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What is self collide? and what is difference between visual and collision?

asked 2018-09-25 00:43:00 -0500

GLV gravatar image

Hey There, I am new to ROS and Gazebo. I want to know what is the self collide what is the use of it (when it true or false) and also the difference between visual and collision? ( I am facing these words while using Link inspector)

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answered 2018-09-25 01:16:03 -0500

Delb gravatar image

From the sdf format spec :

If set to true, all links in the model will collide with each other (except those connected by a joint). Can be overridden by the link or collision element self_collide property. Two links within a model will collide if link1.self_collide OR link2.self_collide. Links connected by a joint will never collide.

From the wiki :

<visual> (optional)

    The visual properties of the link. This element specifies the shape of the object (box, cylinder, etc.) for visualization purposes. Note: multiple instances of <visual> tags can exist for the same link. The union of the geometry they define forms the visual representation of the link. 

<collision> (optional)

    The collision properties of a link. Note that this can be different from the visual properties of a link, for example, simpler collision models are often used to reduce computation time. Note: multiple instances of <collision> tags can exist for the same link. The union of the geometry they define forms the collision representation of the link.

To sum up : Self collide will tell that the elements of the robot can collide with each others, the visual will define the shape of each element in your urdf and the collision their "hitbox".

For further explainations maybe ask on the gazebo forum which is more suitable for your question.

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Thank Mr. Delb, it helps me.

GLV gravatar image GLV  ( 2018-09-25 01:50:27 -0500 )edit

@GLV, if this answered your question, please mark it as correct by clicking the checkmark next to the answer. You can only mark one, so I suggest to mark the one from @Delb, as this is more comprehensive.

mgruhler gravatar image mgruhler  ( 2018-09-25 07:30:05 -0500 )edit
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answered 2018-09-25 01:13:10 -0500

mgruhler gravatar image

See #q199854 about the self collide.

A visual is a tag describing the visualized mesh. This is really only visualization and not used for collision computations.

A collision tag describes the collision mesh.

This seperation allows to have a "high-resolution" mesh for visualization and a "low-resolution" mesh for collision checks, resulting in a faster execution. (Collision checking is expensive, it scales with the amounts of triangles in your mesh, visualization is comparably cheap)

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Thank you Mr. Mgruhler

GLV gravatar image GLV  ( 2018-09-25 01:51:10 -0500 )edit

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Asked: 2018-09-25 00:43:00 -0500

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Last updated: Sep 25 '18