Bracketed exposure models

[ozbigben]

As I mentioned in an earlier post I was thinking of experimenting with bracketed exposures for shooting buildings, with the view of filling in holes in the darker recesses of the building with points derived from images with higher exposures.  I'm not entirely sure whether this is sensible or not, but I shot a quick test yesterday and the result was quite promising.

For now I'm processing the different exposure sets separately and then aligning and merging the point clouds. I'm only shooting one extra exposure (+2 EV). The first test added some extra noise along edges but I only did a quick mask and I can see why this happened. I've set up a separate Photoshop action to mask these images, excluding everything at the top end of the pixel value range (starting with a max of 245 but that's just a number I pulled out of the air)

The initial model from the normal exposures is here: https://sketchfab.com/models/d24123350ae2440086856d1c17193667

[Marcel]

Cool that you tried it, and the results look good.
 After masking you didn't have noise in the  point cloud made of the 2ev+ images?


What do you think, is it quicker and higher quality than doing a bracketed set and merging to HDR? The masking sounds like an annoying step (although HDR merging takes time as well and is not without problems).

[ozbigben]

That one is just the normal exposure as a reference. Didn't make as much difference as I expected, although there are differences, but i haven't uploaded anything yet. Still a few other things I need to understand. I've specifically avoided HDR for now because I wanted a handheld shooting workflow, mainly because i know the people I'll be passing this on to won't have the patience (or possibly time) to be extra careful.

but so far:

Trial 1: Separate chunks for each exposure, dense point cloud, aligned, merged and meshed. Some minor differences but nothing really significant.

Trial 2: All images in one chunk. Aligned photos resulted in fewer points, but sparse point cloud was quite dense in some areas, picked up more detail in trees off to the right of this model (in shade) but lost a lot of the side wall on the left. Not sure if it just ran out of points. I'll have another look at that later.

Trial 3: Similar to trial 1 but building dense point cloud after merging chunks (currently running)

I was shooting with aperture priority but some of the normal frames were a little dark for my liking.  I'll have to take a bit more time for the next shoot and pay more attention to the exposure.  The building that's inspired the test is also my test subject for HDR stuff because the exposure drops significantly in the corners (6-8 stops within 8m) https://www.flickr.com/photos/ben-kreunen/4053467313/ so shooting on manual and just bracketing a single 2 stop increment isn't going to be practical.

Masking the over-exposed image set is possibly quicker than HDR merging as the batch action is pretty quick... but I'll wait until I get a result worth sharing before I call it. The masking did remove the extra white noise along the edge of the roof/chimneys

If this doesn't work the way I expected that will still be worth knowing.

[Marcel]

I've specifically avoided HDR for now because I wanted a handheld shooting workflow, mainly because i know the people I'll be passing this on to won't have the patience (or possibly time) to be extra careful.

In that case I hope you also hand over the processing to them? Otherwise you are going to have a hell of a time trying to get something useful from blurred shots. If you have them process themselves I'm pretty sure they find that patience after the 3rd failed scan.

An option we considered for hand held shooting is a really powerful ring flash, but we are not sure if the shadows  from the flash would cause problems. The shadows from a ring flash are pretty even (it's coming from almost the same position as the lens), but it's still a moving light source. It might be worth renting one and trying it out?

[frank.stremke]

i use 5 stationary flashes along the walls so the shadows stay the same and i can move around freely as long as iam not in front of the flashes :-)
in this way i was even able to vombine very high differences in lighting conditions (high noon in sudan outside, and a completly dark underground quarry inside) i started with the flashed in the entry area and then just kept moving along with my lashes which i repositioned as soon as i came into or out of range....
frank