Dear Dave and mala,
Thank you for your helpful and inspiring replies on my post.
@Dave
- "... it looks like the camera aperture in your illuminating sub-hemisphere is potentially occluding some tens of degrees without illumination."
The minimum diameter of the spyhole results from the insect diameter (here: 8 mm), the distance of the spyhole from the insect (60 mm = radius of the illumination dome) and the distance of the spyhole from the lens entrance pupil in the far position of the camera (32 mm). In the case shown, the spyhole diameter was 3 mm. Therefore, an angle of (3/60) rad or 2.9° is occluded from illumination, whereas the angular distance of neighboring photos is 10°.
- "Can you move the illumination surface further back towards the camera, thus reducing the occluded angle?"
That would indeed reduce the size of the artifacts, but not eliminate the artifacts completely. In addition, the radius of the illumination dome is constrained by geometric and optical considerations.
- "If the dome has to stay where it is, can you fire in some extra light from back near the camera? - e.g. a ringflash-type light close around the lens? - or lights 'over the camera shoulder' but several around the camera/lens?"
Indead, I have also considered to "fill" the hole with some extra light, perhaps by use of a beamsplitter between the lens and the spyhole. But that looked to me as a challenging modification with a questionable result. Any light sources around the spyhole (as a ringlight) would cause additional reflections / artifacts.
- "Can you dispense with the front dome and put the whole rig in a light tent?
I fear that this would cause mirror images of the camera and parts of the whole rig, with even worse artifacts.
- "... I wonder if it might be possible to mask the area in question, as whilst the projection will change, I guess it will always be centre-frame. If that affects the model generation, you could generate the model with the existing images and then substitute the masked ones when generating the texture."
That is an excellent idea! Why did I not hit on this before? Indeed, it is possible to use masked photos for texturing, thus excluding the masked regions from being used for texturing, see the PhotoScan Manual:
"During texture atlas generation (for single mesh model and tiled model), masked areas on the photos
are not used for texturing. Masking areas on the photos that are occluded by outliers or obstacles helps
to prevent the 'ghosting' effect on the resulting texture atlas." The masking needs to be done manually, but as only about 30 of a total of 398 photos show the spyhole reflection, the effort is quite tolerable. I will try that next week and report on
my results.
@mala
- "I would have thought that polarisation would help your project quite a bit, as a good polarised lighting rig combined with a polariser on the camera would give the ability to capture both diffuse and specular images.
... Depending on the quality of the polarisers and position of the lights & camera you should in theory be able to elminate the specular reflection entirely by using cross polarisation.
... If you have the abilty to "toggle" the polarisation angle between 0º and 90º, which as your subject does not move (not alive) you would not have to do very fast, perhaps just one polariser in front of the camera that can be rotated from 0º to 90º."
Indeed, the need to measure with two orthogonal polarisations is a problem. Although our subjects are dead and pinned, that would duplicate the scanning time. As we are aimed ad mass digitization of insects, "time is money". Nevertheless, I will test the use of different polarization filters (also circular).
Thank you again, best regards and a nice weekend
Twister
Topic: Illumination artifact when texturing shiny surfaces (Read 4025 times)
