Creating colour with facing ratio

[roarke80]

Hi, sorry if this has been asked before, but I'm a beginner and am not even sure how to begin searching.

I'm trying to select the points on an object that are at a greater angle away from camera. Basically it would be as though I applied a Fresnel in C4D or a facing ratio shader in Arnold, and then deleted away all the black points which leaves me with the outer rim of the object. If I can control it by somehow shining a light on it to determine where the camera is that would be great. Any tips would be greatly appreciated, many thanks!

 

 

[konstantin magnus]

You can compare the angle of the camera direction towards the points against the surface normals with a dot product.

float thresh = chf('angle_threshold');
vector cam_pos = normalize( chv('camera_position') );
vector dir = normalize(@P - cam_pos);
float angle = abs( dot(cam_pos, @N) );
if(angle > thresh) removepoint(0, @ptnum);

 

 

camera_angle_delete.hipnc

[stickman]

Cool stuff, Konstantin!
If you wanted to just make a group of those points, what would that last line be?
 

[acey195]

1 hour ago, stickman said:

Cool stuff, Konstantin!
If you wanted to just make a group of those points, what would that last line be?
 

@group_delPoints = (angle > thresh);

//or alternatively (needed if want to assign the group to points other than the ones currently ran over):

if(angle > thresh)
	setpointgroup(0, "delPoints", @ptnum, 1);

//or again optimized:
setpointgroup(0, "delPoints", @ptnum, angle > thresh);

 

[konstantin magnus]

With optransform(), cracktransform() you can access a nice object chooser that reads out the camera position for you. I also corrected the code which results in this:

float thresh = chf('threshold');
matrix cam_matrix = optransform(chs("camera"));
vector cam_pos = cracktransform(0, 0, 0, {0,0,0}, cam_matrix);
vector dir = normalize(cam_pos - @P);
float angle = abs( dot(dir, @N) );
setpointgroup(0, "rim", @ptnum, angle > thresh, "set");

camera_angle_delete.hipnc

Edited October 28, 2017 by konstantin magnus
added example file

[roarke80]

Wow this works beautifully, thank you so much for your help!