On Easter weekend we headed out with a small, but very enthusiastic group of students to make some new RTI captures and improve on ones we weren’t satisfied with from the last outing. We found our group of five got along well and we worked quickly and carefully together.
Although there was haze on this unseasonally warm day that brought down the overall light intensity, we needed an umbrella to remove the highlights cast by the sun on our two black snooker balls. It was worth it as the RTI Builder had no problem detecting the correct highlights when we processed our images.
The umbrella added a very picturesque element to our candid shots of the captures for that day!
One of our objectives was to redo an unsuccessful capture of the monument for Absalom Day. The marble surface was heavily eroded with the harder metamorphic areas of the marble persisting. The effect was an almost veinous texture that made reading the stone very difficult. Under the Diffuse Gain enhancement the text could be made out reasonably well.
Specular Enhancement is normally the technique we reach for first when trying to transcribe a stone. In the case of Absalom Day’s monument, Specular Enhancement failed to substantially improve readability. Much work remains to be done on standardizing these enhancement techniques…why are some effective when others yield less than ideal results?
One stone that was of particular interest to us was an obelisk-style monument with detailed reliefs on two sides. While the writing was very clear on visual inspection, the fine decoration work could not easily be discerned. Interesting too was the obvious preferential weathering. The North-facing side was little abraded with the mason’s original tool marks clearly visible, while the West-facing side had virtually no tool-marks visible. Surely a good candidate for an RTI study! Topography, however, did not work in our favour.
Here we can see a good light distribution on the sphere for the side commemorating E.E. Johnston. The Surface Normal Visualization clearly shows the pale-blue colour that generally indicates a clean capture. In other words, the surface normals point towards the camera, just what we would expect from a flat surface parallel with the sensor-plane of the camera.
On the side commemorating Victor Dupont, however, we failed to make a clean capture despite the fact that the light distribution on the blend image was not appreciably different. The problem turned out to be simple. There was a rise in the ground from the base of the stone to the outside of our light radius that made taking low shots with the light all but impossible at a distance approximately four times the region of interest. We will attempt another capture with a tighter radius and better light control.
This stone was an obvious candidate for RTI enhancement. We could make next to nothing on the stone and could not even discern a single name. The monument itself is sandstone and therefore very susceptible to weathering and the effects of salt efflorescence. The Specular Enhancement improved matters, but much work remains to be done with both the RTI image and in the archives to make a full transcription.