Dave Martin

Ah, very sorry - just realised I had clicked on the PTM spec link in Carla's message, not the RTI one. I get the same error as you do for the RTI one. Dave

Just tried and it still downloads OK and then opens OK. If you're having problems, maybe something on your PC/device, or further upstream in your internet connection (corporate policy or ISP?) is blocking it. Dave

Camera setup Re: Steps: Remove ND Filter. Frame the shot. Focus (using centre of image). Adjust F-stop so you have sufficient DOF to get the whole monument face in focus. Do not stop the lens down any further than you have to in order to get everything in focus. Replace ND. Set shutter speed as appropriate for your flash. Adjust ISO if necessary to get sufficient capture, don't use ISO any higher than you have to to avoid introducing any more noise than you have to. Don't use auto-focus / auto-exposure / auto-WB. Conceivably you may get away with auto-ISO.

Illumination For successful RTI, you need a sole (or dominant) isotropic point light source with constant intensity (it only needs to be isotropic within the cone that will hit the subject/monument so maybe a 90 degree cone is enough). Indoor, with no/low ambient light, you can use a studio lamp of some sort. Outdoor, in daylight, you have very limited options other than a powerful flash. You have mentioned modern (LED) torches but I really don’t think they will work. Firstly, it needs to be bright enough to ‘overpower’ the ambient light, and I don’t believe there are many (any?) LED sources that meet the needs of RTI and are price-competitive with a suitable flash. Most powerful LED lamps adopt measures which render them unsuitable for RTI – they either have multiple LED chips/cobs and/or use reflectors. Either of those measures mean that it is no longer a point source, and also that it is no longer isotropic. Multiple LEDs = multiple sources. Reflector means source broader – e.g. a torch with a 100mm reflector effectively means a 100mm dia source - again, no longer a point source. A reflector also means no longer isotropic as light is stronger in the ‘beam’.

Spheres Jon, you mentioned baubles (in jest) but they are unlikely to be up to RTI! Apart from mounting issues, they are unlikely to be sufficiently spherical. Requirements are for a sphere (or, more accurately, a hemisphere) which is geometrically spherical, and has a black (or dark) uniform smooth/polished/glossy surface. For objects of the size of gravestones, black snooker balls are ideal, and they’re cheap enough to be replaced if they get scratched. Mounting the Spheres It needs mounting so that, ideally, the centre of the sphere or the equator of the hemisphere is in the plane of the surface you’re imaging. If you’re imaging something where you can have the spheres to the side then that’s relatively easy, it’s a bit harder of you have to position a sphere in the middle of an artefact. If the monument is recumbent but proud of the ground, I will just place a snooker ball either side of the stone, and pack it up to a suitable height. If it needs help to stop it rolling away, I use either a brass ‘cup washer’ that it used in furniture (from e.g. B&Q) or the cardboard cylinder centre from a roll of insulating tape. Don’t be tempted to use ‘BluTac’ as the residue it leaves on the surface, if on the upper surface next time, can spoil the results. I have also put a flat spot on a snooker ball by hand-held sanding against linisher belt (don’t be tempted to put it in a vice without very good protection!). To mount them on ‘arms’ I drilled and tapped the snooker balls. I started by tapping standard ¼” as used for basic photographic kit, but I found that didn’t fare well (short, fragile thread in the ball material). I then tried 3/8” – worked better, but thread started to wear. I thought about several alternatives such as gluing-in a length of 3/8” studding so I could use a coupler, but what I ended up doing was gluing a standard photographic 3/8” to ¼” reducer which worked almost the same way as a Helicoil insert I would use in Acetal or aluminium. Supporting the Spheres Photographic lightstands are ideal for use with vertical gravestones. You’re ideally looking for an un-interrupted hemisphere facing the same way as the face of the slab, so ideally site the lightstands behind either side of the monument and then bring the arm forward until the centre of the sphere is in the same plane as the face of the monument. To prevent distracting highlights from the lightstand arm, I sleeve the end nearest the sphere in matt black – in fact I literally use old black socks, held in place with black shoelaces. Whatever you do, don’t be tempted to use flexible goosenecks – they will droop! Multiple spheres Even though most processing still only uses one sphere, I will always use two if I can. Partly future-proofing in case dual-sphere processing in future, but mainly as a precaution in case one of the spheres moves – e.g. nudge when moving the flash around, or brush with clothes, or lightstand slowly sinking on grass. Wind can cause things to move, and whilst you can suspend a small ballast (sand) bag from the centre of the lightstand tripod, you need to beware of the wind causing movement of the sphere on the end of the lightstand arm.

Camera tripod Unless it is as perfectly laid out and tended as a Commonwealth War Graves cemetery, getting the camera in the correct position to frame the different monuments can be tedious. Plots can be different heights, different surfaces, kerbed or otherwise, … Adjusting a tripod can take a surprising amount of time. You might think a pan/tilt head would be useful – and it would IF the tripod is level, but it won’t be and it is not worth the faff to level the tripod for each monument plot. A ball head helps, but then you’re usually having to use two hands – one to move and prevent the camera toppling, and the other to lock/unlock the ball head. What I found great for this purpose was a Squeeze-grip Manfrotto 222 head – you can do it all with one hand – squeeze the grip and adjust, let go and it locks, and you can adjust the drag. A cheap bull’s-eye level on top of the camera (or otherwise suitably affixed) is a useful aid to setting the camera. You still have the problem of accommodating monuments which significantly differing height. You may have no alternative but to adjust the tripod leg lengths. One thing I found useful though was having a tripod with a centre column that can be moved up & down by just using a thumb on a spring lock lever. I have much better tripods, but for this feature alone I used to use an old Cullman 2903. For monuments, to make best use of every pixel you’ll probably be shooting mostly in Portrait, so unless your camera is in a cage you’ll need an L-bracket or similar (don’t reply on hanging it off to one side using the ball head as even the slightest droop will spoil the RTI set). To help with setting the camera for each monument, and focusing, it is extremely useful if you have a monitor which you can view from above. Whilst my cameras (Sony Alpha) have modest rear screens with some degree of articulation, I found it was worth mounting a remote HDMI monitor (mine is 7-inch diagonal) to one side of the camera, via a small ‘magic arm’. The monitor’s weight was counterbalanced on the other side of the tripod by the USB ‘power bank’ I used to power the camera via a dummy battery / battery eliminator. Separate message to follow later re spheres and light sources. Dave

Hi Jon, More to follow on equipment, but firstly, I’m a little curious as to your plans to RTI capture 2-4 graveyards in a day - unless they’re just a small number of monuments in each, then I think that might be optimistic. Bear in mind that for each slab you have to (1) Carefully position the sphere(s) on their mounting appropriately; (2) Setup your camera on its tripod, frame the shot so camera is looking at the centre of the slab perpendicularly and includes all of the slab and the sphere(s), and focus (always manual, never AF); (3) Take the photographs (you’re probably looking at a minimum of 25-30); (4) Depending on how you plan on separating the myriad of photographs into one folder per monument, either at the start or end of the batch (or both) for each monument you may at a minimum want to shoot an image in the set of a blank card or a ‘slate’ with monument plot number or similar. Then, periodically, you’ll need to stop and quality-check the images you’re acquiring; and of course pre-preemptively change camera batteries (if it runs-out part way through a set, you need to discard the set and start again because the movement associated with changing battery will mean not all images have exactly the same camera position). You'll also have to change flash batteries / battery packs; and you may have to let the flash'rest' / cool down after a few hundred shots (or have two flash units that you alternate between). I would suggest throughput is more likely to be measured in tens of monuments per day; a slick operation, with an assistant, might just crack a hundred in a long day? Dave

Hi Jon, Yes, RTI can be a great help on weathered gravestone inscriptions: Q1. What lumen should the light source have given picture are taken in daylight? Can the lumens be too high? A: You need the directional light to be many many times brighter than the ambient illumination. A continuous light (be it LED or halogen etc.) is unlikely to be adequate/appropriate. A powerful flash is usually the answer outdoors; but to reduce the ambient impact you need a very strong ND (neutral density) filter on your lens. Q2. What plane should the lightsource spheres be place in given the stone of vertical? Sit on top of the stone/attached to the face of the stone? A: The spheres need to be in the same plane, for an upright slab I would typically have two, either side of the slab, so that the plane of the slab cuts the middle of the sphere. I have several spheres that I have drilled and tapped with 3/8" thread as used for camera kit and then mount the spheres on lightstand arms. Q3. Modern torches can have an adjustable field of focus - should the light source always illuminate the whole picture frame or should it be focused narrowly and move across the lettering. A: The illumination needs to be point source but not constrained light like a narrow torch beam; also the intensity of a torch beam will fall-off as you get off-axis, which likely lead to sub-optimal results. Dave

Re: and and I haven't evaluated this effect using Kintsugi, but whilst fully agree with Carla that disabling scale-containing images would mean as you say that they wouldn't be 'seen' by Kintsugi, they will however still have an effect with potentially introducing reflected illumination in other non-disabled images. I wonder if, in addition to disabling images as Carla suggested, once sufficient frames have been captured with the scales, maybe the scales could be carefully covered with a suitably optically-dead / black material to prevent (or significantly attenuate) any spurious extra light reflected from the scales in the images to be used by Kintsugi? Dave

Rich, not directly answering your Kintsugi 3D question, but if the files aren't 'massively' too big, there may be a short-term workaround. I don't know if it is still the case, but it used to be that the limit on Sketchfab was actually on the physical upload file size - and whilst you can upload the model files as-generated, Sketchfab used to also accept them as a 'zip' archive, which compression could help if the upload was marginally over (but not if it was orders of magnitude too big!). Dave

Michael, Although the specific article links are broken, I've just had a quick look and there is still some info in Marcin's posts on LinkedIn https://www.linkedin.com/in/marcin-klebowski/recent-activity/all/ Dave

Ian, that's brilliant, thanks for sharing.

As you suspected, you definitely don't want to stitch images before processing. One thread I've found is: https://forums.culturalheritageimaging.org/topic/542-stitching-together-multiple-rtis/ I seem to recall a discussion (but can't find the thread right now) on merging RTIs of columns, but can't spot that thread right now. As well as the long-standing RTI generator, there is the new 'Relight Lab' under development, would suggest you enquire there (sub-forum here as well). Dave

Hi Annalisa, first few quick thoughts. GCP targets can be coded or un-coded; I guess from your question you're thinking about un-coded. First thing I would suggest is that you need to work from your proposed flight(s) and know the GSD and the accuracy you aspire to, that will help guide the size of target you plan to lay. I don't know what background or substrate you plan to lay your targets on, but from the air I would be concerned that the red carpet might not be that distinct? You mention using CDs - whilst the playing surface of a CD can 'blink' when, say, the sun strikes it, at other times they can be quite dark from other directions; the un-printed top white surface of a printable CD/DVD is though visible against a dark background. A very significant proportion of targets used for UAS/drone aerial photogrammetry use black and white for maximum contrast, and the chequer board pattern of two quarters white and two quarters black, with the reference point at the central intersection - that type of target is distinctive and also draws the eye in when working with them on-screen. (If you were using coded targets, then that centre can be found automatically to pretty good accuracy). If you are wanting to use carpet as your target material, could I suggest investing in cans of black and white paint and a roll of masking tape? In terms of logistics, you also need to remember that as well as a suitably-distributed set of GCPs, you also need a similar number of check points in your area that won't be used to constrain the resultant model, but will be used to assess the accuracy of the survey/model. As this is an issue wider than (just) cultural heritage, I might also suggest reading/searching/joining groups on UAS/drone aerial photogrammetry and photogrammetry more generally? Three that I could suggest on FaceBook are: Drone Photogrammetry Drone mapping, 3D modelling and GIS Agisoft Metashape (not sure if you're using Metashape, but target info will transcend software used) (disclosure: I'm admin/moderator for those groups) Dave

Thanks René, let us know how you get on - ideally some side-by-side screen grabs of the same area to show the impact. That would be a great service to the community too! cheers Dave