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Panorama stitching errors

aeollos

New member
Hi guys,

am having an issue with my pano images while I try to stitch everything together the resulting image shows vertical lines.
would that be from the difference in exposure and if yes is this possible to be corrected in post?
I.e. I don't encounter the same when I process the same stack of images in LRC.

thanks
 

drevil

Well-known member
Staff member
Do the images have different eposures?
what program are you using to stitch?
 

dchew

Well-known member
Are you talking about something like this?



If so, then I think @pegelli is right about vignetting correction. Been so long since I did a pano by rotating the camera, I initially forgot about having to correct for lens vignetting. In C1, the default for "Light Falloff" in the Lens Correction tool is 0. After setting it to 100, I get this much improved result:



These were all shot at the same exposure settings; if you didn't do that, you might have some additional corrections to make. Images processed in C1 then exported to PS for the pano stitch.

Dave
 
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MGrayson

Subscriber and Workshop Member
Warning: Pompous lecture ahead.

Panorama stitching is a hard problem. This is why shift lenses (or better, backs) are so good. If you keep exposure and f-stop the same, and nothing out there moves, the captures are actually part of the same image (Imagine chopping an 8x10 negative into pieces - except that they overlap...), so stitching them is just a matter of aligning the pieces, and that's really easy (I could go into the math, but I doubt anyone is interested).

But when the panorama is made from separate captures where the lens changes aim, like rotating the head 10 degrees between shots, then you add geometry changes and vignetting (and possibly color shifts with an older digital back) to the differences between each piece. Say you have a horizontal panorama. What happens "under the hood" is that each image is projected onto a cylinder. The computer has to know the exact FoV to do this, and it usually figures it out by matching the distortion of a pair of adjacent images. You could see this in old stitching programs as a string of overlapping bubbles. Now all the pieces will line up, so the geometry is solved. The final stitch is either left as cylindrical and unwrapped or projected back onto a plane. The former is more natural for landscapes, but straight horizontal lines, like roads, will appear curved. The latter keeps all straight lines straight, but gets very distorted near the edges - you're effectively creating a super-wide lens. If you cover a full 180-degrees, the projected image will be infinitely large. Bad.

The hard part, and what PS doesn't do perfectly, is matching the exposure, vignetting, and color of the different pieces. That's hard and I don't know the current state of the art. It used to be that specialized software had to be used, e.g., AutoPano or the venerable PTGui where you lined everything up by hand. In C1, if you took an LCC (same image but with a white translucent piece of plastic in front of the lens) for each piece, then the software could correct more or less perfectly. But the latest and greatest can figure out on-the-fly and give a smooth stitch. Without those final corrections, you get dips in the brightness between the pieces, hence the vertical lines.

If you do a multi-row stitch, the computer uses a sphere to match up the pieces and then projects - somehow - back to a plane. It's the same problem map makers have when they try to cover a significant part of the globe, and why there are so many different global map types.

Matt

Edit: I unfairly maligned PS. It should be able to do all the tone and color alignment. Was that box unchecked?
 
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aeollos

New member
Warning: Pompous lecture ahead.

Panorama stitching is a hard problem. This is why shift lenses (or better, backs) are so good. If you keep exposure and f-stop the same, and nothing out there moves, the captures are actually part of the same image (Imagine chopping an 8x10 negative into pieces - except that they overlap...), so stitching them is just a matter of aligning the pieces, and that's really easy (I could go into the math, but I doubt anyone is interested).

But when the panorama is made from separate captures where the lens changes aim, like rotating the head 10 degrees between shots, then you add geometry changes and vignetting (and possibly color shifts with an older digital back) to the differences between each piece. Say you have a horizontal panorama. What happens "under the hood" is that each image is projected onto a cylinder. The computer has to know the exact FoV to do this, and it usually figures it out by matching the distortion of a pair of adjacent images. You could see this in old stitching programs as a string of overlapping bubbles. Now all the pieces will line up, so the geometry is solved. The final stitch is either left as cylindrical and unwrapped or projected back onto a plane. The former is more natural for landscapes, but straight horizontal lines, like roads, will appear curved. The latter keeps all straight lines straight, but gets very distorted near the edges - you're effectively creating a super-wide lens. If you cover a full 180-degrees, the projected image will be infinitely large. Bad.

The hard part, and what PS doesn't do perfectly, is matching the exposure, vignetting, and color of the different pieces. That's hard and I don't know the current state of the art. It used to be that specialized software had to be used, e.g., AutoPano or the venerable PTGui where you lined everything up by hand. In C1, if you took an LCC (same image but with a white translucent piece of plastic in front of the lens) for each piece, then the software could correct more or less perfectly. But the latest and greatest can figure out on-the-fly and give a smooth stitch. Without those final corrections, you get dips in the brightness between the pieces, hence the vertical lines.

If you do a multi-row stitch, the computer uses a sphere to match up the pieces and then projects - somehow - back to a plane. It's the same problem map makers have when they try to cover a significant part of the globe, and why there are so many different global map types.

Matt
thanks for the response its quite a mouthfull and i have to digest all the info provided here.
 

pegelli

Well-known member
Thanks, looking at those "sharp" vertical stripes I think they're caused by slight exposure differences between the frames and not by the absence of vignetting correction.

Maybe you can tweak the exposures of some of the frames to make them more uniform (usually easiest when shot in raw, but sometimes it can work with jpg's as well) before you let photoshop create the panorama. Another option might be to use Microsoft ICE, which is a freeware panorama program and see how that handles these slight exposure differences.
 

MGrayson

Subscriber and Workshop Member
Thanks, looking at those "sharp" vertical stripes I think they're caused by slight exposure differences between the frames and not by the absence of vignetting correction.

Maybe you can tweak the exposures of some of the frames to make them more uniform (usually easiest when shot in raw, but sometimes it can work with jpg's as well) before you let photoshop create the panorama. Another option might be to use Microsoft ICE, which is a freeware panorama program and see how that handles these slight exposure differences.
It's obvious on the right side you can see all the steps that camera took
Agreed. And that's odd, as I thought PS could correct simple exposure differences. In fact, I thought it could do everything. Was "Seamless Tones and Colors" checked on the Auto Blend Layers?
 

drevil

Well-known member
Staff member
i think there should be some options that needs to be ticked, you mentioned with a different program this doesnt occur, try also PTGUI, imho, the best stitcher out there
 

rdeloe

Well-known member
Warning: Pompous lecture ahead.

Panorama stitching is a hard problem. This is why shift lenses (or better, backs) are so good. If you keep exposure and f-stop the same, and nothing out there moves, the captures are actually part of the same image (Imagine chopping an 8x10 negative into pieces - except that they overlap...), so stitching them is just a matter of aligning the pieces, and that's really easy (I could go into the math, but I doubt anyone is interested).

But when the panorama is made from separate captures where the lens changes aim, like rotating the head 10 degrees between shots, then you add geometry changes and vignetting (and possibly color shifts with an older digital back) to the differences between each piece. Say you have a horizontal panorama. What happens "under the hood" is that each image is projected onto a cylinder. The computer has to know the exact FoV to do this, and it usually figures it out by matching the distortion of a pair of adjacent images. You could see this in old stitching programs as a string of overlapping bubbles. Now all the pieces will line up, so the geometry is solved. The final stitch is either left as cylindrical and unwrapped or projected back onto a plane. The former is more natural for landscapes, but straight horizontal lines, like roads, will appear curved. The latter keeps all straight lines straight, but gets very distorted near the edges - you're effectively creating a super-wide lens. If you cover a full 180-degrees, the projected image will be infinitely large. Bad.

The hard part, and what PS doesn't do perfectly, is matching the exposure, vignetting, and color of the different pieces. That's hard and I don't know the current state of the art. It used to be that specialized software had to be used, e.g., AutoPano or the venerable PTGui where you lined everything up by hand. In C1, if you took an LCC (same image but with a white translucent piece of plastic in front of the lens) for each piece, then the software could correct more or less perfectly. But the latest and greatest can figure out on-the-fly and give a smooth stitch. Without those final corrections, you get dips in the brightness between the pieces, hence the vertical lines.

If you do a multi-row stitch, the computer uses a sphere to match up the pieces and then projects - somehow - back to a plane. It's the same problem map makers have when they try to cover a significant part of the globe, and why there are so many different global map types.

Matt

Edit: I unfairly maligned PS. It should be able to do all the tone and color alignment. Was that box unchecked?
Not pompous! I found that very helpful. I knew the concerns intuitively, but you explained them clearly. Thanks!

P.S. Lightroom does a good job stitching too now. It doesn't automatically correct exposure (or at least not all the time because once when I forgot to lock the exposure in for all frames, it was quite obvious where the join was based on the exposure change.
 

AreBee

Member
To add to the above, if each gradient edge is indicative of each image edge, it seems an excessively large overlap in neighbouring images was adopted. A value of one-third is typically suggested for the proportion of overlap in neighbouring images. In reality it can be significantly less, depending on image content.
 

MGrayson

Subscriber and Workshop Member
To add to the above, if each gradient edge is indicative of each image edge, it seems an excessively large overlap in neighbouring images was adopted. A value of one-third is typically suggested for the proportion of overlap in neighbouring images. In reality it can be significantly less, depending on image content.
Yes. Many panoramas improve by *removing* some of the images in the stack.
 
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