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Sigmund

Camera Angle

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Hi,

 

in the mode Specular Enhancement some of my PTMs look "grainy" or "sandy":

 

post-233-0-53445200-1369418970_thumb.jpg

 

Now, I realize that this appears to depend on the camera angle. The camera should be at 90 degrees from the subject. In the "grainy" cases I had to change the camera angle in order to catch figures on the upper part of my erected carved stones.

 

Can anyone confirm this observation or does it rather depend on the power of the flash light (too much light?)?

 

Thank you!

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hi Sigmund,

 

What kind of rock are you documenting? Curious.

 

I have seen 'shinny' rock, like volcanic material, present itself in a similar manner. Be sure to try out other viewing modes — the 'diffuse' mode could be a interesting view on this material. Tho, again, I'm not sure what I'm looking at, so at this point, I'm just guessing.

 

You also might want to consider 'not processing' certain images in your data set if that could be introducing an undesirable result. Perhaps you could try reprocessing and exclude some of the 'brightest' images.

 

Camera positioning is recommended to be 90º from the subject. But this is not a requirement. The 'area on the subject' will dictate the camera angle. The area of interest in your composition should always be in Focus.

 

You also really need to be sure that the string length is consistent. Be sure and look at the the Capture Guide. Check out the 4th 'bullet' on page 18 of the RTI Highlight Capture Guide.

http://culturalheritageimaging.org/What_We_Offer/Downloads/Capture/index.html

 

I know that this is a fairly general reply to your questions ... but I hope that it helps.

 

marlin.

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Sigmund, 

 

I'm not sure this is a camera angle issue...in my experience this grainy quality is due to lots of bad surface normals. It may have a lot to do with the light distribution and the light distance. 

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Dear Marlin and George,

 

thank you! It is good to know, that it has nothing to do with the angle. First I will try to kick out the brightest images. Next week I can do the entire capture again.

 

I have to add, that I made the capture outside and from time to time the sun came out. Could the sun be the reason? Usually, I work indoors. I really hate these grainy results, I can not work with them.

 

Concerning Marlin's question: These are Viking stone slabs made of lime stone. The depictions are made in a quite low and primitive relief, today it is hardly weathered. The stones were painted by researchers in recent times.Therefore, I have to turn out the colour in order to see the original carving. This normally works with Specular Enhancement. Unfortunately, with Specular Enhancement the grainy PTMs look terrible.

 

Sigmund

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Yes, Sigismund, we are experiencing the same problem imaging the surface of a piece of timber. We are controlling exposure, distance, etc closely, but still getting blown highlights. The problem is visible only in specular enhancement mode. We are examining faint teeth marks on the surface of a piece of smooth pine - the timber is quite smooth with a visible sheen. Will look further into this and report back.

 

J.A.

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Sigmund, 

 

If the sun is coming in and out of clouds during the shoot the total amount of light entering the scene is going to change substantially. In principal only your flash unit should be changing the lighting. The sun will consequently cause problems in the estimation of normals. There are a couple of ways of dealing with this: either shade the entire relief or use a powerful flash to effectively overpower the sun. The latter technique will require you to use a neutral density filter to bring down the aperture/shutter speed to numbers that will produce the best spatial resolution (no diffraction at very small f-stops) and will sync with your flash unit (the remotes we use won't permit a sync with any speed above 1/400s). The former technique -- manually shadowing the scene -- often requires someone to hold a large photographic reflector or umbella, sometimes up on a ladder. 

 

Outdoor RTI in changing light conditions, or very bright light is extremely challenging...I think there is discussion elsewhere on the forum on these challenges. I'd appreciate hearing from other forum members about their experiences. 

 

Could you and John post the "surface normal visualizations" of both your PTMs? 

 

George

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oy. I couldn't agree with George any more on his comments.

 

The rule is either shoot in Full sun, or Full Shade. The sun playing peek a boo would certainly introduce unpredictable and undesirable results in the final product.

 

At our training in Berlin, a group of egyptologists had to hire a person whose only job was to 'shade' the composition during test shots and during the capture sequence — his name was, "The Shade Man". (quite difficult if you consider, the hot sun and standing with your arms up in the sky fighting the wind, holding a black out flag for minutes on end). Perhaps a C-stand, sand bags and some flags would work as well.

 

Movie crew -- 'grip' type supply companies have many shade structures available for rent, purchase or inspiration. Tho costly, its good to see what they do to create shade when on location.

 

we own one of these to make shade:

 

Caravan 10 x 10 Alumashade Black-Out Canopy Package Deal + 4 Zippered Sidewalls

 

http://www.ecanopy.com/ca10x10alblc.html?productid=ca10x10alblc&channelid=FROOG&gclid=CJ3F2ZHstrcCFc9xQgodgFwAAQ

 

never actually used this product in the field, or in windy conditions, but this could be a good solution for a 'dark room' and dinning hall as well!

 

Marlin.

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post-214-0-77778000-1369761860_thumb.jpgWe have been attempting to RTI the surface of a small (2,5in x 0,5in) piece of softwood to find toothmarks (the timber is evidence in a food adulteration case involving alleged contaminated fries).

 

The softwood was affixed on a copy stand, the camera (Canon 5D2 + macro lens) mounted above. A total of 40 carefully-controlled exposures were captured, the light source a 100W halogen lamp fitted with a diffuser.

 

The image stack was processed as a PTM, and the result is attached - completely blown-out highlights when viewed in Specular Enhancement mode - otherwise OK. We have noticed, however, the illumination source is critical. A diffuser must be used. We attach an opal diffusing glass (Edmund Optics) to try to achieve as close a Lambertian distribution as possible. Point-source illumination (i.e. a naked light bulb) makes the situation far worse.

 

The only other factor is a slight sheen in the surface of the timber.

 

Anyone any ideas?

 

John Anderson.

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John,

 

your data set looks over exposed. You might want to try and EV comp down your original images (maybe a whole stop?- just a guess) export the jpegs and then re process to the final product.

 

If you shoot the majority of your sample shots from the higher 65º angle, you've got a lot of light on your subject. Suggestion, perhaps populate your subject light samples with numerous lower angle shots, go for the raking light angles — esp if your subject is naturally brighter.

 

Hard to tell what 'view' you're viewing from in the image. If the 'lighting' is directly overhead, as in high noon, the view you will experience is 'hot' and blown out. Be sure to adjust the sliders and manipulate your lighting model. 

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This is very similar to a couple of inscriptions I was working on recently. On the advice of Kathryn Piquette I dropped the exposure by about 0.8EV to get a better result in specular (very similar situation to what you have). It worked rather nicely. As a matter of course I usually over-expose my RTI shoots, but in this case I overdid it. Dropping exposure is something I generally avoid since it can cause more internal shadowing in low-angle shots, and thus more inaccuracies in surface normal generation. 

 

I think sort of grainy specular enhancement Sigmund has is from a different source.

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Thanks, Marlin and George. Actually, anticipating blown highlights, we reduced exposure by 1,0 EV from that indicated by the camera's exposure system. The image is underexposed, rather than over. We don't like underexposing for the very same reason George mentioned. In response to Marlin, the image I posted is a low angle, PTM specular enhancement snapshot; that's when the problem manifests. High angle exposures are also affected, but to an acceptable extent.

 

We found that the source of illumination is critical when imaging even slightly specular surfaces. A point-source, such as a naked light-bulb or even a 5W LED, increases the problem, diffused light lessens it.

 

We haven't tried altering the camera's exposure mode - that's next. We'll carry out further experiments to see if we can resolve the problem. Maybe we should use a flashlight with the batteries reversed!

 

Thanks for both your input.

 

 

J.A.

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J,

 

on the note of Lighting, strobe, continuous light source, what ever it may be, the idea is to go for wide, broad, even illumination on your subject. We frequently shoot with the the Canon 580EXII as our light source. The 'beam spread' is set to 24mm. --- think broad light source, stay away from narrow spot lights or narrow cones of light — that is NOt what you want.

 

If you're using a continuous lamp, make sure that it stay very still, you don't want that highlight to be wandering around ...

 

M

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Hi Marlin, We use several light sources, including small LED flashlights - ideal for very small objects. Our illuminant for the timber was a 100W halogen mini spotlight fitted with an additional opal glass diffuser, which guarantees uniform light distribution. The beam angle is adjustable, we set it to its widest - around 65-degrees. For something as small as our timber sample, we use a simple rig for holding the light in position, so that's not the problem.

 

We haven't experimented with the camera's metering settings. We are currently using spot-metering, but we'll try the others. I'm not too concerned about the problem, as we can use our structured light source (a laser fitted with a line generator) to capture a raking-light image of our evidential sample. However, if we can crack this one, it may prove useful to other RTI users.

 

I'll let you know what happens.Many thanks.

 

John.

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Concerning my "grainy" results I agree with George: This is a different problem. I tried it again and again but the result is always grainy. However, other PTMs I made during the same working day on the same location are OK! In addition, I realized that in a single case I obtained grainy PTMs also in the Museum (indoors).

 

My conclusion for now: It appears to depend on the surface structure. Some of my stones course grainy PTMs, most of them not. Not satisfying but apparently true.

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Sigmund, a further test which might help is to image the surface of the stone with a macro lens at 1:1 or 1:2 magnification with low-angle raking illumination. A single LED flashlight is good for this (unless you have a laser/line generator). You need take only 1 exposure. Open in Photoshop, ImageJ, or whatever, desaturate, apply enough sharpening to reveal structure and examine the result. If it appears grainy, it's your sample's surface.

 

Regards, J.

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Hey John,

 

I have seen this problem frequently with varnished paintings. The solution is to restrict the maximum light sample inclination. Try not lighting above 45-50 degrees. Reduce the the maximum inclination until the sheen disappears from your photos. If only a few images have sheen, discard them.

 

Best of luck.

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I would agree with Mark that this problem can occur with varnished paintings.  There was also a previous discussion on this topic:  http://forums.culturalheritageimaging.org/index.php?/topic/243-lrgb-or-rgb-files-for-painting-with-varnish/

 

Using the HSH fitter or selecting RGB instead of LRGB in the last screen of the PTM fitter can improve the results with specular surfaces, or avoiding high light angles is also sometimes helpful.  Since one doesn't necessarily know in advance which light positions will cause the problem, I'd suggest collecting the data in the usual way, including higher-angle captures (up to 60 degrees).   Then, as Mark and Marlin said, you can remove the photos that have a sheen from the image sequence during processing with the RTIBuilder.  This way, you're likely to get the most useful data.

 

It's interesting that many materials do exhibit a sheen when viewed from certain angles, including some that one wouldn't expect to be specular.  These light angles might be randomly distributed (e.g., some rocks), or can be more pronounced at low angles (off-specular effects), or concentrated at certain positions (e.g., minerals, due to their crystal structure).  I've seen apparent halo reflections off surfaces such as sandstone, for example.  These effects are discussed in Section 3.7 of Malzbender et al. (2001): 

http://www.hpl.hp.com/research/ptm/papers/ptm.pdf

 

This is another reason to capture images at a range of light angles from 15 to 60 degrees, and then discard the problematic images by inspection when building the RTI.  It might help explain the problems that Sigmund was observing when the camera was not perpendicular to the surface (this is a separate issue from varying light from intermittent cloud cover), and also with RTIs of certain materials such as mineral surfaces, bone, and wood that exhibit non-diffuse reflectance properties at certain light angles.  Over-exposure tends to exaggerate the problem when it's present, and experimentation will usually lead to better results.

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