New Relightable (RTI) Images Now Available

June 23, 2020

A new way of generating and viewing relightable images has been applied to all Accurate Color images of Ambrosiana C73inf, which includes the Latin (Arian) Commentary on Luke and Latin Moses (Latin Jubilees and the Testament of Moses). The new Relight images can be found alongside the old WebRTI images in the Mirador annotations. A less polished but more direct portal to the available relight html (and supporting) files is available here:

Earlier images were processed using Hemispherical Harmonics. The new images should offer higher fidelity. Feedback is always appreciated, especially about quality for purposes of scholarly analysis.

Relight Fitter

The same institution that brought us WebRTI has brought us “Relight,” including a fitter and a viewer: Visual Computing Lab of CNR-ISTI, The fitter is the part that combines images captured with the light at many positions into a single image. That single image simulates the appearance when lit from any light position, not only the ones captured. The cost of this interactivity and the ability to extrapolate and interpolate can be image fidelity, with some methods better than others. The images processed in 2017 used the Hemispherical Harmonics method. The Accurate Color images from C73inf were reprocessed in 2020 using the bilinear interpolation method with 27 planes. For C73inf, the fidelty was much better with Bilinear than RBF or RBF in YCC. The bilinear method was also tested at 36 and 18 planes. More planes did not produce noticably better results to justify loading additional data. Color fidelity was noticably poorer with 18 planes (most evident in the color checker).

Relight Viewer

Along with the new options for fitting relightable images, the good folks at Visual Computing Lab also offered some improvements over the WebRTI viewer. The most noticable improvement for casual users is the addition of the option of “visualize normals.” The default mouse/touch behavior is moving the light, rather than panning the image. Web developers have many more useful options for determining the initial view. I prefer to start with a dramatic raking angle. I can also set that angle to match one of the captured positions to facilitate fidelity comparisons.

IIIF Tile Source

One of the biggest improvements for the Jubilees Palimpsest Project from the perspective of data management is support for IIIF as a source of tiles. Tiles are important because a screen can never show all the available pixels at once. It saves time transfering data to load only the broad view at first and then greater detail only for the area to which the user zooms. In WebRTI the only option for storing tiles used static pregenerated tiles. For many users without an image server, this was fast and easy. However, the tens of thousands of tiny images for each RTI image was cumbersome for data management on the backend. With IIIF support, 9 JPEG2000 files (3 planes per file) contain all the data from which derivatives can be generated dynamically. However, as of June 20, 2020, the relight-viewer.js code available on GitHub seems to assume IIP Server in IIIF mode, not the actual IIIF Image API. The simple fix is to replace two lines (or segments of lines working from the minified files) and add two parameters to the HTML that calls the script.

The first substitution is to replace:
t.metaDataURL = t.server + "?IIIF=" + t.path + "/" + t.img + "/info.json";
t.metaDataURL = t.iiifServer + t.iiifPath + '/' + t.img + '.jp2/info.json';

The second substitution is to replace:
return t.server+`?IIIF=${t.path}/${t.img}/${xr},${yr},${wr},${hr}/${ws},${hs}/0/default.jpg`};
return t.iiifServer + t.iiifPath + '/' + image + '.jp2/'+`${xr},${yr},${wr},${hr}/${ws},${hs}/0/default.jpg`;
If your IIIF server suppresses the .jp2 extension then remove it in both replacements.

Then you will want to define iiifServer and iiifPath when invoking the script. For example, see the source code in:

Future Directions

Although the option to tune for fidelity is much appreciated, RTI is fundamentally a simulation mediated by mathematical formulas. The benefits of interactivity, extrapolation, interpolation, and enhancement are significant. However, for scholarship, particularly judgments of whether a mark is a shadow, something on the surface, or a digital artifact, it remains necessary to consult the raking images on which the RTI fitting is based. The Jubilees Palimpsest makes all these images available in the archive: in folders labeled "RTI", for example: Future work will consider options other than RTI for visualizing this large set of raking images (56 images). Meanwhile, four raking corners are available in Mirador and the complete set is available in the archive.