[yt-dev] Differences between on-axis projections and off-axis projections

[Nathan Goldbaum]

On Sun, Aug 17, 2014 at 6:58 PM, Cameron Hummels <chummels at gmail.com> wrote:

> Hey everyone,
>
> Now that the righthandedness has gone into the PlotWindow objects, making
> slices and projections of isolated disk galaxy simulations yields the
> y-projection having a vertically oriented disk as seen here in an image
> generated by Mike Butler:
>
> http://www.ics.uzh.ch/~butler/Enzo_nofeedback_1000Myr_Sigma.png
>

There was a change in the way slices and projections through the y axis are
oriented to make the plot axis a right-handed coordinate system.  This
happened in the 3.0 branch sometime in the spring.

 It might be the case that the script was written before we made these
changes and needs to be updated (probably by calling transpose on the raw
image array for the y projections).


>
> I suppose we could modify the ProjectionPlot framework to allow a keyword
> to specify the x-axis and y-axis vectors, but I was just trying to make
> equivalent projections using the OffAxisProjectionPlot object since that
> allows for one to explicitly specify a north_vector.  Unfortunately, I'm
> finding some inconsistencies between the ProjectionPlot (on-axis) images
> and the OffAxisProjectionPlot images (specifying vectors to be on-axis).
>
> This simple script demonstrates what I'm talking about:
>
> http://paste.yt-project.org/show/5049/
>
> Running this script yields 4 images:
>
> off_axis_proj.png - http://i.imgur.com/CKbq5iO.png
>

It looks like something is going wrong with the raw image values in this
plot.  This seems like a pretty clear bug.

Those values make sense for a volume density, perhaps the projection
conversion is not being correctly applied?


> on_axis_proj.png - http://i.imgur.com/2iBucuv.png
> off_axis_weighted_proj.png - http://i.imgur.com/RQV2REP.png
> on_axis_weighted_proj.png - http://i.imgur.com/CNWHyQS.png
>

I'm not terribly surprised that the image extrema aren't exactly equal here
since the algorithms used to calculate the projections are very different.
I think you might also need to set the no_ghost parameter to False to get a
more accurate off-axis projection.

I'm not sure whether the off axis projection machinery will ever perfectly
agree with the QuadTree projection, someone who is more knowledgeable about
the volume renderer would need to comment on this.


>
> As you can see, the off_axis_proj is using significantly different limits
> than the on_axis_proj, which makes me think it isn't doing a true line
> integral, but I haven't investigated this closely.  While the weighted
> projections are better, they still have slight differences in their limits.
>  Is this expected and I'm just messing something up here, or is this a true
> bug?
>
> Cameron
>
> --
> Cameron Hummels
> Postdoctoral Researcher
> Steward Observatory
> University of Arizona
> http://chummels.org
>
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>
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