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#51856 by Nick Pyner
Wed Jan 19, 2011 1:29 am
I always have a difficulty getting a kosher match in perspective between a model and a photograph. The procedre is simple enough, camera position is measured on site and brought into DataCad via survey, but getting it right first time seems more down to luck than management.

The camera position is defined in the model by a fat post of appropriate height and this is the eyepoint for the perspective. This can be tested in DataCad and then the package taken through to a renderer, where the post is used again for the camera therein. Needless to say, if it is decided to relocate the camera because the perspective doesn't match the photo, you really need another post. If you are slack and set the perspective, and indeed have what you want, but without a new post, salvation is at hand.

The current eyepoint is offered to you when you select S1 SetPersp and the screen scale will be at something obvious, like 1:500. You can record this with the PrintScreen key, and paste this as a new image in a graphics programme. You may then trim this to show just your drawing area. It pays to bucket the background white, so that you can tell it apart from the screen....

With the plan view of the model at say 1:500, you can insert the cropped bitmap and size it to fill the whole screeen. I find it difficult to jam the second cursor point into the bottom corner but hitting Enter seems to fix it. This can then be moved over the model, with a minor scale adjustment if deemed necessary, and thus giving course and distance in order to copy the post into the new camera position. This is verifiable by going into the perspective view and stepping back 300. Part of the post should then come into view.

I can't get the pictures in order but the intent should be clear enough!
The Dcad model inserted into the picture
work1.jpg (225.26 KiB) Viewed 7005 times
The camera position verified
posttop.jpg (75.03 KiB) Viewed 7005 times
The PrtScn image back in DataCad
Dcadv.jpg (73.53 KiB) Viewed 7005 times
#51863 by Mark F. Madura
Wed Jan 19, 2011 11:53 am
For what it's worth, Bill Riseman used to incorporate what he called a 'Stadia' (a 30"x40" reference board) at the center of his site photographs. He would then use this as a reference to match the DataCAD wire-frame perspective with the scanned photo.
#51884 by Nick Pyner
Wed Jan 19, 2011 11:45 pm
The real problem is not what you are aiming at, but where you are aiming from. The centre of the photo is the centre of the photo. I use a cricket ball on its own layer as a target in the model and generally get a neighbouring gutter etc for additional reference.

The above is for those with unsufficient discipline to rigorously track their eyepoints.

For all the theory, I have never gotten the perspectives to match on the first attempt and, the wider the lense, the more difficult this can be. The above was shot with 27mm equiv digital and actually came together pretty well. On two occasions I have been obliged to use my 20mm Krasnogorsk on 35mm film. I had a bad time with both and have never been able to work out why. I will sort it out when I retire........
#51888 by Mark F. Madura
Thu Jan 20, 2011 9:16 am
A CAD perspective does not have any lens distortion. Wide angle lenses tend to have the most distortion. Have you tried using a program like DxO to correct the geometry in the digital photo?
#51894 by Nick Pyner
Thu Jan 20, 2011 7:33 pm
I'm not talking about distortion per se. PaintShop Pro has satisfactory barrel distortion correction which I occasionally use on the 27mm (equiv.) Olympus. The 20mm is so good that I have never bothered.

The problem I have encountered is the relationships in perspective i.e. the juxtaposition of objects and the eyepoint. The focal length of the lense has no bearing on perspective but it compounds the problem because it can see more. Having said that, I never model something the camera can't see. Most of my stuff is done at 35mm.

All this is hard to explain, which is why I haven't pursued it too far. I have now encountered the problem again in the above job. The house is pretty right but the garage in the foreground doesn't line up properly and I have just found out that there is alot of new work going on there. I will have to fake it with a separate model.

I was thinking again about this under the shower last night. I will go onto a football field and set up a test with the goalposts and some wooden stakes - all carefully set out and modelled in DataCad. I can even see the possibility of plotting the wirefame onto 5x4 clear film and putting that in the back of the Graflex.

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