matching real camera info in Houdini

[ghoshix]

Hello,

I am trying to understand how to match a real camera on a drone with a 3D camera inside Houdini.  I am not a photographer so only know basics about cameras and I have found previous posts here and the SideFX documentation - https://www.sidefx.com/docs/houdini/ref/cameralenses.html - listing various formulas for calculations but those confused things even more:

" You can obtain a good fit between the Houdini camera and a real world camera by matching a measured lens’s horizontal angle of view, and deriving a Houdini focal length value that reproduces it with the default aperture 41.4214."

I have the real camera specifications: it has a 20 mm lens, a f/2.8 aperture and a dimension of 4056 x 3040 - how does that apply?  Their "basic mathematics" formulas are pretty much over my head - I really wish they would include examples of things like this in the documentation

I see a few blender and maya tutorials online for similar questions so I guess I'll see if those help but if anyone can offer clearer instructions than the "help" documentation did I would be really grateful!

Thanks

Edited September 1, 2022 by ghoshix

[evanrudefx]

5 hours ago, ghoshix said:

Hello,

I am trying to understand how to match a real camera on a drone with a 3D camera inside Houdini.  I am not a photographer so only know basics about cameras and I have found previous posts here and the SideFX documentation - https://www.sidefx.com/docs/houdini/ref/cameralenses.html - listing various formulas for calculations but those confused things even more:

" You can obtain a good fit between the Houdini camera and a real world camera by matching a measured lens’s horizontal angle of view, and deriving a Houdini focal length value that reproduces it with the default aperture 41.4214."

I have the real camera specifications: it has a 20 mm lens, a f/2.8 aperture and a dimension of 4056 x 3040 - how does that apply?  Their "basic mathematics" formulas are pretty much over my head - I really wish they would include examples of things like this in the documentation

I see a few blender and maya tutorials online for similar questions so I guess I'll see if those help but if anyone can offer clearer instructions than the "help" documentation did I would be really grateful!

Thanks

I spent time trying to figure this out a while back. First I am not sure why it is called aperture since it is really the horizontal dimension of the film sensor. What ever your sensor size is, put the horizontal dimension as the value here instead of 41.4214. if you have a full frame it would be 36. You can test this out by exporting a camera from blender or maya as a abc. They will import with a aperture of 36 mm because that is typically the default everywhere. then you can match the focal length value with the value of your camera.  

edit: I see you are using a camera on a drone, so I am guessing it isn't full frame. Just google the dimensions of your sensor on the camera.

Edited September 2, 2022 by ejr32123

[ghoshix]

Thanks @ejr32123, I appreciate the response - really surprised there isn't more clearer information out there about any of this stuff but I guess people just use matchmoving software for movies so nobody really needs to do it themselves. .

I'm sorry to say I don't understand what you're suggesting - I looked up the sensor size and it is 1/2.3 (which is 0.43478) and actual dimensions 6.17mm x 4.55mm but again i don't see where those numbers would make sense anywhere in these settings, they aren't anywhere close to that default recommendation by Sidefx unless I shift the 0.43478 by two decimal places.  The specified focal length on the camera is "20mm (with 35mm equivalent)" - is that where you're getting the 36 you recommended? Very very confusing!

[evanrudefx]

2 hours ago, ghoshix said:

Thanks @ejr32123, I appreciate the response - really surprised there isn't more clearer information out there about any of this stuff but I guess people just use matchmoving software for movies so nobody really needs to do it themselves. .

I'm sorry to say I don't understand what you're suggesting - I looked up the sensor size and it is 1/2.3 (which is 0.43478) and actual dimensions 6.17mm x 4.55mm but again i don't see where those numbers would make sense anywhere in these settings, they aren't anywhere close to that default recommendation by Sidefx unless I shift the 0.43478 by two decimal places.  The specified focal length on the camera is "20mm (with 35mm equivalent)" - is that where you're getting the 36 you recommended? Very very confusing!

So the setting in houdini called "aperture" that has a value of 41.4214 is really the horizontal dimension of your sensor. A full frame 35mm camera has a sensor size of 24mm x 36mm. That is why most software has 36 as the default. I was told (by different sources) the reason houdini has a value of 41.4214 is because that paired with a default 50mm lens produces a perfect 45 degree fov. If your camera has a 20mm focal range with 35mm equivalent, I suppose that means when compared to a standard 35mm sensor your focal length would be 20 mm. 

I would try two things and see which works better.

1. try setting the aperture value to 36 (because that matches the standard 35mm sensor size horizontal dimension) and your focal length to 20mm

2. try putting the horizontal dimension of your sensor for the aperture value (6.17 I think you said) then find what the actual focal length of your lens is (not the 35mm equiv) and use that for your focal length value. 

from my experience using either of those should work.