“TOASTing”: Tiling full spheres

To view full-sphere data in AAS WorldWide Telescope — either all-sky maps or all-planet maps — the input imagery needs to be tiled into the special TOAST projection. It is not necessary to use this projection for images that may contain many pixels but do not cover a substantial fraction of the sphere. In those cases, you should prepare a study instead.

The intent of the toasty package is to make it so that most tiling workflows can be executed on the command line using the toasty program. This section will demonstrate this way of using the software.

Toasting a single full-sphere image

Sometimes, you already have a large image mapping the entire sphere — it just needs to be reprojected, broken into tiles, and described in a WTML file. The workflow in this case is as follows:

  1. Choose and/or create a directory on your computer in which you’ll be working.

  2. Download the source image to your work directory. For concreteness, we’ll call it fullsize.tif here, but multiple image formats are supported.

  3. Determine the projection used by the source image. This is the mechanism by which the curved surface of the sphere is mapped onto a 2D image. For full-sphere imagery, only a few choices are ever used: probably the main thing to check is whether your image is in equatorial (RA/Dec) or galactic (l/b) coordinates. Consult the toasty tile-allsky documentation (or toasty tile-healpix) for the supported choices. If your image uses an unsupported projection, please file a request with the developers.

  4. Determine the tiling depth appropriate for your use case. The depth is a number that specifies the highest resolution that your final map will attain. Consult the toasty tile-allsky documentation for the quantitative definition. The best choice will depend on your individual circumstances. But as a general guideline, you should probably choose the depth that yields a number of pixels equal to, or just greater than, the number of pixels in your source image.

  5. Do the initial tiling with:

    $ toasty tile-allsky --projection={PROJECTION} \
        --outdir=tiled fullsize.tif {DEPTH}

    where {PROJECTION} and {DEPTH} should be replaced by the values you determined in the previous steps. (Here, the backslashes are used because the command spans multiple lines of the shell prompt. If you type it all on one line, no backslashes should be used.)

    It might make sense to use some of the Standard image-loading options to control how the source image is processed. Along with the tiles, this command will generate a file tiled/index_rel.wtml that describes the imagery.

  6. Generate the higher-level tiles with:

    $ toasty cascade --start={DEPTH} tiled

    where {DEPTH} is the same value as used in the previous command.

  7. Examine the image tiled/0/0/0_0.png to see if it looks like a reasonable TOAST-ification of your source image. The equator of your map will be translated to a diamond touching the midpoints of the edges of the square tile.

  8. Review the appearance of your image in WWT. Use the wwtdatatool command provided by the wwt_data_formats Python package to start up an HTTP server that will make your WTML and tile data accessible to WWT (either Windows or web clients):

    $ wwtdatatool serve tiled

    This command will print a URL to a synthetic index.wtml file that you can open up in WWT to view your tiled image. Hopefully everything will be fine, but at this point you can tune and/or fix the tiling procedure if something isn’t right.

  9. Fill in proper metadata in the index_rel.wtml file. Items to consider are:

    • The <Credits> XML element with proper credit text.

    • The <CreditsUrl> XML element with a link to the image source and/or more information about it.

    • The <Description> XML element with text describing the image. If editing the XML manually, make sure to properly escape the magic XML characters & (to &amp;), < (to &lt;) and > (to &gt;).

    • The Name attributes of the <ImageSet>, <Place>, and <Folder> elements. These should generally all be the same.

    • Other metadata like the Bandpass, etc.

  10. When the index_rel.wtml file is all finalized, it needs to be transformed to have absolute rather than relative URLs. To do this transformation, you need to know the URL from which users will be accessing your data. When you know that base URL, the command to use is of this form:

    $ wwtdatatool wtml rewrite-urls \
        tiled/index_rel.wtml \
        http://myserver.org/datasetname/ \

    (Here, the backslashes are used because the command spans multiple lines of the shell prompt. If you type it all on one line, no backslashes should be used.)

  11. Finally, upload the complete contents of your tiled subdirectory to your web server. In this case, the upload location should be such that the url http://myserver.org/datasetname/index.wtml will yield the index.wtml file created in the previous step.