3d coordinates of the scene to 2d image coordinates

[traiano]

Hi im in houdini with a geometry that has a texture.

i render with a camera that has transform information such as translate, rotate, also the pixel aspect, focal lenght and aperture etc

 i render in 1920 x 1080

if i know the geometry points position in the 3d scene, how can i calculate in which coordenates in 2D 1920x1080 those points gonna finish at?

 

 

[localstarlight]

I'm not sure if Houdini has a quick and easy way to do this...?

Here's an article with a lot of detail about the process in a more general sense: https://www.scratchapixel.com/lessons/3d-basic-rendering/computing-pixel-coordinates-of-3d-point/mathematics-computing-2d-coordinates-of-3d-points

Don't know if that helps at all though.

[konstantin magnus]

This might not be entirely correct (it's late), but you can use the transform node in VOPs to convert from world space to camera space. Then setup a wrangle node like this:

vector res = chv('res'); // Screen resolution
float focal = radians(chf('Focal_length')); // Cameras focal length in radians
float angle = 2 * atan(0.5 / focal); // View angle

float frustum = 2 * -@P.z * tan(focal * 0.5); // Cameras frustum size depending on the distance
vector screen = set( (@P.x / frustum) + 0.5, (@P.y / frustum) + 0.5, 0 ); // Relative screen positions
v@screen_pixel = set(res * screen); // Absolute screen positions in pixels

screen_pixel.hipnc

Edited June 16, 2017 by konstantin magnus

[traiano]

thank you very much konstantin and steve, i will proceed with these stufff you mention.

[konstantin magnus]

Hmm ok, after reading jsmack`s post in your other thread about this on the SideFX forums this would be the code, I guess:

string cam = chs('Camera');
vector res = chv('Resolution');
v@pos = toNDC(cam, @P);
v@pix = @pos * res;

What are you trying to do, anyways?

[traiano]

I'm doing sinthetic data to train neural networks with cg rendering

[traiano]

Konstantin the coordenates doesnt seem to match in the render output, screen pixel x is almost similar to what it should be, screen pixel y seems to be inverted

[traiano]

For houdini the lower right corner of a render output seems to be 1280 x 0 instead of 1280 x 720

[traiano]

toNDC is not predicting well the pixel in the output

[konstantin magnus]

I guess its assuming a square shaped image. probably the height value has to be multiplied by the aspect ratio.

[traiano]

the aspect radio is 1