Houdini Solaris set camera focus distance

houdini_solaris_attribute_wrangle_lop_set_cam_focus_distance.vex

string targetpath = "/focus_point";  // Target prim to focus on

// Compute Z-plane distance to target prim transform from camera transform
matrix cam = usd_worldtransform(0, @primpath);
matrix target = usd_worldtransform(0, targetpath);
vector origin = set(0, 0, 0);
matrix local_diff = target * invert(cam);
vector local_pos = ptransform(origin, local_diff);
float dist = -local_pos.z; // inverted, because -Z is direction camera looks at

// Set focus distance on the camera
f@focusDistance = dist;
printf("%s\n", dist); 

houdini_solaris_python_lop_set_cam_focus_distance.py

from pxr import UsdGeom

node = hou.pwd()
stage = node.editableStage()

cam = stage.GetPrimAtPath("/cameras/camera1")
target = stage.GetPrimAtPath("/focus_point")

cache = UsdGeom.XformCache()
cam_xform = cache.GetLocalToWorldTransform(cam)
target_xform = cache.GetLocalToWorldTransform(target)

target_local_xform = target_xform * cam_xform.GetInverse()
distance = -target_local_xform.ExtractTranslation()[2]

UsdGeom.Camera(cam).CreateFocusDistanceAttr().Set(distance)

Note that a lot of examples on the web set the focus distance by taking just the distance between object and camera, but that's technically incorrect. It should be the z-depth distance to the focal plane - so the z-depth depth for the target object from the camera.

Also see other workflows:

• https://www.sidefx.com/forum/topic/41552/#post-282697
• https://www.sidefx.com/forum/topic/7088/#post-374564
• https://vfxbrain.wordpress.com/2020/07/07/camera-auto-focus/ - this also mentions that focus distance is not just distance to the object, but is actually the z-depth (straight forward) distance to the object's depth because the focal distance defines a distance to the focal plane.
• https://youtu.be/fPozKBPG7kM?t=1032 - Tim van Helsdingen tip on setting focus distance from camera to object for OBJ level context.