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Brightness/contrast in Photoshop

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#1
Hans van der Maarel

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Got a basic Photoshop question:

I have a dataset split up in 6 separate tiff files, slightly overlapping that I need to tweak using the brightness/contrast settings. However, when I do the various tiles I notice that the exact same settings still result in slightly different colors in the overlap. Is this normal behaviour? I would expect something like Autocontrast to exhibit this, but I always thought the brightness and contrast were 'absolute' values. Or am I just being very naive and dense here? :unsure:
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#2
CHART

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Hans,

Assuming this is aerial photos (could also apply to other imagery)
This is related to radiometric balancing.... which in definition means that because the light does not reflect the same way on each photo, colour intensity (the canadian way of spelling color) will vary from frame to frame.

PhotoShop will not do the job... you need to look into more specific software (= expensive).

VR balancing is one standalone tool that can adjust radiometry. You can look it up at.

http://www.cardinalsystems.net/

Numerous other software apply radiometric balancing ... PCI Ortho, ERMapper all in different ways.

Note that, there is no perfect solution, and this has always been a hot topic in photogrammetry.

http://gis.esri.com/...PAP306/p306.htm

Regards,

... if your are talking about raster maps .... then I believe there is a new function in PhotoShop CS2 ... match color or something like that ... but again not applicable to aerial photography...
Chart

#3
Hans van der Maarel

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Jacques,

Thanks for the tips. I ended up combining the 6 tiles into one, changing the brightness/contrast on that one and then cutting it back into the 6 individual tiles. Bit of a long way around, but it works with the software I have in-house at the moment.

Data is not aerial photos, but Tom Patterson's Natural Earth, which suffers from the same problem.
Hans van der Maarel - Cartotalk Editor
Red Geographics
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#4
Dennis McClendon

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GraphicConverter (shareware from LemkeSoft) is handy to have around for two reasons: it's now about the only way to view MrSID images on the Mac, and it will cut large images into tiles automatically. You just tell it you want three rows and four columns, or whatever. I haven't figured out how to get Photoshop to do that.
Dennis McClendon, Chicago CartoGraphics
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#5
benbakelaar

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I haven't figured out how to get Photoshop to do that.


Dennis, do you have any programming in you? This is a tile cutter for Google Maps. It's a JSX (Javascript file) that you load in to Photoshop as a custom script. If you know any programming, I think you could easily modify it. If you can't, let me know and I'll give it a shot.

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#6
frax

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Hans - maybe Photoshop is not the best tool for the job? Maybe a software such as imagemagick? (not for the mosaicking, but for doing exact brightness/contrast changes)

Sometimes I prefer to do some raster/image processing the barebones way as well - another solution - split up the three bands as three different raster datasets (one stack in ArcInfo GRID lingo), and manipulate them using math (one just have to make sure that values are within 0-255 for an 3x8 bit image). When you are done, just export back out as a tiff
Hugo Ahlenius
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#7
Matthew Hampton

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I haven't figured out how to get Photoshop to do that.


I think you would use Adobe ImageReady for slicing and dicing. This application has been bundled since Photoshop 7.

co-cartographic creator of boringmaps.com


#8
Hans van der Maarel

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Hans - maybe Photoshop is not the best tool for the job?


I don't know, it seemed to work well. The only problem really was the fact that the combined image was about 1.95 Gb, so it took a couple of minutes to do the brightness/contrast change.

Apart from that, propagating this change throughout the entire production process, 6 times, is going to take a lot of time. But I don't really see a way to do that any faster (into Manifold, reprojecting, exporting to Illy, replacing the raster layer in another Illy document)
Hans van der Maarel - Cartotalk Editor
Red Geographics
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