November 16th, 2007
I’d like to comment and elaborate on a discussion thread that is going on in the Photography For Real Estate flickr group. I’ve noticed quite a number of folks using stitched images for real estate photos.First I need to admit that when I first discovered stitching I used an image of a large master bedroom on a flyer layout. My wife (my Realtor client) and the seller both went ballistic about the curved lines. I recall not agreeing with my clients on this criticism at the time because I was fascinated by the technology of stitching. I thought this was a wonderful way to show this large room but I changed my flyer to use rectilinear images.The example image above is six images (shot with a Canon 10-22 mm lens on a Canon 20D) stitched together with the photomerge feature of Photoshop CS3 (fantastic stitching software by the way). I think the image above is a wonderful image except for the fact that the lines of the windows are curved. These curved lines create a very similar distraction that barrel distortion causes. That is, you know very well that it is unlikely that this wall is curved yet the wall is curved is curved in the photo. It’s a visual contradiction that grabs the viewers attention and distracts from the real purpose of the photo; not what you want in a marketing photo.The reason the lines are curved is that this image is a cylindrical projection of the 3D space of this room onto a 2D surface. Photomerge will try make a rectilinear projection if you use the “perspective” option. However, if you try to make a rectilinear projection of an image that has a field of view (FOV) wider than 120 degrees the perspective gets wildly distorted and the distortion is all on the left and right edges. For more interesting reading and examples on the subject of cylindrical vs rectilinear projections see this article. The conclusion of this article is:
“deformation is evenly distributed in a cylindrical panorama while it’s concentrated near image sides when rectilinear mode is used. 360° is possible when using cylindrical projection. 90° to 120° is acceptable (depending on the subject) when using rectilinear projection.”
Interestingly this straight line problem with using cylindrical panoramas for interiors almost never happens when you use them for landscapes. That’s because your eye usually can’t even spot straight lines being rendered as curves if there aren’t long straight lines.The bottom line on using stitched images for interiors or exteriors with long straight lines is that if you use them, use rectilinear stitching and keep the horizontal field of view less than 120 degrees and probably closer to 100 degrees so the perspective distortion on the left and right side of the image is not too objectionable.
Nov 18 Update: I want to point out the excellent comment below that Jon May made today. It points out a article by Georges Lagarde and a photo in Jon’s flickr photo stream where Jon has used the technique described by Lagarde. In short, what this means is there are in fact techniques to “fix” the perspective distortions I described above and the seventeenth century venetian masters understood and used these techniques. These non-classical perspective “tricks” that the venetian masters used can be used for photographic images.
This is what I love about this medium (collaborative Internet discussions). When you carry on a global discussion like we are here you end up with far than you would in any other form of interaction! Thanks Jon for this insight!