Creating morph target systems in C++

Creating a morph target system in C++ involves setting up a framework that allows you to blend between different versions (targets) of a 3D model’s mesh to achieve realistic animations or deformations. This is typically used in character animation for facial expressions, muscle movements, or other subtle transformations.

Here’s a basic outline for creating a morph target system in C++:

1. Understanding Morph Targets

Morph targets (also known as blend shapes) are different versions of a mesh where vertices are displaced to represent different expressions or deformations. When blending between these targets, each vertex moves towards its target position by a certain weight factor.

2. Data Structure Setup

You will need to set up a structure to store the original mesh and its associated morph targets.

• Base Mesh: The original static mesh.

• Morph Targets: Additional meshes that represent different states or deformations.

• Weights: Control the interpolation between the base mesh and each morph target.

Define a Vertex Structure

cpp
CopyEdit
struct Vertex {
    glm::vec3 position; // 3D position of the vertex
    glm::vec3 normal;   // Normal vector for lighting
    glm::vec2 uv;       // UV texture coordinates
};

Define a Mesh Structure

cpp
CopyEdit
struct Mesh {
    std::vector<Vertex> vertices;
    std::vector<unsigned int> indices; // Index list for rendering

    // Optionally, you could include normals, tangents, and other attributes
};

Define a Morph Target System

cpp
CopyEdit
class MorphTargetSystem {
public:
    Mesh baseMesh; // Original mesh
    std::vector<Mesh> morphTargets; // Morph target meshes
    std::vector<float> weights; // Weights for blending morph targets

    MorphTargetSystem(Mesh base) : baseMesh(base) {}

    void AddMorphTarget(Mesh target) {
        morphTargets.push_back(target);
        weights.push_back(0.0f); // Default weight is 0 (no influence)
    }

    void SetWeight(int targetIndex, float weight) {
        if (targetIndex >= 0 && targetIndex < weights.size()) {
            weights[targetIndex] = std::clamp(weight, 0.0f, 1.0f);
        }
    }

    Mesh GetBlendedMesh() {
        Mesh blendedMesh = baseMesh; // Start with the base mesh
        for (size_t i = 0; i < morphTargets.size(); ++i) {
            BlendMeshes(blendedMesh, morphTargets[i], weights[i]);
        }
        return blendedMesh;
    }

private:
    void BlendMeshes(Mesh& targetMesh, const Mesh& morphTarget, float weight) {
        for (size_t i = 0; i < targetMesh.vertices.size(); ++i) {
            targetMesh.vertices[i].position = glm::mix(targetMesh.vertices[i].position, morphTarget.vertices[i].position, weight);
            // Optionally, blend normals and other attributes here
        }
    }
};

3. Blending Logic

The core of the morph target system is blending. For each vertex in the base mesh, you will interpolate its position between the base and the corresponding vertex in the morph target. This interpolation is controlled by the weight, typically between 0.0 (no influence from the morph target) and 1.0 (full influence from the morph target).

The glm::mix function (from GLM, the OpenGL Mathematics library) is used here to perform a linear interpolation between two vectors.

cpp
CopyEdit
glm::vec3 blendedPosition = glm::mix(basePosition, targetPosition, weight);

4. Performance Considerations

• Vertex Count: Blending between morph targets involves manipulating vertex positions for all vertices, so the number of vertices can affect performance, especially for high-resolution meshes.

• Vertex Caching: If you’re doing multiple morph target blends, caching the resulting mesh might be more efficient than recalculating every time.

• Optimization: Consider using GPU-based approaches like shaders for real-time morph target blending in graphics applications.

5. Implementing Rendering

Once you’ve blended your morph targets, the resulting mesh needs to be rendered. If you’re using a graphics API like OpenGL or DirectX, you’ll send the blended vertices to the GPU for rendering.

Simple Rendering Code (Example with OpenGL)

cpp
CopyEdit
void RenderBlendedMesh(Mesh& blendedMesh) {
    glBindVertexArray(vertexArrayObject); // Bind VAO

    // Upload vertex data to GPU (you'd usually have buffers for position, normal, etc.)
    glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObject);
    glBufferData(GL_ARRAY_BUFFER, blendedMesh.vertices.size() * sizeof(Vertex), blendedMesh.vertices.data(), GL_DYNAMIC_DRAW);

    // Draw the mesh
    glDrawElements(GL_TRIANGLES, blendedMesh.indices.size(), GL_UNSIGNED_INT, 0);
}

6. Handling Multiple Morph Targets

You can blend multiple morph targets together by using a weighted average. For example, if you want to blend three morph targets (A, B, and C) with weights of 0.5, 0.3, and 0.2 respectively, you can modify the blending function:

cpp
CopyEdit
void BlendMultipleTargets(Mesh& targetMesh, const std::vector<Mesh>& morphTargets, const std::vector<float>& weights) {
    for (size_t i = 0; i < targetMesh.vertices.size(); ++i) {
        glm::vec3 blendedPosition = targetMesh.vertices[i].position;

        for (size_t j = 0; j < morphTargets.size(); ++j) {
            blendedPosition = glm::mix(blendedPosition, morphTargets[j].vertices[i].position, weights[j]);
        }

        targetMesh.vertices[i].position = blendedPosition;
    }
}

7. User Interface for Controlling Weights

You may want to expose the ability to control the weight of each morph target to the user, such as through a GUI slider or during an animation sequence.

cpp
CopyEdit
// Example code to adjust weight based on user input:
float userWeightInput = GetUserInput(); // Get input from the user (e.g., slider value)
morphTargetSystem.SetWeight(0, userWeightInput); // Update weight for the first target

8. Animation Integration

If you want to animate morph targets (e.g., facial expressions changing over time), you can interpolate the weights over time:

cpp
CopyEdit
void AnimateMorphTargets(float deltaTime) {
    for (size_t i = 0; i < morphTargetSystem.weights.size(); ++i) {
        morphTargetSystem.weights[i] = std::sin(deltaTime + i); // Simple animation example
    }
}

Conclusion

Creating a morph target system in C++ involves setting up a flexible system for storing mesh data, defining the logic for blending between targets, and integrating with a rendering system. It can be enhanced with various optimizations like GPU-based processing and real-time animations.

This is just a starting point; more features could be added such as more efficient blending algorithms, additional vertex attributes (e.g., normals, tangents), and integration with animation systems.