Writing a Custom File Loader for Skeletons

Creating a custom file loader for skeletons involves reading and parsing a file format that stores skeletal data (like bones, joints, animations, etc.), and transforming that data into a usable structure within a 3D engine or application. Below is a step-by-step guide for creating a simple custom file loader for skeletons, assuming you’re working in a game engine or 3D graphics context like Unity or Unreal Engine, or even with a low-level API like OpenGL or Vulkan.

Step 1: Define the Skeleton Structure

First, you need to define the structure of your skeleton. Typically, a skeleton is made up of bones (or joints) which are linked together hierarchically. Each bone has a name, a transformation matrix (for position, rotation, scale), and possibly additional data such as weight or parent-child relationships.

Skeleton Data Structure

Here’s an example of how you might structure your skeleton:

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struct Bone {
    std::string name;          // Name of the bone
    glm::mat4 transformation;  // Transformation matrix (position, rotation, scale)
    int parentIndex;           // Index of the parent bone in the skeleton
};

struct Skeleton {
    std::vector<Bone> bones;   // List of all bones in the skeleton
    // Additional properties, like animations, can go here
};

In this example, we’re using glm::mat4 for a 4×4 matrix that represents a bone’s transformation, and each bone keeps track of its parent in the hierarchy.

Step 2: Design the Custom File Format

The next step is to decide how you want to store the skeleton data. For simplicity, let’s assume we are going to use a custom text-based format such as JSON or a custom binary format. Here’s an example using JSON:

Sample Skeleton JSON Format

json
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{
  "skeleton": {
    "bones": [
      {
        "name": "root",
        "transformation": [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1],
        "parentIndex": -1
      },
      {
        "name": "spine",
        "transformation": [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 5, 0, 1],
        "parentIndex": 0
      },
      {
        "name": "head",
        "transformation": [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 10, 0, 1],
        "parentIndex": 1
      }
    ]
  }
}

In this example, the transformation array is a flat representation of a 4×4 matrix, and parentIndex refers to the bone’s parent in the hierarchy. The root bone has no parent, so its index is -1.

Step 3: Implement the Loader

Now, let’s implement the custom file loader to read this JSON file and convert it into our Skeleton data structure.

Example Code for Loading Skeleton from JSON

Here’s an example of how you might load the skeleton data from a JSON file using C++ and a JSON library (e.g., nlohmann/json):

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#include <iostream>
#include <fstream>
#include <string>
#include <nlohmann/json.hpp>
#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>

using json = nlohmann::json;

struct Bone {
    std::string name;
    glm::mat4 transformation;
    int parentIndex;
};

struct Skeleton {
    std::vector<Bone> bones;
};

Skeleton LoadSkeletonFromFile(const std::string& filename) {
    std::ifstream file(filename);
    json skeletonData;
    file >> skeletonData;

    Skeleton skeleton;

    for (const auto& boneData : skeletonData["skeleton"]["bones"]) {
        Bone bone;
        bone.name = boneData["name"];
        bone.transformation = glm::make_mat4(boneData["transformation"].get<std::vector<float>>().data());
        bone.parentIndex = boneData["parentIndex"];

        skeleton.bones.push_back(bone);
    }

    return skeleton;
}

int main() {
    Skeleton skeleton = LoadSkeletonFromFile("skeleton.json");

    // Print out the skeleton data
    for (const auto& bone : skeleton.bones) {
        std::cout << "Bone: " << bone.name << ", Parent Index: " << bone.parentIndex << std::endl;
    }

    return 0;
}

Explanation:

• Loading the JSON: We load the file using std::ifstream and then parse it using the nlohmann::json library.

• Matrix Parsing: The transformation matrix is a flat list of floats in the JSON file, which we convert to a glm::mat4 matrix using glm::make_mat4.

• Bone Hierarchy: The parentIndex in each bone connects the bone to its parent in the hierarchy.

Step 4: Use the Loaded Skeleton Data

After loading the skeleton data, you can manipulate the bones or apply transformations. For example, you might want to calculate the world space transformations for all bones based on the hierarchy:

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void ComputeWorldTransforms(Skeleton& skeleton) {
    for (size_t i = 0; i < skeleton.bones.size(); ++i) {
        if (skeleton.bones[i].parentIndex == -1) {
            // The root bone has no parent, so its world transform is just its own transformation
            skeleton.bones[i].worldTransform = skeleton.bones[i].transformation;
        } else {
            // Otherwise, combine the parent’s world transform with this bone's local transformation
            skeleton.bones[i].worldTransform = skeleton.bones[skeleton.bones[i].parentIndex].worldTransform * skeleton.bones[i].transformation;
        }
    }
}

In this function, you iterate over the bones and compute their world transformations. The root bone’s transformation is used directly, while child bones have their parent’s world transformation multiplied by their own local transformation.

Step 5: Handle Animations (Optional)

If your skeleton also includes animations (keyframes, bone transformations over time), you’d likely need to store animation data separately, but you can extend your loader to include animation data from the same file or a separate file.

For example, an animation might look like this:

json
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{
  "animations": [
    {
      "name": "walk",
      "keyframes": [
        {
          "time": 0.0,
          "boneTransformations": {
            "root": [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1],
            "spine": [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 5, 0, 1]
          }
        },
        {
          "time": 0.5,
          "boneTransformations": {
            "root": [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 1],
            "spine": [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 6, 0, 1]
          }
        }
      ]
    }
  ]
}

This would involve more logic for interpolating between keyframes and applying them to the bones at each time step.

Step 6: Optimize (Optional)

For performance, especially in games or real-time applications, you might want to consider loading your skeleton in a binary format for faster reading. You can also implement data compression techniques, caching, or multithreading for loading large datasets.

Conclusion

This guide provides a basic approach to creating a custom file loader for skeletons. By storing bone data and hierarchical relationships, you can import and manipulate 3D skeletons in your own custom format. If animations are required, you can extend the loader to support keyframe-based transformations.