How Can I Render a Mesh in OpenGL with Separate Vertex and Normal Indices?

OpenGL Index Buffer Woes

When working with custom file formats containing 3D mesh data, it's not uncommon to encounter the challenge of separate indices for vertices and normals. However, OpenGL requires a single set of indices.

Overcoming the Dilemma

The key to resolving this issue is to create an OpenGL vertex for each unique pair of vertex and normal indices.

Algorithm Elaboration

Assuming you have arrays of vertex coordinates (inVertices) and normals (inNormals), you can utilize a STL map to map each unique vertex-normal pair (key(vertexIdx, normalIdx)) to a combined vertex index (combinedIdx). Here's a simplified pseudo-code outline:

nextCombinedIdx = 0
indexMap = empty
for each triangle in input file:
  for each corner:
    read vertexIdx and normalIdx
    if indexMap.contains(key(vertexIdx, normalIdx)):
      combinedIdx = indexMap.get(key(vertexIdx, normalIdx))
    else:
      combinedIdx = nextCombinedIdx
      indexMap.add(key(vertexIdx, normalIdx), combinedIdx)
      nextCombinedIdx++
      combinedVertices.add(inVertices[vertexIdx], inNormals[normalIdx])
    combinedIndices.add(combinedIdx)

This algorithm constructs a new array of combined vertices (combinedVertices) and a corresponding index list (combinedIndices). Each element of combinedVertices represents a complete vertex-normal pair, allowing you to render using glDrawArrays(GL_TRIANGLES,...).

Although this approach involves some vertex duplication, it effectively solves the problem of incompatible vertex and normal indices while ensuring correct mesh rendering.

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