Sketchup Blog - News and Notes from the Sketchup folks

Cropped pictures need not apply

Posted by Aidan Chopra, Product Evangelist

I received an interesting question from a gentleman in Greece the other day; he was trying to use SketchUp's nifty photo-matching feature to build a model based on this rendered image of a building. Needless to say, he didn't have any luck. To get the photo-matching feature to work, your image needs to be uncropped. That's why lots of renderings, magazine images and pictures from architecture books don't work; they've been re-framed. Why should this matter? It has something to do with re-positioning the center of the image. SketchUp's photo-matching feature works by assuming that the center of your image is also the image's "center of projection" (where the camera was pointing when you took the picture). When you crop an image, you move its center.

There is, of course, an exception to the "No Cropped Pictures" rule when matching a photo: If your picture has been cropped by an even amount on each side, or by an even amount top and bottom, the center of projection is maintained, and thus everything will work just fine.

Update Clarified the exception to the "No Cropped Pictures" rule in the last paragraph.

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Eiger said...

Aiden- On this subject, any tips on using photo-match in conjuction with a Google Earth terrain snapshot? In each of my attempts the building footprint dimensions from GE conflicted with what photo match indicated.

db said...

Please correct the article so it is clear for the literal ones. A picture has to be cropped by an even amount on opposite sides, across axis. (ie: the top and bottom crops need to be the same, and the left/right crops need to be the same.) It does not have to be cropped by an even amount on all four sides.

Anonymous said...

i don't get your explanation, i mean i don't get the _technical_ reason why Photo-Match does not work with such cropped pictures...
Does Photo-Match extract some information from the EXIF header of the picture file, to apply perspective correction or whatever?
Because without reading the EXIF header (that is to say without having any information about the focal length used, ...), a cropped picture is simply a change of the point of view of the photographer, am i wrong ?

Anonymous said...

Ok i think i got it! This is all about perspective and horizon
Tell me if i'm right...
(From wikipedia:)
"Perspective is also seen in the way the parallel lines of how railway tracks appear to be meeting at a distant point (the vanishing point) on the horizon. When used in this sense, _the 'horizon' is always at the level of the viewer's eye_."

edaniels said...

So... photos taken with a bellows-style view camera (where the lens is offset to keep vertical lines looking parallel) would be a problem too, I'm guessing...?

SMcQ said...

Yes, a shift lens on any format, whether a view camera or 35mm, would displace the vanishing point from the center of the image frame. I would guess that one should also avoid perspective correction in Photoshop, which distorts the vanishing point lines. Let SketchUp do the correction.

Anonymous said...

Now let's see if you look at your image you should be able to determine the original central view point (or at least close) so in a photo editing program simply add blank area to the side or sides that were cropped until you have re-centered the perspective.

Of course this is not always easy and as an artist that often combines different images to create one. I may have a better trained eye then most for doing this but I think it can be done by anyone especially with images of buildings in which you have straight lines to follow.

Much harder to do when you want to add a image of a person to another image. It never looks right if you don't adjust for the original view point no matter how well you blend them with the new background they just do not fit. And then you have to also match the lighting as well at least with this you just need to find the original central view point not match it to another.

Now of course it would be better if Sketchup would allow you to determine the actual perspective center of your image manually instead of assuming that the center of the photo is the central view point. But try this work around for now and let us know how it works.

db said...

Seems like it makes more sense for Google/SketchUp to allow other apps to redefine the center, for the few that have that need and understanding. They also likely have an app that would suffice.

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