Resolving distortion in a stitched wide-angle panorama

<p>I was in a situation where 24mm (on full frame) was the widest focal length that I was carrying, but I needed something a bit wider to "fit in" the composition that I wanted to capture.</p>

<p>I mounted the camera in vertical (portrait) orientation and snapped four shots while rotating the camera through the scene. The camera was level for all four shots (not tilted upward or downward). Focus, white balance, and exposure are all spot on and perfectly matched from frame to frame.</p>

<p>When I Merge the images in Photoshop CS6 using the default settings (plus clicking the Fix Perspective Distortion option), the output is predictably football shaped. The center of the image is tall, and it tapers off to the left and right sides due to the wide angle distortion of the more distant corner objects. There are no issues with ghosting or misalignment of near objects. Everything in the image is at least a couple of hundred feet away. </p>

<p>I want to transform the football-shaped composite image into something more rectilinear, i.e. to expand the left and right sides a bit to have them match the size and lines of the center of the image. I tried all of PS CS6's transform tools, but I couldn't find one that would do what I needed. Any tips would be appreciated.</p>

<p>I tried the Adaptive Wide Angle filter after watching some video tutorials, but it seems to be very tricky to use. I'll give it another go if you think that it would help.</p>

<p>I tried to redo the stitched panorama using the Merge's Perspective option, but this did something very strange. After several minutes, the process created a highly distorted panorama based on the extreme right side image. Is there a way to designate which image should be in the center and have the panorama take perspective cues from that center image instead of one of the extreme left or right edges?</p>

<p>Thanks in advance for your time and suggestions. Reshooting the scene is not practical.</p>

<p>You might give Microsoft's ICE a try, it has the possibility of changing the pano by just dragging the edges up or down.<br>

Another program is Hugin, it comes with a lot of options for different projections. It has a steep learning curve though.<br>

Both are freeware so you can give them a try.</p>

<p>In order to merge frames into a panorama, the program must first convert the images into a cylindrical projection. If you leave it in this format, horizontal lines will converge away from the viewer as you move toward the edges. Vertical lines will be preserved. An image in cylindrical projection will be roughly rectangular, with bulges and jagged edges which can be cropped away. The closer you estimate the focal length, keep the camera level and rotate around the front node, the less you will need to crop.</p>

<p>If the horizontal angle is less than 180 degrees, you can convert the results to rectilinear projection, similar to the results from non-fisheye wide angle lenses. The image is stretched into a bow tie shape before cropping, expanding the ends vertically. This will keep booth horizontal and vertical lines straight, but distort spherical objects into egg shapes at the sides and corners.</p>

<p>These corrections will generally be centered, but you can shift the perspective vertically or horizontally. You can aim the camera up or down to adjust the horizon, causing vertical lines to converge. Shifting the perspective point can remove this convergence.</p>

<p>Most of these controls are available in Photoshop 6, but a dedicated program for panoramas makes the job much easier. I use PtGUI, which costs about $100, but makes blending and perspective much easier. Life is too short to take baby steps.</p>

<p>You indicated you tried all the transform tools. Did you try stitching using the Cylindrical Projection setting? Also, did you perform lens correction and CA correction on all of the frames before stitching?</p>

Did you try a rectilinear rendering? Have you experimented with puppet warp? Depending on wide the view is you may

have to accept some warping as the price you pay for fitting an extremely wide angle of view to a flat two dimensional

medium.

 

I have also found that it helps to over shoot (shoot additional frames on the ends of your intended final composition) and

stitch to give you more headroom on the ends of the final composition.