Question for Max about an Archived Image

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WWG
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Question for Max about an Archived Image

Post by WWG » Thu Oct 04, 2012 3:02 pm

Max, I like scanning your on-line archive

http://www.tawbaware.com/maxlyons/cgi-b ... and=Search

occasionally to see your new work and to try to understand more about projections and your choice of projections. Your recent image, Hallgrimskirkja Church of Hallgrimur, is a mystery to me. It is a 12 image pano 116 deg wide and you chose to render it in rectilinear and leave the horizon somewhere in the middle, as though it had been taken with an ultra wide lens pointed up at the church. Can you discuss your rationale for these choices?
Werner

maxlyons
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Re: Question for Max about an Archived Image

Post by maxlyons » Fri Oct 05, 2012 8:05 am

Werner,

The short answer to your question is that it was partly driven by aesthetic concerns and partly by pragmatism.

I wanted to use rectilinear projection to avoid any of the strong lines (e.g. the floor and building) in the picture being curved, which would result from any other projection. Ordinarily, I would have also chosen to render the scene so that vertical elements (like the lampposts in the scene) were displayed as vertical. (This can be done by placing the horizon line at base of the church or using vertical line control points and optimizing the project including pitch and roll for all images). However, in this case, I felt that the top of the building appeared to be "too large", and the building felt "too tall" compared to my recollection of the scene. Here is a screenshot from PTAssembler's preview screen showing what that looks like:

Image

If I was willing to accept some curvature in the lines in the image, I probably would have used PTAssembler's configurable "rectilindrical" projection to achieve a compromise somewhere between cylindrical and rectilinear. In the example below I used a yaw factor of 0.78 and a pitch factor of 0.54. This means that the image is stretched along the x axis almost as much as a rectilinear projection (0.78 compared to 1), but compressed quite significantly along the y axis (0.54 compared to 1). I left the horizon line unchanged so that all the vertical lines in the scene are still rendered as vertical, but there is some (subtle) curvature to the lines in the scene, and while the top of the church doesn't feel "too tall" anymore, it does feel "too wide":

Image

So, in the end I chose to use rectilinear projection, but place the horizon a little higher in the scene to simulate as if the image had been taken with the camera pointing upwards at the church:

Image

This isn't completely unprecedented for me. I don't do it too often, but sometimes I like the effect when the vertical are not rendered as vertical...it can help (in my opinion) to give a sense of scale to the scene. Here are a couple of other examples rendered this way:

Image
Details here

Image
Details here

Three other quick points:

First, I really don't think there is a right or wrong choice here. There are a huge number of different ways that this could have been displayed via PTAssembler, and I bet that other people would have made different choices.

Second, sometimes I change my mind about these things...I revisit an image after a while and decide that I like an alternate projection better. Maybe that will happen here!

Third, I have some more images taken of this church that I will process eventually. Some of them were taken from further away, and I suspect that those may work better with a rectilinear projection where the vertical elements are rendered as vertical (because the field-of-view will be less extreme, and so the stretching/enlarging effect at the top of the church will be less pronounced).

Max

WWG
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Re: Question for Max about an Archived Image

Post by WWG » Fri Oct 05, 2012 8:28 am

Thank you for your detailed reply. I get the "too tall" problem. This is not a pano but a single image taken at 12mm on a cropped sensor camera (18 equiv) and leveled.

Image

The church is impossibly tall and thin and after cropping back to a rectangle much of the surrounding environment got lost. I ended up undoing the correction and leaving the tilts alone. But I wished that I had shot this as a pano with enough extra space to allow for other projections but now I see your point that sometimes there aren't any good choices.

Thanks again. I do enjoy your albums, they are very educational.
Werner

ATucker
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Re: Question for Max about an Archived Image

Post by ATucker » Sat Oct 06, 2012 10:59 am

Max,

Thanks for taking the time to give a detailed reply to Werner. Much learned.

Tom

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