Problems Scanning in Strong Sunshine

In September 2018 we (students and myself) will be taking the scanner to Northern Italy to hopefully survey some Palladian buildings in Vicenza (Teatro Olimpico, Basilica, Villa Capra and the likes).   At that time of the year the sun will still be high in the sky which sort of got me concerned.

To date most of my scanning work had been done in quite gloomy or low sun conditions but I had noticed that if the sun was blasting on the machine I would often get poorer registration results or ‘Unable to Register’ notifications..

One of the main selling points of the BLK360 is the ‘mobile’ tie in with Recap Pro on the iPad and the automatic registration this offers – I could risk doing one-touch scans and then registering everything later but it’s good to get the stuff registered ‘on-board’, get it off to the laptop to see what we have and then start again with confidence the next day.

We have just had an amazing spell of very sunny weather here in Dundee so I used some of that time to test the scanner in very sunny conditions … I got a bit of a shock.

Here’s the subject building (photo from previous visit please note):

 

20180419_112700_Richtone(HDR)

This is a standalone pavilion in Baxter Park in Dundee – it is used for civil wedding ceremonies and also has a small café for park visitors.   It’s a lovely building – designed by George Henry Stokes (son-in-law of Sir Joseph Paxton no less!) in 1863.   Notice how it sits basically by itself – to the rear there are mature trees roughly 20m away but not much ‘solid’ in front – just a few shrubs and bollards.

Here’s the scanning sequence I tried first:

baxters_first_scan

Overlap results were acceptable (in the green) but balance results were very very low (in the range of 0.0 to 5.0%) which concerned me but the plan view of the registration you saw on the iPad was generally OK … until I got to scan 10 at the rear of the building where the strong shadows and glare made registration generally impossible – you can see above how the facades are all at angles.

This is what scan 10 looks like in the 360 image firstly as a screen grab and then as how that sits in the full 360 image.

glare_poor_reg2

glare_poor_reg1

You can see just how the software would struggle to make any sense of this at all – the ‘caustics’ generated by the glass of the lenses makes everything distorted, add to this the contrast and there’s no way the software could operate.

I read that using HDR images for the registration might help … nope … the glare was still there and the registration was actually worse.

Mmm … I was more than a bit concerned now – the Italy trip (for me) is going to hinge on being able to scan outside buildings.   Inside could still be awkward as the large window openings might cause glare but lets deal with one issue at a time.

I thought a shading device might help so next day I dragged along a photographic reflector on a stand … yes it shaded the machine but it was too cumbersome, it caught the wind easily and so I had to stand and hold it (and get in the way of the scans even more) and I was worried that it would fall and knock the scanner over.

rrem

Something a bit smaller would suffice – something just a bit bigger than the scanner unit – held closer.   I looked at the type of thing that holds a tablet or mobile phone but the bracketry was more suited to table-tops or bed headboards and not tubular parts like the BLK360’s tripod.   Also the reach of these ‘holders’ is not great.

shade2

I remembered my wife had seen something she thought might be useful to me … she was right.   Flexible foam covered wire ties – really big ones though.

shade3

I lashed two together with cable ties to get a bit more rigidity, made a card shading device that can slip on to the wire ties, cable tied the whole thing to the vertical part of the tripod in two places to get good stability …

rrem

and then watched it bob around in the light breeze.   Oh.   It still needed my presence to keep it positioned but it was less stressful on the arms than holding something up for sure.   If you want to see it in action click on this YouTube link.

Here is the how the 360 HDR photo (from position 10) looks using the home-made shading device as a close up and the full 360 image:

no_glare_good_reg2

no_glare_good_reg3

You can see there is some tearing because the shading object is very close to the cameras but it still made a big difference and the software on the iPad was able to register without any major problems.   If you were doing a single scan then you would miss part of the building but since I am moving the scanner (approx. 6-8m) between scans you get the whole building covered.

The balance results as you can see below are really low – but it came out OK so I am much more optimistic about getting some good results in Italy.

report

You can see by comparing the two images below how dark the scan is on the back of the building compared to the front – it’s the same coloured stone it’s just the shadow that’s the problem.   There is a bit of slippage in the accuracy – approx. 100 to 200mm I would say so that is something to be aware of.   This might be cure-able by doing manual registrations if it was critical.

C:UsersrichyDesktopBAXTERS PARK PAVILIONbaxters_hdri_test L

baxters_pavilion_front_perspective

baxters_pavilion_rear_perspective

Here is a link to an A2 presentation sheet that shows one of the building sides and plan too.