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Author Topic: is photo pre-processing useful?  (Read 7928 times)

ajg-cal

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is photo pre-processing useful?
« on: August 02, 2012, 01:55:57 PM »
Was just wondering if pre-processing photos (noise reduction etc) is beneficial?

Also if I wanted to achieve a more even exposure when using mosaic blending under ortho export, would preprocessing photos with auto levels / exposure adjustment help?

thanks very much for the help =]

lostboy

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Re: is photo pre-processing useful?
« Reply #1 on: January 12, 2013, 02:38:27 AM »
+1

Wishgranter

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Re: is photo pre-processing useful?
« Reply #2 on: January 12, 2013, 03:41:33 AM »
Hello preprocessing is very usefful, like you say noise, higher dynamic range and other stuff should be done. Pscan should then process data and not play with something for what it was not created.

Im preparing some info about it, how  to process and what to expect from that...... But it will take some time..
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ju523m

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Re: is photo pre-processing useful?
« Reply #3 on: January 12, 2013, 08:13:00 AM »
Hi,

yes, indeed, preprocessing can be very useful, and depending on the quality of your input  images it is a must. We are specialised on the processing of aerial and satellite images and first step is always to check the images for issues like:

- exposure variation from image to image
- light fall-off / vignetting
- athmospheric influences like haze
- spheric aberration and defocus in border regions
- image moise

Please find attached some examples for effects and correction of:
- vignetting
- haze correction
- changing envoronmental conditions, it started raining during flight

For more info and examples, please look here:

http://www.giscat.de

http://www.aerogis.de

If you need assistance for your data processing please contact us.


Cheers
« Last Edit: January 12, 2013, 08:33:52 AM by ju523m »

RalfH

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Re: is photo pre-processing useful?
« Reply #4 on: January 12, 2013, 03:38:35 PM »
Interesting post.

The mosaicing/orthophoto issue is a very obvious case where preprocessing can improve the resulting texture. Perhaps this is also a field for possible improvements in PhotoScan: for example, even some very simple panorama stitching software is able to adjust for image-to-image brightness difference (but PhotoScan isn't yet able to do this - if you create a mosaic from images with different brighness, you get very blotchy results).

Quite another question is whether preprocessing has an influence on the geometric quality of the resulting model - does anyone have experience with this?
« Last Edit: January 12, 2013, 04:19:06 PM by RalfH »

ju523m

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Re: is photo pre-processing useful?
« Reply #5 on: January 12, 2013, 04:05:25 PM »
Hi,

I found that there seems to be a "magick" block size when exporting orthos: 6000x6000px. So when you export large orthos there are blocks visible of this size.. So I wrote a python script that exports the ortho in smaller but overlapping blocks into tif with alpha and did the final mosaicking with opensource tools and some scripting externally of PS.

besides improvements in mosaicking, I made the following experience:

When you have hazy images, the pointcloud seems to become very "unprecise" and tends to scatter the ponts alot around z ==> noisy appearance.
When doing the haze correction, the images become clearer, more "crispy" and textures seem not to be disturbed by the noise. ==> less noisy, more smooth point cloud that describes surfaces better.

But you shuld be careful with image filterering to reduce noise,because you also may have effects on the appearance of texture, e.g. when using mean or median filters or things like Kuwahara Filter, Meanshift and that kind of stuff.
« Last Edit: January 12, 2013, 04:08:50 PM by ju523m »

ajg-cal

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Re: is photo pre-processing useful?
« Reply #6 on: January 14, 2013, 01:36:55 PM »
Thanks for the info guys - very interesting.  :)

You're right about the mottling effect ju523m, but I think this feature doesn't appear so much in 0.9.0 in my experience - perhaps it was only a problem in 0.8.5's mosaic mode. Also the final orthophotos in 0.9.0 often have a more 'tonemapped' feel, which suggests to me that there is more exposure equalisation going on under the hood than before.

I imagine like everything there is a balance to be found. Too much noise reduction might impair the ability of PS to detect features, but none at all might produce poor quality noisy imagery if the lighting conditions weren't great.

Perhaps sharpening might improve the geometry in some cases, if the images are slightly blurry that is - it's not something I've tested yet.  :)

George

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Re: is photo pre-processing useful?
« Reply #7 on: January 14, 2013, 03:59:58 PM »
Hi all,
what are the standart pre-postprocessing operations you do to your imagery?
George

... and what software do you guys use for that?
« Last Edit: January 15, 2013, 04:32:50 PM by George »