Topic: Does Image Post-Processing Improve the Density of Point Clouds?

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Nathan.Craig (Over 1 year ago)
Dear Synthers,

I am an archaeologist interested in PhotoSynth as a tool to generate point clouds for rapidly recording sites and their surroundings. My primary aim is generating high quality point clouds of architecture and topography. I have been wondering if image post-processing, like histogram adjustments and perhaps sharpen filters, would make for point clouds that are more dense.

From my first experiment, which involves comparing unmodified and histogram adjusted images, image post-processing does not seem to have much impact on point cloud quality. Does anyone else have any insight or suggestions regarding this issue?



Thank you,
Nathan Craig
jimcseke (Over 1 year ago)
Hello Nathan, Jim here. I haven't been synthing very long, but I might have some useful information for you. I have experimented with post-processing a little, and have used certain tools in photo editing programs to improve certain aspect of my synths. Firstly, I have found that brightening a photo will improve a pointcloud. I used backlight enhance in one of my programs, which will actually bring out parts hidden by shadow. False colors sometime help because photosynth does not do well with whites. Natural lighting /bright sunlight or a good source of natural type lighting can really make a difference. But I think one of the most important tool is planning your photographs.Check out some of my favorites and of course check out their favorites. The majority of my favorites have beautiful pointclouds and it amazes me how well they work when you see how few photos are taken. Look the photos and notice the angles they use. I'm just a newbie but I hope this was useful.
Nathan.Craig (Over 1 year ago)
Thank you this is very useful information. I will make note of your suggestions and try them out. 1) Brighten dark areas by applying a backlight enhance, 2) try using false colors, 3) plan photographs carefully.
TonyErnst (Over 1 year ago)
Hey guys, The specifics on this intrigued me so I talked to a few of the true experts on this around the office and came up with the following:

Color - the first thing the synther does is convert to grayscale, so color has no effect at all.  This is why you can synth scenes in radically different lighting conditions and have everything match

Brigtness - increasing this doesnt really improve the point cloud, but may make the points more visible because, well, they're brighter

Contrast/Curves - may have a small effect if it makes features stand out more, but it probably wont be a noticable difference

Given all this I would suggest you post process your images to make them look great, and not worry how it will effect the synther.
jimcseke (Over 1 year ago)
Hey Nathan, Jim here again. It is strange that you posted your question, because I was experimenting with some of my old synths to see if I could get any amout of improvement with post processing. Also the weather and my job have made it hard for me to get out and enjoy my new hobby. So I have been screen capturing shots from older synths (lower resolution) and editing them to see what effect it would have. If anything, it was fun experimenting.

Starting with an original and 3 screen captures I edited with descriptions:

Again check out my favs (amazing stuff)
Also look for TonyErnst Curiosity Table. One of my fav favs!
Let me know how you make out.
jimcseke (Over 1 year ago)
Hey again Nathan. I'm sorry, I gave you embeded links and when you click them the descriptions are not shown on the synth screen. So just go to my page and the synths are there for you to check out. The original Snails and Stones is a few pages back from the post process tests. 
Sorry again for the relatively new guy mistake.
Hey it's way past my bedtime. G'night and keep on synthing cause it's fun!
Nathan.Craig (Over 1 year ago)
Dear Jim and Tony,

Sorry, it has been some time since I checked back on this forum. Thank you both for your feedback. The information is very useful. During the interim, I've been trying to work out methods to get the point clouds into real world coordinates. Any thoughts on what would make the best photo targets. I'm not having much luck with cones.
Nathan.Craig (Over 1 year ago)
Drat, 1) I failed to add a question mark when one was required and 2) I posted the WRONG LINK. I don't see a means to edit an old post, so I'll post the correct link here. Please forgive my errors.

This is the link I meant to post in the prior entry.

Thank you,
sir_ivar (Over 1 year ago)
One thing I've discussed with others that might improve the point cloud is to undistort each image for radial lens distortion before processing in Photosynth. I have a Matlab script that can do this if anyone is interesting in trying it.  You need to know the distortion parameters for your lens to use it though.

As for translating the coordinate system in Photosynth to a real world system, that is a little tricky.  I have done this with some of my KAP imagery and Photosynth by transforming points in the synth to points with known real world locations.  Geomagic and VRMesh Studio are capable of this but I'm not positive of the accuracies of the points after conversion. I'm hoping to compare a laser scan to synth conversion soon.

Here is a recent example of such a conversion:

christoph_hausner (Over 1 year ago)
As far as I know, Photosynth already does radial undistortion internally. Correct me if I'm wrong.