blotchy blue sky  

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pskrep
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blotchy blue sky

by pskrep » Thu Dec 09, 2010 1:21 am

Has anyone else noticed that blue skies often end up blotchy looking after stitching?
This seems to be caused by the panorama software,as usually it's not apparent in the source images.
Just to be clear,I"m talking about blobs that often have a sharply defined edge and are lighter/darker than the surrounding areas and sometimes have a color cast,not noise per se,or posterization.
It's most noticeable in the red channel,being usually the darkest.
I've seen this with other stitchers as well,so it's not unique to autopano pro,as my version of ptassembler does it as well,it just makes blotches in different locations.
The latest ptgui seems to have improved on this using it's own blender,as opposed to the smartblend or enblend options,but I only have a demo and it's hard to see with all the watermarks.
What I've been doing is outputting using smartblend,as it stitches much better for wide angle shots compared to multiband,then I make a mask over the sky,then output again using multiband and layer in the sky from the multiband pic...it's a pita though...
so,other than correcting for vignetting before stitching,is there anything else I can do from within autopano to improve on this?
I've been trying the 2.5beta and it seems roughly the same when multiband is set to -2,worse at other levels (smartblend is no longer available in ver2.5?).
The blotches are visible on 3 different lcds i'm using,which while not high end units,have been calibrated,and also sometimes in prints as well,although that depends on paper type and settings,gamut of the ink,etc....but if it's obvious on screen I find it will usually be visible in print as well.
Any suggestions?

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by GURL » Thu Dec 09, 2010 12:13 pm

pskrep wrote:This seems to be caused by the panorama software, as usually it's not apparent in the source images.

my version of ptassembler does it as well,it just makes blotches in different locations.

What I've been doing is outputting using smartblend,as it stitches much better for wide angle shots compared to multiband,then I make a mask over the sky,then output again using multiband and layer in the sky from the multiband pic...it's a pita though...

If said blotches are visible next to the seams only, the blender should be the culprit but if they are visible everywhere in the blue sky this could be caused by another problem.
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by pskrep » Thu Dec 09, 2010 12:41 pm

it seems random to me.
Sometimes there will be a vertical dark patch on an imperfect blend,but that is a bit different to what I'm talking about.
I should mention that I shoot raw and convert to 8 bit tiff usually,sometimes 16bit if there is noticeable posterization.
I thought at first that is was caused by the canon slrs I've been using,but my d700 does it as well,although maybe not as noticeable.
If the sky is exposed such that there is no or little adjustment in raw,then the result will be better,if there is underexposure,which is hard to avoid in mountain scenery with snow,then it will be worse,although the source images still appear fairly clean to me.
This is at base iso as well.
If I output as layers,the layers themselves seem much cleaner than the underlying blended image,but with wide angle pics I find it hard to blend the seams well,which is after all what autopano is for, really.
I'm no expert,so I suppose it's possible the way my monitors are set are exaggerating the problem.
So...am I the only one to experience this?If so,then I must be doing something wrong.....

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by pskrep » Thu Dec 09, 2010 12:49 pm

I've noticed as well that autopano files don't have a color space assigned even though i started in adobe rgb....
So,if I open in cs4 and my settings are "convert to working color space",is there a change in color space,or is it merely tagged as argb with no actual adjustment?Could this be part of the issue if there is an actual change?
Or should the settings be "perserve imbedded profile"?(never actually tried this...)

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by GURL » Thu Dec 09, 2010 6:17 pm

Here is a blue sky example.

On the left a "straigh from camera" source image where the blue sky next to zenith is underexposed. On the right a corresponding crop of the zenith region (at 100%) from the stitched spherical pano: I can see some blotches ...but don't care as, when I watch the pano in a viewer, I don't notice them. Even on a large print, I believe they would not be a problem.

That being said, not any blotch is visible in the TIFF I processed in ACR to brighten the sky and then used as source image for the panorama...


Last edited by GURL on Thu Dec 09, 2010 6:18 pm, edited 1 time in total.
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by pskrep » Fri Dec 10, 2010 3:07 am

I see what looks like compression artifacts,so it's hard to see very much in that crop,but I can see ( I think) some purplish blotches,which is what I was talking about,but sometimes I get them much more noticeable than that....and I have had instances where they are visible in a print,and hard (for me) to get rid of....
Bottom line is,if it's not visible in the source images,then I would like a way to not have them show up in a finished panorama...if possible.
When I have time,i'll try some more in ptgui to compare.....
It may not be so much of an issue for other people,certainly if you are doing interiors,or if your skies are light blue,usually it won't show up,but I would think people shooting mountain scenery with blue sky in morining with snow and glaciers in the background would have the same issues I'm having.
I thought my canon's were maybe more subceptible to this,but my d700 seems about the same.

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by GURL » Fri Dec 10, 2010 12:58 pm

pskrep wrote:I can see ( I think) some purplish blotches,which is what I was talking about,but sometimes I get them much more noticeable than that....and I have had instances where they are visible in a print,and hard (for me) to get rid of....

The first thing I would attempt: to brighten a dark blue sky in a source image using an image post-processor. If no blotch is visible in the resulting image but blotches are visible in a corresponding panorama then the cause is in the stitcher. If blotches are visible in the post-processed image the camera is the culprit.

I doubt any stitcher blender would add blotches in noise-free source images. Pre-existing noise being handled differently by different blenders and interpolators (try Bicubic Smoother) would not surprise me.

:( With my first digital camera - end of the previous century - any blue sky image suffered from visible blotches.
:) With the present one I tend to find blue sky being good enough even when some post-processing is needed to brighten it.
Last edited by GURL on Fri Dec 10, 2010 1:02 pm, edited 1 time in total.
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by DrSlony » Sat Dec 11, 2010 10:14 pm

pskrep: AP doesn't convert your images to any different color space, just re-apply the color space tag in photoshop/whatever once you've stitched it.

Please post some screenshots, or even better render a small layered pano, because you wrote, "If I output as layers,the layers themselves seem much cleaner than the underlying blended image".
You can upload it to http://filebin.ca/


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