Neither is correct.
I learned about inverse kinematics.
Is correct, both words should be written in lowercase.
Inverse kinematics (I capitalized here as it is the start of a sentence) is a mathematical process, it's a similar class of thing as... algebra. We wouldn't capitalize algebra in the middle of a sentence for the same reason.
You can see some example sentences over on the Wikipedia entry:
Inverse kinematics (Wikipedia)
Inverse kinematics is the mathematical process of recovering the
movements of an object in the world from some other data, such as a
film of those movements, or a film of the world as seen by a camera
which is itself making those movements. This is useful in robotics and
in film animation.
In robotics, inverse kinematics makes use of the kinematics equations
to determine the joint parameters that provide a desired position for
each of the robot's end-effectors. Specification of the movement of
a robot so that its end-effectors achieve the desired tasks is known
as motion planning. Inverse kinematics transforms the motion plan into
joint actuator trajectories for the robot.
A recent edit to the question asks about physically based rendering.
The same rule applies as above, all lower case in the middle of a sentence.
Physically based rendering, appears to have an accepted abbreviation (PBR).
You could refer to the model as PBR once you have introduced it. For example:
"Physically based rendering (PBR) is a model in computer graphics...
PBR is often characterized by..." etc.
See here for more examples of physically based rendering being used in sentences.