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Asked: 2025-05-20T10:14:13+05:30

How can I effectively use vertex, texture, and normal indices from an OBJ file without generating multiple EBOs?

I’m working on an OBJ parser that’s mostly finished, but I’ve hit a bit of a roadblock. I can access the vertex, texture, and normal indices from the OBJ file, but I’m trying to figure out how to effectively use all of them without creating multiple Element Buffer Objects (EBOs).

Here’s the situation: I can render the mesh using just the vertex indices, which is great, but I’m scratching my head over how to incorporate the texture and normal indices. My initial thought was to generate separate EBOs for each type of index, but that feels like it could lead to performance issues or just complicate everything too much.

I’m leaning towards the idea of creating a single array that contains all necessary data, but I’m worried that might end up being way more complicated than it’s worth. It seems like I should be able to somehow match up the vertex indices with their corresponding texture and normal indices to produce a unified output for the EBO buffer. But to be honest, I’m getting lost in how I would approach that.

Is there a way to effectively combine the indices or map them out so they can all work together without generating multiple EBOs? I’ve read a bit about shared vertices and how a single vertex might need to use multiple texture and normal indices, but the implementation part is where I’m getting stuck.

If you’ve tackled this kind of problem before or have insights on how I can efficiently utilize all these indices in a streamlined manner, I’d really appreciate any guidance. I’m trying to keep the rendering fast and straightforward, but I don’t want to miss out on using texture and normal mapping effectively. Any tips or strategies would be super helpful! Thanks for reading my ramble. 😊

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    2 Answers

    1. The most straightforward and efficient approach to handling OBJ data when dealing with vertex, texture, and normal indices is to generate a single, unified vertex structure. Essentially, you would iterate through each face in your OBJ file and create unique vertex definitions for each unique combination of vertex/texture/normal indices encountered. To implement this, use a hash-based mapping (e.g., a hashmap or dictionary) where the key is a tuple or combination derived from the specific vertex, texture, and normal indices. Each unique combination becomes a distinct vertex in your final vertex array, complete with positions, textures, and normals. This approach ensures you only need one Element Buffer Object (EBO) referencing these unique combined vertices, greatly simplifying your rendering pipeline and reducing complexity.

      This method addresses the challenge of shared vertices effectively. Even though OBJ files can reference identical vertex positions with different texture or normal coordinates, your hash-based mapping ensures each unique combination gets its own index in the unified array. While this slightly increases memory usage (as positions might repeat), it’s typically negligible in performance impact. Moreover, this simplifies rendering significantly, enabling fast and concise GPU uploads. Modern graphics applications often take this approach precisely for its balance between simplicity and GPU-friendly performance. Thus, you’ll have a streamlined EBO paired with a single consolidated vertex buffer, ultimately keeping your renderer efficient and easy to maintain.

    2. Combining Vertex, Texture, and Normal Indices in an OBJ Parser

      It sounds like you’re on the right track by wanting to avoid multiple EBOs! The idea of combining vertex, texture, and normal indices into a single structure can definitely help streamline your rendering process.

      You could consider creating a single vertex structure that holds all the necessary data for each vertex. For instance, you might set up a structure like this:

          struct Vertex {
        glm::vec3 position;   // Vertex position
        glm::vec2 texCoord;   // Texture coordinate
        glm::vec3 normal;     // Vertex normal
    };

      After defining your vertex structure, you’ll want to create a single array of vertices instead of separate arrays for positions, texture coordinates, and normals.

      When you parse the OBJ file, you’ll read the vertex, texture, and normal indices and then build your vertex array accordingly. Instead of having separate index buffers for vertex positions, texture coordinates, and normals, you can do something like this:

          std::vector vertices;
    for (const auto& face : faces) {
        for (const auto& index : face.indices) {
            Vertex vertex;
            vertex.position = positions[index.vertexIndex];
            vertex.texCoord = texCoords[index.textureIndex];
            vertex.normal = normals[index.normalIndex];
            vertices.push_back(vertex);
        }
    }

      This way, you can create a single VBO (Vertex Buffer Object) from your vertex array and a single EBO that uses the vertex indices. When binding your vertex data with a VAO (Vertex Array Object), just make sure to set the attributes correctly:

          glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, position));
    glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, texCoord));
    glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*)offsetof(Vertex, normal));

      In this way, you are effectively mapping each vertex to its attributes in one unified structure, making it easier to work with and potentially more efficient for rendering.

      Ultimately, this combination approach can simplify the management of your meshes while ensuring that you can still use texture and normal mapping effectively. Good luck with your OBJ parser!