Stereographic projection lens shader ?!

[NNois]

Hi,

 

I need to create a lens shader in VOP but can't found any info/guide about how to create one.

 

What is the correct procedure or do someone has an example file somewhere to start with ?

 

This would be a lens shader similar to the "dome master" one on orbolt https://orbolt.com/asset/nhat::DomeMaster

But mine would be to produce Little Planet result like this

 

 

The two projection differs because the stereographic one (above picture) deform the picture because it's a polar projection shooted from the pole of a globe, so maybe this involve a 2 pass projection... I hope not

 

 

I know i can do that in post:

1. Shoot a Polar Projection of the scene

2. Map that to a sphere then

3. Shoot the sphere from the inside the cam placed on the north pole

 

My main problem is that it involve shooting extreme resolution pictures to cover the entire revolution.

 

 

 

Cheers

Edited October 22, 2015 by NNois

[mestela]

I went through a similar journey to hack up a stereo pano shader, should get you started. If you have a reference implementation with the correct maths, should be relatively easy to do:

 

http://www.tokeru.com/mayawiki/index.php?title=HoudiniOculusWip

[NNois]

Great !

Thank you very much for these CVEX example.

 

Now i managed to make my own fisheye and my own panoramic lens. So now i need to do the real work but i'm stuck...

 

My problem is implementing the stereographic projection http://mathforum.org/mathimages/index.php/Stereographic_Projectionwitch refers to "Points on a sphere"

I assume these points are the results of the panoramic lens who needs to be reprojected on a plane right ?

 

Can I do just that in a vex shader, projecting two times in one op ?

 

here's a ref of the projection

 

++

Noël

[mestela]

Wait a sec, isn't that basically what Matt Ebb has already done?

 

http://mattebb.com/weblog/houdini-fisheye-camera/

 

Just set the fov to 360, it gets that little planet effect:

 

 

 

[NNois]

well sadly no, look how your trees are Squashed in Y, a proper stereographic projection don't do that

[mestela]

A power curve to bias the y vales more towards the center of the image should fix that right? Haven't looked closely, but I reckon Matts code should get you 90% the way there.

[NNois]

Well the problem is 3d isn't alone and i had to repro that in nuke where i do can get a 100% projection match.

 

Had to invest time converting the math but seems pretty hard 

Edited October 27, 2015 by NNois

[mestela]

Bit of warping worked.  The line where Matt calculates the radius:

float r = sqrt(x*x + y*y);

I changed so it goes through a power function, which should values towards the center if you raise to a power less than 1:

float r = pow(sqrt(x*x + y*y),warp);

Playing with the fov and warp values gives this image:

 

 

 

fisheye_little_planet.hip

 

 

[NNois]

Right,

But notice how the center is "pinched" compared to the "right" one

 

Fisheye mod:

 

Stereoscopic Projection:

[NNois]

Hi,

I've managed to implement the real Stereographic projection ;-)

(http://www.ipb.uni-bonn.de/fileadmin/publication/pdf/Steffen2005Fish.pdf)

i = set(
    (x+x)/(1+(x*x)+(y*y)),
    (y+y)/(1+(x*x)+(y*y)),
    (1-(x*x + y*y))/(1+(x*x)+(y*y))
    );

 

This is for the 180° dome, just multiply x and y before and you have a "zoom" factor.

 

I was just wondering, what could be the right math to get exactly the full 360 dome ?

 

testdomeshader_v3.hipnc

 

Thanks !

Edited October 29, 2015 by NNois

[mestela]

Nice one!

 

Re 180 vs 360, not sure, you might have to go read some other implementations. Does the domemaster stuff have a stereographic projection? I ended up looking through their arnold/mentalray shader code for clues when I was trying to get answers to my stereo pano stuff.