Lining up to a photographic backplate without any survey data?

[Rich O]

I've been given backplates for compositing by an architect, however they don't provide any data like camera location. All I have to go by is the '35mm focal length' property in the jpeg's meta data (which is 28mm). It also says the 'focal length' is 5mm, which seems bizarre

 

I'm using Vray's camera, how do these relate its "film gate" and "focal length" options? Any tips would be great.

 

Using Maya and Vray, btw.

Edited February 12, 2012 by Richard7666

[410]

All I have to go by is the '35mm focal length' property in the jpeg's meta data (which is 28mm). It also says the 'focal length' is 5mm, which seems bizarre

 

I'm using Vray's camera, how do these relate its "film gate" and "focal length" options? Any tips would be great.

 

Hi there Richard,

 

I can't help you with the lining up of the back plate, I did see a tutorial online about lining up cameras and backplates without detailed camera data, but I can't seem to find it right now.

 

Someone else will undoubtedly know more; but to my knowledge it is possible to eyeball it and make guesstimate calculations based on perspective lines (vanishing points, horizon lines, etc.) but to a degree it does depend on the image that you've been provided with.

 

With regards to your film gate/focal length question I explored this subject in depth fairly recently in another thread here:

 

http://forums.cgarchitect.com/68634-architectural-interiors-sizes-proportions-units-setup-problem.html#post348800

 

But based on the data you've provided, your film gate should be approximately

6.43mm and focal length should be 5mm.

 

By the sound of it, this image was taken on a digital compact camera with a very small 'film gate' or sensor size- if the data is to be believed, the image was taken with a 5mm focal length lens (which projected over a "full 35mm" frame equivalent sensor size of 36.0mm would be an extreme wide angle; but because the sensor is so much smaller, this equates to the equivalent of a 28mm lens's modestly wide field of view).

 

Now, I should say that my calculation isn't exact and it's based on the assumption that the 5mm lens on this camera sensor gives almost exactly a 28mm equivalent field of view (65.4705° measured horizontally) if you had the camera information, ie "Camera Maker" and "Camera Model" you could find out in the technical specifications for that camera exactly what size the sensor is and plug that measurement into the 'film gate' option on your V-Ray Camera. But my numbers may be accurate enough for your purposes.

 

If you don't have the camera make and model and want to do your own exact calculations, check out that thread I posted in- although I'm using 3ds Max, you should be able to apply everything I've done to work out the numbers to any other formidable 3d program like Maya.

[frog_a_lot]

Always try and get the type of camera so you can check the sensor size to be sure. but with the info you have.

 

1 - Set camera (vray physical is what i use) film gate to 35mm and focal length to 28mm

2 - Get google earth or maps and place your model roughly where it would be in real life.

3 - Look at photo and find some key elements and move camera to around about where you think photo was taken from.

4 - Try and match your horizon line to the horizon line of the photo.

5 - Wiggle..wiggle.. move the camera bit by bit left.. right.. up down.. etc until you are happy.

 

It is always good to have the google earth image on the bottom so you can get reference points, like trees or other buildings. Put in some cylinders for trees, rough boxes for buildings etc

[410]

Always try and get the type of camera so you can check the sensor size to be sure. but with the info you have.

 

1 - Set camera (vray physical is what i use) film gate to 35mm and focal length to 28mm

2 - Get google earth or maps and place your model roughly where it would be in real life.

3 - Look at photo and find some key elements and move camera to around about where you think photo was taken from.

4 - Try and match your horizon line to the horizon line of the photo.

5 - Wiggle..wiggle.. move the camera bit by bit left.. right.. up down.. etc until you are happy.

 

It is always good to have the google earth image on the bottom so you can get reference points, like trees or other buildings. Put in some cylinders for trees, rough boxes for buildings etc

 

This is great advice Cameron.

 

And for CG work, you can definitely substitute the "35mm equivalent" focal length for the actual lens on a compact digital camera- it undoubtedly makes your job in Max or Maya a whole lot simpler.

 

But as a former camera technician, I love to niggle at the details- and if you're using the "35mm equivalent"/"full-frame equivalent" the film gate should be 36.0mm - a standard 35mm film stills camera has a gate measuring 36mm x 24mm.

 

Will it make a difference if you use a 35.0mm gate as opposed to a 36.0mm gate? Uhh well, no it's basically negligible. Half a degree of horizontal field of view.

 

And I'm guessing if it was critical for the work to have an exact camera match you'd have all the camera & lens data.

[Rich O]

It's been taken with a Pentax Optio P80, which is a point-and-click, yeah. That's really good advice guys, thanks.

 

Giving it a try now.

[Scott Dombrowski]

Here's how I approximate where a photo was taken. It requires relatively large landmarks that can be seen in a Google Maps aerial view.

 

First, in the backplate image, I find two objects that line up. Then, in the aerial image, I draw a line between those two objects. I repeat the process a few times, and usually end up with those lines intersecting in one spot. It's not pinpoint accurate, but it's usually pretty close. Certainly close enough to use as a starting point for placing your camera in 3ds Max.

 

[ATTACH=CONFIG]47149[/ATTACH][ATTACH=CONFIG]47150[/ATTACH]

[Stephen Thomas]

But as a former camera technician, I love to niggle at the details- and if you're using the "35mm equivalent"/"full-frame equivalent" the film gate should be 36.0mm - a standard 35mm film stills camera has a gate measuring 36mm x 24mm.

 

Quoted for agreement, this one always causes confusion 35mm film is so-called because that is the width of the strip of film, i.e. 24mm height of the frame plus the sprocket holes either side. The 36mm wide frame lies horizontally along the strip, as you can see from looking at a set of negatives.

[Rich O]

So once you've drawn the lines, you use the Google image as a reference plane in Max/Maya/whatever, place your model where it would be on this reference plane, and then the camera where the lines intersect and rotate it so it looks at your model from the correct angle? That would work.

[Scott Dombrowski]

So once you've drawn the lines, you use the Google image as a reference plane in Max/Maya/whatever, place your model where it would be on this reference plane, and then the camera where the lines intersect and rotate it so it looks at your model from the correct angle? That would work.

 

Yup, that's pretty much it. Just make sure your Google image plane is to scale.

[Bwana Kahawa]

If you need to know sensor sizes, I find http://www.dpreview.com has a great camera database - you can find most models on there (get the camera name from the image metadata), and you'll find the sensor size on the database.