文章目录

  • OBJ格式解析
  • 一、OBJ格式
  • 二、vtkOBJReader源码解析
    • 1.主要部分
  • 总结

OBJ格式解析

通过解读vtk源码中对OBJ文件的读取代码，讲解如何获取obj模型的顶点(vertex)信息，法线信息，纹理信息以及点(Point)、线、面等主要元素的提取。

一、OBJ格式

1.OBJ文件示例

# 一些注释mtllib cube.mtl
g default
v -0.500000 -0.500000 0.500000
v 0.500000 -0.500000 0.500000
v -0.500000 0.500000 0.500000
v 0.500000 0.500000 0.500000
v -0.500000 0.500000 -0.500000
v 0.500000 0.500000 -0.500000
v -0.500000 -0.500000 -0.500000
v 0.500000 -0.500000 -0.500000
vt 0.001992 0.001992
vt 0.998008 0.001992
vt 0.001992 0.998008
vt 0.998008 0.998008
vt 0.001992 0.001992
vt 0.998008 0.001992
vt 0.001992 0.998008
vt 0.998008 0.998008
vt 0.001992 0.001992
vt 0.998008 0.001992
vt 0.001992 0.998008
vt 0.998008 0.998008
vt 0.001992 0.001992
vt 0.998008 0.001992
vt 0.001992 0.998008
vt 0.998008 0.998008
vt 0.001992 0.001992
vt 0.998008 0.001992
vt 0.001992 0.998008
vt 0.998008 0.998008
vt 0.998008 0.998008
vt 0.001992 0.998008
vt 0.998008 0.001992
vt 0.001992 0.001992
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vn 0.000000 1.000000 0.000000
vn 0.000000 1.000000 0.000000
vn 0.000000 1.000000 0.000000
vn 0.000000 1.000000 0.000000
vn 0.000000 0.000000 -1.000000
vn 0.000000 0.000000 -1.000000
vn 0.000000 0.000000 -1.000000
vn 0.000000 0.000000 -1.000000
vn 0.000000 -1.000000 0.000000
vn 0.000000 -1.000000 0.000000
vn 0.000000 -1.000000 0.000000
vn 0.000000 -1.000000 0.000000
vn 1.000000 0.000000 0.000000
vn 1.000000 0.000000 0.000000
vn 1.000000 0.000000 0.000000
vn 1.000000 0.000000 0.000000
vn -1.000000 0.000000 0.000000
vn -1.000000 0.000000 0.000000
vn -1.000000 0.000000 0.000000
vn -1.000000 0.000000 0.000000
s 1
g pCube1
usemtl file1SG
f 1/1/1 2/2/2 3/3/3
f 3/3/3 2/2/2 4/4/4
s 2
f 3/13/5 4/14/6 5/15/7
f 5/15/7 4/14/6 6/16/8
s 3
f 5/21/9 6/22/10 7/23/11
f 7/23/11 6/22/10 8/24/12
s 4
f 7/17/13 8/18/14 1/19/15
f 1/19/15 8/18/14 2/20/16
s 5
f 2/5/17 8/6/18 4/7/19
f 4/7/19 8/6/18 6/8/20
s 6
f 7/9/21 1/10/22 5/11/23
f 5/11/23 1/10/22 3/12/24

2.格式分析

  • 注释以#开头；
  • v表示模型的顶点坐标，表示为: v x y z
  • vn：法线坐标,表示为:vn x y z
  • vt：纹理坐标，一般每个坐标包含两个值,表示为:vt u v w
  • p：Point元素；
  • l：线
  • f：面，可以有多个顶点表示;
    • f v1 v2 v3 … ,仅由三个以上顶点索引组成；
    • f v1/vt1 v2/vt2 v3/vt3 …,由顶点和纹理索引组成；
    • f v1//vn1 v2//vn2 v3//vn3 …,由顶点和法线索引组成；
    • f v1/vt1/vn1 v2/vt2/vn2 v3/vt3/vn3 …,由顶点,纹理和法线索引组成；
      索引的形成是按照顶点、法线，纹理信息在文件中的先后顺序从1开始编号，可以采用负索引，表示倒数第几个顶点，法线，纹理，一般由三个顶点组成一个面，也有多个顶点的情况。

其它更多信息请查看补充部分。

补充：
http://netghost.narod.ru/gff/graphics/summary/waveobj.htm#WAVEOBJ-DMYID.3
https://blog.csdn.net/qq_26900671/article/details/81739977
https://www.cnblogs.com/daofaziran/p/11540517.html

二、vtkOBJReader源码解析

1.主要部分

/*---------------------------------------------------------------------------*\This is only partial support for the OBJ format, which is quite complicated.
To find a full specification, search the net for "OBJ format", eg.:http://en.wikipedia.org/wiki/Objhttp://netghost.narod.ru/gff/graphics/summary/waveobj.htmWe support the following types:v <x> <y> <z>vertexvn <x> <y> <z>vertex normalvt <x> <y>texture coordinatef <v_a> <v_b> <v_c> ...polygonal face linking vertices v_a, v_b, v_c, etc. whichare 1-based indices into the vertex listf <v_a>/<t_a> <v_b>/<t_b> ...polygonal face as above, but with texture coordinates foreach vertex. t_a etc. are 1-based indices into the texturecoordinates list (from the vt lines)f <v_a>/<t_a>/<n_a> <v_b>/<t_b>/<n_b> ...polygonal face as above, with a normal at each vertex, as a1-based index into the normals list (from the vn lines)f <v_a>//<n_a> <v_b>//<n_b> ...polygonal face as above but without texture coordinates.Per-face tcoords and normals are supported by duplicatingthe vertices on each face as necessary.l <v_a> <v_b> ...lines linking vertices v_a, v_b, etc. which are 1-basedindices into the vertex listp <v_a> <v_b> ...points located at the vertices v_a, v_b, etc. which are 1-basedindices into the vertex list\*---------------------------------------------------------------------------*///部分代码float xyz[3];//按行来解析数据...// in the OBJ format the first characters determine how to interpret the line://为定点信息if (strcmp(cmd, "v") == 0){// this is a vertex definition, expect three floats, separated by whitespace://获取坐标值if (sscanf(pLine, "%f %f %f", xyz, xyz + 1, xyz + 2) == 3){points->InsertNextPoint(xyz);}else{vtkErrorMacro(<< "Error reading 'v' at line " << lineNr);everything_ok = false;}}//纹理坐标else if (strcmp(cmd, "vt") == 0){// this is a tcoord, expect two floats, separated by whitespace:// 获取坐标if (sscanf(pLine, "%f %f", xyz, xyz + 1) == 2){tcoords->InsertNextTuple(xyz);}else{vtkErrorMacro(<< "Error reading 'vt' at line " << lineNr);everything_ok = false;}}//法线坐标else if (strcmp(cmd, "vn") == 0){// this is a normal, expect three floats, separated by whitespace://获取坐标if (sscanf(pLine, "%f %f %f", xyz, xyz + 1, xyz + 2) == 3){normals->InsertNextTuple(xyz);hasNormals = true;}else{vtkErrorMacro(<< "Error reading 'vn' at line " << lineNr);everything_ok = false;}}//点元素else if (strcmp(cmd, "p") == 0){// this is a point definition, consisting of 1-based indices separated by whitespace and /pointElems->InsertNextCell(0); // we don't yet know how many points are to comeint nVerts = 0; // keep a count of how many there arewhile (everything_ok && pLine < pEnd){// find next non-whitespace characterwhile (isspace(*pLine) && pLine < pEnd) { pLine++; }if (pLine < pEnd)         // there is still data left on this line{int iVert;if (sscanf(pLine, "%d", &iVert) == 1){pointElems->InsertCellPoint(iVert - 1);nVerts++;}else if (strcmp(pLine, "\\\n") == 0){// handle backslash-newline continuationif (fgets(rawLine, MAX_LINE, in) != NULL){lineNr++;pLine = rawLine;pEnd = rawLine + strlen(rawLine);continue;}else{vtkErrorMacro(<< "Error reading continuation line at line " << lineNr);everything_ok = false;}}else{vtkErrorMacro(<< "Error reading 'p' at line " << lineNr);everything_ok = false;}// skip over what we just sscanf'd// (find the first whitespace character)while (!isspace(*pLine) && pLine < pEnd) { pLine++; }}}if (nVerts < 1){vtkErrorMacro(<< "Error reading file near line " << lineNr<< " while processing the 'p' command");everything_ok = false;}// now we know how many points there were in this cellpointElems->UpdateCellCount(nVerts);}//线元素else if (strcmp(cmd, "l") == 0){// this is a line definition, consisting of 1-based indices separated by whitespace and /lineElems->InsertNextCell(0); // we don't yet know how many points are to comeint nVerts = 0; // keep a count of how many there arewhile (everything_ok && pLine < pEnd){// find next non-whitespace characterwhile (isspace(*pLine) && pLine < pEnd) { pLine++; }if (pLine < pEnd)         // there is still data left on this line{int iVert, dummyInt;if (sscanf(pLine, "%d/%d", &iVert, &dummyInt) == 2){// we simply ignore texture informationlineElems->InsertCellPoint(iVert - 1);nVerts++;}else if (sscanf(pLine, "%d", &iVert) == 1){lineElems->InsertCellPoint(iVert - 1);nVerts++;}else if (strcmp(pLine, "\\\n") == 0){// handle backslash-newline continuationif (fgets(rawLine, MAX_LINE, in) != NULL){lineNr++;pLine = rawLine;pEnd = rawLine + strlen(rawLine);continue;}else{vtkErrorMacro(<< "Error reading continuation line at line " << lineNr);everything_ok = false;}}else{vtkErrorMacro(<< "Error reading 'l' at line " << lineNr);everything_ok = false;}// skip over what we just sscanf'd// (find the first whitespace character)while (!isspace(*pLine) && pLine < pEnd) { pLine++; }}}if (nVerts < 2){vtkErrorMacro(<< "Error reading file near line " << lineNr<< " while processing the 'l' command");everything_ok = false;}// now we know how many points there were in this celllineElems->UpdateCellCount(nVerts);}//面元素else if (strcmp(cmd, "f") == 0){// this is a face definition, consisting of 1-based indices separated by whitespace and /polys->InsertNextCell(0); // we don't yet know how many points are to cometcoord_polys->InsertNextCell(0);normal_polys->InsertNextCell(0);int nVerts = 0, nTCoords = 0, nNormals = 0; // keep a count of how many of each there arewhile (everything_ok && pLine < pEnd){// find the first non-whitespace characterwhile (isspace(*pLine) && pLine < pEnd) { pLine++; }if (pLine < pEnd)         // there is still data left on this line{int iVert, iTCoord, iNormal;//1.获取顶点、纹理、法线索引if (sscanf(pLine, "%d/%d/%d", &iVert, &iTCoord, &iNormal) == 3){polys->InsertCellPoint(iVert - 1); // convert to 0-based indexnVerts++;tcoord_polys->InsertCellPoint(iTCoord - 1);nTCoords++;normal_polys->InsertCellPoint(iNormal - 1);nNormals++;if (iTCoord != iVert)tcoords_same_as_verts = false;if (iNormal != iVert)normals_same_as_verts = false;}//2.获取顶点、法线索引else if (sscanf(pLine, "%d//%d", &iVert, &iNormal) == 2){polys->InsertCellPoint(iVert - 1);nVerts++;normal_polys->InsertCellPoint(iNormal - 1);nNormals++;if (iNormal != iVert)normals_same_as_verts = false;}//3.获取顶点、纹理索引else if (sscanf(pLine, "%d/%d", &iVert, &iTCoord) == 2){polys->InsertCellPoint(iVert - 1);nVerts++;tcoord_polys->InsertCellPoint(iTCoord - 1);nTCoords++;if (iTCoord != iVert)tcoords_same_as_verts = false;}//4.获取顶点索引else if (sscanf(pLine, "%d", &iVert) == 1){polys->InsertCellPoint(iVert - 1);nVerts++;}else if (strcmp(pLine, "\\\n") == 0){// handle backslash-newline continuationif (fgets(rawLine, MAX_LINE, in) != NULL){lineNr++;pLine = rawLine;pEnd = rawLine + strlen(rawLine);continue;}else{vtkErrorMacro(<< "Error reading continuation line at line " << lineNr);everything_ok = false;}}else{vtkErrorMacro(<< "Error reading 'f' at line " << lineNr);everything_ok = false;}// skip over what we just read// (find the first whitespace character)while (!isspace(*pLine) && pLine < pEnd) { pLine++; }}}// count of tcoords and normals must be equal to number of vertices or zeroif (nVerts < 3 ||(nTCoords > 0 && nTCoords != nVerts) ||(nNormals > 0 && nNormals != nVerts)){vtkErrorMacro(<< "Error reading file near line " << lineNr<< " while processing the 'f' command");everything_ok = false;}// now we know how many points there were in this cellpolys->UpdateCellCount(nVerts);tcoord_polys->UpdateCellCount(nTCoords);normal_polys->UpdateCellCount(nNormals);// also make a note of whether any cells have tcoords, and whether any have normalsif (nTCoords > 0) { hasTCoords = true; }if (nNormals > 0) { hasNormals = true; }}else{//vtkDebugMacro(<<"Ignoring line: "<<rawLine);}

总结

熟悉vtk框架，将obj转换为其他模型文件以及提取有效信息。

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