iphone - 球体上的iOS OpenGL ES 2.0广告牌对象并随着球体旋转

我在iOS的OpenGL ES 2.0中有一个球体(地球)。我也有一些标记，希望将其放置在地球上的经度/纬度上，但我希望标记始终面对用户(广告牌)，但当地球通过触摸旋转时也随地球一起移动。因此，我尝试研究广告牌，并将以下代码放在一起。该代码来自我创建地球后调用的公告牌函数(在Z轴上向后平移6个单位)。我似乎无法使广告牌的平面始终面向相机，但随着地球旋转也随之移动。我怎样才能做到这一点？

    // Get the current modelview matrix
    GLKMatrix4 originalMat = self.effect.transform.modelviewMatrix;
    GLKMatrix4 currMat = self.effect.transform.modelviewMatrix;

    // Define the buffer designators
    GLuint billboardVertexArray;
    GLuint billboardVertexBuffer;
    GLuint billboardIndexBuffer;

    glGenVertexArraysOES(1, &billboardVertexArray);
    glBindVertexArrayOES(billboardVertexArray);

    // Now draw the billboard
    glGenBuffers(1, &billboardVertexBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, billboardVertexBuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Billboard_vertex), Billboard_vertex, GL_STATIC_DRAW);

    glGenBuffers(1, &billboardIndexBuffer);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, billboardIndexBuffer);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Billboard_index), Billboard_index, GL_STATIC_DRAW);

    // u0,v0,normalx0,normaly0,normalz0,x0,y0,z0
    glEnableVertexAttribArray(GLKVertexAttribPosition);
    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 8*sizeof(float), BUFFER_OFFSET(5));
    glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
    glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 8*sizeof(float), BUFFER_OFFSET(0));
    glEnableVertexAttribArray(GLKVertexAttribNormal);
    glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE, 8*sizeof(float), BUFFER_OFFSET(2));

    // Enable the Earth texture
    self.effect.texture2d0.name = _billBoardTextureInfo.name;
    self.effect.texture2d0.target = _billBoardTextureInfo.target;

    // Bind the earth vertex array
    glBindVertexArrayOES(billboardVertexArray);

    // Now put a billboard at a specific Lat Lon - so first
    // calculate XYZ from lat lon
    XYZ xyz;
    xyz.x = 0; xyz.y = 0; xyz.z = 0;
    [self LLAtoXYZwithLat:0 andLon:0 andXYZ:&xyz];
    //NSLog(@"XYZ after: %f %f %f",xyz.x,xyz.y,xyz.z);

    // Move the billboard back so we can see it
    GLKMatrix4 modelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, -6.0f);

    // In update, we convert the quaternion into a rotation matrix, and apply it to the model view matrix as usual.
    GLKMatrix4 rotation = GLKMatrix4MakeWithQuaternion(_quat);
    modelViewMatrix = GLKMatrix4Multiply(modelViewMatrix, rotation);

    // First create the variation translations to anchor the billboard
    GLKMatrix4 translationXYZ = GLKMatrix4MakeTranslation(xyz.x, xyz.y, xyz.z);
    GLKMatrix4 translationForLatLonWithTranslation = GLKMatrix4Multiply(modelViewMatrix,translationXYZ);

    // Scale this object as well
    GLKMatrix4 scaledWithTranslationAndRotation = GLKMatrix4Scale(translationForLatLonWithTranslation, scale, scale, scale);

    // Remove the Translation portion of the matrix
    // | xx xy xz xw |
    // | yx yy yz yw |
    // | zx zy zz zw |
    // | wx wy wz ww |
    //
    // | R       T |
    // | (0,0,0) 1 |
    //
    // d = sqrt( xx² + yx² + zx² )
    //
    // | d 0 0 T.x |
    // | 0 d 0 T.y |
    // | 0 0 d T.z |
    // | 0 0 0   1 |
    //
    // union _GLKMatrix4
    // {
    //     struct
    //     {
    //         float m00, m01, m02, m03;
    //         float m10, m11, m12, m13;
    //         float m20, m21, m22, m23;
    //         float m30, m31, m32, m33;
    //     };
    //     float m[16];
    // }
    // typedef union _GLKMatrix4 GLKMatrix4;

    // Construct the rows in the new matrix
    float d = sqrt( pow(currMat.m00,2) + pow(currMat.m10,2) + pow(currMat.m20,2) );
    GLKVector4 columnToInsert0 = GLKVector4Make(d, 0, 0, currMat.m03+xyz.x);
    GLKVector4 columnToInsert1 = GLKVector4Make(0, d, 0, currMat.m13+xyz.y);
    //GLKVector4 columnToInsert2 = GLKVector4Make(0, 0, d, currMat.m23-6+xyz.z);
    GLKVector4 columnToInsert3 = GLKVector4Make(0, 0, 0, 1);

    // Build the new Matrix
    GLKMatrix4 noTranslationInfo = GLKMatrix4SetRow(currMat, 0, columnToInsert0);
    noTranslationInfo = GLKMatrix4SetRow(noTranslationInfo, 1, columnToInsert1);
    //noTranslationInfo = GLKMatrix4SetRow(noTranslationInfo, 2, columnToInsert2);
    noTranslationInfo = GLKMatrix4SetRow(noTranslationInfo, 3, columnToInsert3);

    [self printMatrix:noTranslationInfo];

    // Assign the new matrix to draw with - no translation
    self.effect.transform.modelviewMatrix = noTranslationInfo;

    // Render the object with GLKit
    [self.effect prepareToDraw];

    // Draw elements from the index array and uv / vertex / normal info
    glDrawElements(GL_TRIANGLES,Billboard_polygoncount*3,GL_UNSIGNED_SHORT,0);

    // Restore the original matrix
    self.effect.transform.modelviewMatrix = originalMat;

最佳答案

这似乎工作得很好:

// Get the current modelview matrix
GLKMatrix4 originalMat = self.effect.transform.modelviewMatrix;
GLKMatrix4 currMat = self.effect.transform.modelviewMatrix;

// Print the original matrix for comparison
//NSLog(@"Original Matrix:");
//[self printMatrix:currMat];

// Define the buffer designators
GLuint billboardVertexArray;
GLuint billboardVertexBuffer;
GLuint billboardIndexBuffer;

glGenVertexArraysOES(1, &billboardVertexArray);
glBindVertexArrayOES(billboardVertexArray);

// Now draw the billboard
glGenBuffers(1, &billboardVertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, billboardVertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Billboard_vertex), Billboard_vertex, GL_STATIC_DRAW);

glGenBuffers(1, &billboardIndexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, billboardIndexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Billboard_index), Billboard_index, GL_STATIC_DRAW);

// u0,v0,normalx0,normaly0,normalz0,x0,y0,z0
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 8*sizeof(float), BUFFER_OFFSET(5));
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 8*sizeof(float), BUFFER_OFFSET(0));
glEnableVertexAttribArray(GLKVertexAttribNormal);
glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE, 8*sizeof(float), BUFFER_OFFSET(2));

// Enable the Earth texture
self.effect.texture2d0.name = _billBoardTextureInfo.name;
self.effect.texture2d0.target = _billBoardTextureInfo.target;

// Bind the earth vertex array
glBindVertexArrayOES(billboardVertexArray);

// Now put a billboard at a specific Lat Lon - so first
// calculate XYZ from lat lon
XYZ xyz;
xyz.x = 0; xyz.y = 0; xyz.z = 0;
[self LLAtoXYZwithLat:35 andLon:-97 andXYZ:&xyz];
//NSLog(@"XYZ after: %f %f %f",xyz.x,xyz.y,xyz.z);

// Scale this object as well
//GLKMatrix4 scaledWithTranslationAndRotation = GLKMatrix4Scale(translationForLatLonWithTranslation, scale, scale, scale);

// Remove the Translation portion of the matrix
// | xx xy xz xw |
// | yx yy yz yw |
// | zx zy zz zw |
// | wx wy wz ww |
//
// | R       T |
// | (0,0,0) 1 |
//
// d = sqrt( xx² + yx² + zx² )
//
// | d 0 0 T.x |
// | 0 d 0 T.y |
// | 0 0 d T.z |
// | 0 0 0   1 |
//
// union _GLKMatrix4
// {
//     struct
//     {
//         float m00, m01, m02, m03;
//         float m10, m11, m12, m13;
//         float m20, m21, m22, m23;
//         float m30, m31, m32, m33;
//     };
//     float m[16];
// }
// typedef union _GLKMatrix4 GLKMatrix4;

// Construct the rows in the new matrix
float d = sqrt( pow(currMat.m00,2) + pow(currMat.m10,2) + pow(currMat.m20,2) );
GLKVector4 columnToInsert0 = GLKVector4Make(d, 0, 0, xyz.x);
GLKVector4 columnToInsert1 = GLKVector4Make(0, d, 0, xyz.y);
GLKVector4 columnToInsert2 = GLKVector4Make(0, 0, d, xyz.z);
GLKVector4 columnToInsert3 = GLKVector4Make(0, 0, 0, 1);

// Build the new Matrix
GLKMatrix4 noTranslationInfo = GLKMatrix4SetRow(currMat, 0, columnToInsert0);
noTranslationInfo = GLKMatrix4SetRow(noTranslationInfo, 1, columnToInsert1);
noTranslationInfo = GLKMatrix4SetRow(noTranslationInfo, 2, columnToInsert2);
noTranslationInfo = GLKMatrix4SetRow(noTranslationInfo, 3, columnToInsert3);

// Print out the 'no translation' matrix
//NSLog(@"No Translation Matrix:");
//[self printMatrix:noTranslationInfo];

// Get a rotation matrix from the quaternion we last rotated the globe with
GLKMatrix4 rotationMatrixFromQuaternion = GLKMatrix4MakeWithQuaternion( _quat );
//NSLog(@"Rotation Matrix from Quaternion: ");
//[self printMatrix:rotationMatrixFromQuaternion];

// Now use the matrix produced from our Quaternion to rotate the global coordinates
// of the billboard object
GLKMatrix4 rotatedNoTranslationInfo = GLKMatrix4Multiply(rotationMatrixFromQuaternion, noTranslationInfo);

//NSLog(@"rotatedNoTranslationInfo:");
//[self printMatrix:rotatedNoTranslationInfo];

// Throw the world translation coordinates in the matrix
noTranslationInfo.m30 = ( rotatedNoTranslationInfo.m30 );
noTranslationInfo.m31 = ( rotatedNoTranslationInfo.m31 );
noTranslationInfo.m32 = ( rotatedNoTranslationInfo.m32 + GLOBAL_EARTH_Z_LOCATION );

//NSLog(@"Final Matrix:");
//[self printMatrix:noTranslationInfo];

// Assign the new matrix to draw with - no translation
self.effect.transform.modelviewMatrix = noTranslationInfo;

// Render the object with GLKit
[self.effect prepareToDraw];

// Draw elements from the index array and uv / vertex / normal info
glDrawElements(GL_TRIANGLES,Billboard_polygoncount*3,GL_UNSIGNED_SHORT,0);

// Restore the original matrix
self.effect.transform.modelviewMatrix = originalMat;