mikktspace.h

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// ----------------------------------------------------------------------------
// -                        Open3D: www.open3d.org                            -
// ----------------------------------------------------------------------------
// Copyright (c) 2018-2024 www.open3d.org
// SPDX-License-Identifier: MIT
// ----------------------------------------------------------------------------
 
/* # MikkTSpace
A common standard for tangent space used in baking tools to produce normal maps.
More information can be found at http://www.mikktspace.com/.
https://github.com/mmikk/MikkTSpace  [Oct 2025]
*/
#ifndef __MIKKTSPACE_H__
#define __MIKKTSPACE_H__
 
#ifdef __cplusplus
extern "C" {
#endif
 
/* Author: Morten S. Mikkelsen
 * Version: 1.0
 *
 * The files mikktspace.h and mikktspace.c are designed to be
 * stand-alone files and it is important that they are kept this way.
 * Not having dependencies on structures/classes/libraries specific
 * to the program, in which they are used, allows them to be copied
 * and used as is into any tool, program or plugin.
 * The code is designed to consistently generate the same
 * tangent spaces, for a given mesh, in any tool in which it is used.
 * This is done by performing an internal welding step and subsequently an
 * order-independent evaluation of tangent space for meshes consisting of
 * triangles and quads. This means faces can be received in any order and the
 * same is true for the order of vertices of each face. The generated result
 * will not be affected by such reordering. Additionally, whether degenerate
 * (vertices or texture coordinates) primitives are present or not will not
 * affect the generated results either. Once tangent space calculation is done
 * the vertices of degenerate primitives will simply inherit tangent space from
 * neighboring non degenerate primitives. The analysis behind this
 * implementation can be found in my master's thesis which is available for
 * download --> http://image.diku.dk/projects/media/morten.mikkelsen.08.pdf Note
 * that though the tangent spaces at the vertices are generated in an
 * order-independent way, by this implementation, the interpolated tangent space
 * is still affected by which diagonal is chosen to split each quad. A sensible
 * solution is to have your tools pipeline always split quads by the shortest
 * diagonal. This choice is order-independent and works with mirroring. If these
 * have the same length then compare the diagonals defined by the texture
 * coordinates. XNormal which is a tool for baking normal maps allows you to
 * write your own tangent space plugin and also quad triangulator plugin.
 */
 
typedef int tbool;
typedef struct SMikkTSpaceContext SMikkTSpaceContext;
 
typedef struct {
    // Returns the number of faces (triangles/quads) on the mesh to be
    // processed.
    int (*m_getNumFaces)(const SMikkTSpaceContext* pContext);
 
    // Returns the number of vertices on face number iFace
    // iFace is a number in the range {0, 1, ..., getNumFaces()-1}
    int (*m_getNumVerticesOfFace)(const SMikkTSpaceContext* pContext,
                                  const int iFace);
 
    // returns the position/normal/texcoord of the referenced face of vertex
    // number iVert. iVert is in the range {0,1,2} for triangles and {0,1,2,3}
    // for quads.
    void (*m_getPosition)(const SMikkTSpaceContext* pContext,
                          float fvPosOut[],
                          const int iFace,
                          const int iVert);
    void (*m_getNormal)(const SMikkTSpaceContext* pContext,
                        float fvNormOut[],
                        const int iFace,
                        const int iVert);
    void (*m_getTexCoord)(const SMikkTSpaceContext* pContext,
                          float fvTexcOut[],
                          const int iFace,
                          const int iVert);
 
    // either (or both) of the two setTSpace callbacks can be set.
    // The call-back m_setTSpaceBasic() is sufficient for basic normal mapping.
 
    // This function is used to return the tangent and fSign to the application.
    // fvTangent is a unit length vector.
    // For normal maps it is sufficient to use the following simplified version
    // of the bitangent which is generated at pixel/vertex level. bitangent =
    // fSign * cross(vN, tangent); Note that the results are returned unindexed.
    // It is possible to generate a new index list But averaging/overwriting
    // tangent spaces by using an already existing index list WILL produce
    // INCRORRECT results. DO NOT! use an already existing index list.
    void (*m_setTSpaceBasic)(const SMikkTSpaceContext* pContext,
                             const float fvTangent[],
                             const float fSign,
                             const int iFace,
                             const int iVert);
 
    // This function is used to return tangent space results to the application.
    // fvTangent and fvBiTangent are unit length vectors and fMagS and fMagT are
    // their true magnitudes which can be used for relief mapping effects.
    // fvBiTangent is the "real" bitangent and thus may not be perpendicular to
    // fvTangent. However, both are perpendicular to the vertex normal. For
    // normal maps it is sufficient to use the following simplified version of
    // the bitangent which is generated at pixel/vertex level. fSign =
    // bIsOrientationPreserving ? 1.0f : (-1.0f); bitangent = fSign * cross(vN,
    // tangent); Note that the results are returned unindexed. It is possible to
    // generate a new index list But averaging/overwriting tangent spaces by
    // using an already existing index list WILL produce INCRORRECT results. DO
    // NOT! use an already existing index list.
    void (*m_setTSpace)(const SMikkTSpaceContext* pContext,
                        const float fvTangent[],
                        const float fvBiTangent[],
                        const float fMagS,
                        const float fMagT,
                        const tbool bIsOrientationPreserving,
                        const int iFace,
                        const int iVert);
} SMikkTSpaceInterface;
 
struct SMikkTSpaceContext {
    SMikkTSpaceInterface* m_pInterface;  // initialized with callback functions
    void* m_pUserData;  // pointer to client side mesh data etc. (passed as the
                        // first parameter with every interface call)
};
 
// these are both thread safe!
tbool genTangSpaceDefault(
        const SMikkTSpaceContext*
                pContext);  // Default (recommended) fAngularThreshold is 180
                            // degrees (which means threshold disabled)
tbool genTangSpace(const SMikkTSpaceContext* pContext,
                   const float fAngularThreshold);
 
// To avoid visual errors (distortions/unwanted hard edges in lighting), when
// using sampled normal maps, the normal map sampler must use the exact inverse
// of the pixel shader transformation. The most efficient transformation we can
// possibly do in the pixel shader is achieved by using, directly, the
// "unnormalized" interpolated tangent, bitangent and vertex normal: vT, vB and
// vN. pixel shader (fast transform out) vNout = normalize( vNt.x * vT + vNt.y *
// vB + vNt.z * vN ); where vNt is the tangent space normal. The normal map
// sampler must likewise use the interpolated and "unnormalized" tangent,
// bitangent and vertex normal to be compliant with the pixel shader. sampler
// does (exact inverse of pixel shader): float3 row0 = cross(vB, vN); float3
// row1 = cross(vN, vT); float3 row2 = cross(vT, vB); float fSign = dot(vT,
// row0)<0 ? -1 : 1; vNt = normalize( fSign * float3(dot(vNout,row0),
// dot(vNout,row1), dot(vNout,row2)) ); where vNout is the sampled normal in
// some chosen 3D space.
//
// Should you choose to reconstruct the bitangent in the pixel shader instead
// of the vertex shader, as explained earlier, then be sure to do this in the
// normal map sampler also. Finally, beware of quad triangulations. If the
// normal map sampler doesn't use the same triangulation of quads as your
// renderer then problems will occur since the interpolated tangent spaces will
// differ eventhough the vertex level tangent spaces match. This can be solved
// either by triangulating before sampling/exporting or by using the
// order-independent choice of diagonal for splitting quads suggested earlier.
// However, this must be used both by the sampler and your tools/rendering
// pipeline.
 
#ifdef __cplusplus
}
#endif
 
#endif