Blender Merging Bones: Retaining Weight Paint

Merging bones in Blender while retaining their weight paint is a common issue for users of the software. While there is no native way to do this, there are a few workarounds. One method is to use a vertex weight mix modifier to add the desired weights to the bone that will absorb them, then apply the modifier and remove the appropriate vertex group from the mesh. Another method is to select both objects and press Ctrl+J to join them, keeping the weight paint. However, this may result in the bones no longer being linked to the weight painted parts, requiring them to be reconnected to vertex groups manually.

Use a vertex weight mix modifier to transfer weights to the bone

The Vertex Weight Mix Modifier allows you to mix a second vertex group or a simple value into the affected vertex group, using different operations. This process can be used to transfer weights to a bone.

The Vertex Weight Mix Modifier panel allows you to define the default weight to assign to all vertices not in the given vertex group (Vertex Group A) and the second vertex group (Vertex Group B) to mix into the affected one. You can leave Vertex Group B empty if you only want to mix in a simple value. The default weight for vertices not in Vertex Group B can also be set.

The Mix Set defines which vertices will be affected by the modifier. If you select "All Vertices", all vertices will be affected, regardless of their vertex group membership. Other options include affecting only vertices from Group A, Group B, one of the groups, or both groups.

The Mix Mode defines how the vertex group weights are affected by the other group's weights. You can choose to replace the affected weights with the second group's weights, add or subtract the values, multiply or divide them, or use the smallest or largest weight values.

Additionally, you can use a texture mask to limit the modifier's effect to certain areas of the mesh. The texture mask multiplies the global influence value for each vertex, with black representing 0.0 and white representing 1.0. This allows for more precise control over the weight distribution.

By using the Vertex Weight Mix Modifier, you can transfer weights between bones by mixing the weights of the vertex groups associated with each bone. This provides a flexible way to adjust and fine-tune the weighting of your skeleton rig in Blender.

Use additional modifiers to nullify the weight

When merging bones in Blender, retaining the weight paint can be achieved through various methods, including the use of additional modifiers to nullify the weight. This approach offers a flexible way to manage weights during the merging process. Here's a detailed guide on how to utilize additional modifiers effectively:

Understanding Weights and Vertex Groups

Before delving into modifiers, it's essential to grasp the concept of weights and vertex groups in Blender. Weights refer to the influence that bones exert on vertices within a mesh. Vertex Groups are collections of vertices that are influenced by specific bones. When merging bones, the challenge lies in preserving these weight relationships without disrupting existing animations or weight distributions.

Vertex Weight Mix Modifier

The Vertex Weight Mix modifier serves as a valuable tool in managing weights during bone merging. This modifier enables you to add or subtract weights from specific bones, allowing you to fine-tune the weight distribution. By applying this modifier, you can selectively adjust the weights you want to nullify or transfer to another bone. This targeted approach ensures that you maintain the desired weight distribution while merging.

Practical Application

To illustrate the usage of the Vertex Weight Mix modifier, let's consider a scenario. Suppose you have two bones, A and B, and you intend to keep Bone A while merging Bone B into it. By setting Bone A as the target bone and Bone B as the source, you can use the Vertex Weight Mix modifier to transfer the weights from Bone B to Bone A. This ensures that Bone B can be safely dissolved or removed without affecting the overall weight distribution.

Additional Modifiers for Weight Nullification

In some cases, you may want to nullify specific weights associated with particular bones. This can be achieved by employing additional modifiers in conjunction with the Vertex Weight Mix modifier. For instance, you can use modifiers like normalization and group locking to isolate and nullify specific weights. By normalizing weights, you ensure that all deforming weights assigned to a single vertex sum up to 1, providing a standardized framework for weight management. Group locking, on the other hand, prevents accidental edits or changes to specific vertex groups, allowing you to protect certain weights from being influenced during the merging process.

Multi-Paint Option

Another valuable tool at your disposal is the Multi-Paint option. This feature treats multiple selected bones as a single entity, allowing you to adjust their combined weight while preserving the ratio within the group. By utilizing locking in conjunction with Multi-Paint, you can fine-tune the balance between specific sets of bones without affecting others. This option is particularly useful when dealing with bone loops, such as those found in a mouth or an eye, as it enables you to expose the falloff between the loop and surrounding bones, facilitating precise weight adjustments.

In conclusion, utilizing additional modifiers provides a versatile approach to nullifying weights during bone merging in Blender. By employing the Vertex Weight Mix modifier, normalization, group locking, and the Multi-Paint option, you can effectively manage weights, prevent unwanted influences, and retain the desired weight paint while merging bones. These techniques empower you to create seamless animations and maintain the integrity of your 3D models.

Use normalisation and group locking

Normalisation is a process that ensures that all deforming weights assigned to a single vertex add up to 1. This is often required for functions like deformation. While Blender's Armature modifier automatically normalises weights, it is sometimes preferable to work with normalised weights from the painting stage. This allows the use of certain tools designed for normalised weights and makes it easier to understand the final influence of the current group without needing to know the weights in other groups on the same vertex.

To start working with normalised weights, you must first normalise the existing weights using the "Normalise All" tool. However, it is important to select the right mode and disable "Lock Active" to avoid unexpected mesh movement.

Group locking is a feature in Blender that allows you to lock certain groups of bones. When working with normalised weights, group locking can be used to balance between one set of bones and the rest, excluding a third set that remains unaffected. The Lock Relative option displays unlocked groups as if they were re-normalised with the locked groups deleted, making it appear as if the locked groups did not exist.

By combining normalisation and group locking with the Multi-Paint option, you can treat multiple selected bones as if they were a single bone. This allows you to change the combined weight while preserving the ratio within the group. This technique can be used to merge bones and their weights without affecting a third set of bones.

Use Python scripts to copy/paste weight paint data

Python scripts can be used to copy and paste weight paint data between bones in Blender, allowing for more efficient and automated workflows. Here's a step-by-step guide on how to use Python scripts for this purpose:

Enabling the Corrective Shapekeys Add-on

Before using Python scripts, ensure you have the 'Corrective Shapekeys' add-on enabled. Go to "File > User Preferences > Add-ons > Corrective Shapekeys" and activate it. This add-on provides additional tools for managing weight paint data.

Selecting the Source and Destination Bones

Enter 'Weight Paint Mode' and select the bone with the assigned weight that you want to copy. This is your source bone. Then, Shift-select the bone you want to paste the weight data onto. This is your destination bone.

Copying and Pasting Weight Data

With the source bone selected, click ''Copy Vertex Group' in the Corrective Shapekeys addon menu. This will create a copy of the weight data. Now, select the destination bone, enter 'Edit Mode' (TAB), and press 'A' to deselect everything. Then, Shift-select the vertex you want to copy the weight data to. Open the Properties Panel by pressing 'N' and click the 'paste down arrow' in the Vertex Weights panel. This will paste the copied weight data onto the selected vertex of the destination bone.

Using the Transfer Weights Feature

Another method is to use the 'Transfer Weights' feature in Blender. With your source and destination bones selected, hit 'Transfer Weights' in the Weight Paint Mode. This will open the 'Transfer Mesh Data' tab. Hit 'Transfer Weights' again, and you will be prompted to select the source and destination layers, as well as the mix mode. Ensure that the vertex group names match between the bones.

Scripting for Mirroring Weight Data

If you are looking to mirror weight paint data from one side to the other, there might be a built-in function in Blender for this. However, if you need more control or want to overwrite one side, you can create a Python script. This allows you to mirror the weight painting and also mirror the names of the groups. The scripting approach provides a more customizable solution for mirroring weight data.

Using Python scripts to copy/paste weight paint data offers flexibility and automation when working with bones and weight painting in Blender. It streamlines the process, making it easier to manage complex rigs and ensure consistent weight distribution.

Select both objects and press Ctrl+J to join them

To merge bones in Blender while keeping weight paint, you must first select both objects. To do this, hold Shift and click on both objects. You should see both objects highlighted in orange. Once both objects are selected, press Ctrl+J (or Command + J on Mac) to join them. This will merge the objects into a single shape while retaining the weight paint.

It is important to note that all objects must be of the same type, such as mesh, curve, surface, or armature. Additionally, make sure that you have an active object selected. The active object is the last object you selected and will have a brighter or slightly lighter outline compared to the other selected objects. If you do not have an active object selected, Ctrl+J may not function as intended.

By following these steps, you can successfully merge bones in Blender while retaining the weight paint information.

If you encounter any issues with the Ctrl+J command, you can try troubleshooting by checking your keybindings. In Blender, go to “Input” and enter "join" in the search bar under "Object Mode" to view the keybind for the join function. If the keybind is not set to Ctrl+J, you can restore the default settings by clicking “Restore” and saving your user settings.

Frequently asked questions