Scott Feister, Ph.D.
>> Postdoctoral Researcher, Flash Center for Computational Science
>> University of Chicago, Department of Astronomy and Astrophysics
>>
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>>
>
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