iphone - 无法在 OpenGL ES 2.0 中通过 glDrawElements 绘制三角形

Question

我正在尝试绘制引用 here 的程序球体.

我稍微修改了一下，以便可以使用 OpenGL ES 2.0 的 glDrawElements 方法

这是我的 createSphere 版本:

GLfloat sphereVerticies[10000]={0.0};
GLubyte triangleIndices[15000]={0};

int createSphere (GLfloat spherePoints[], GLubyte triangleIndices[], GLfloat fRadius, GLfloat step)
{
    int points = 0;

    GLfloat uStep = DEGREES_TO_RADIANS (step);
    GLfloat vStep = uStep;

    unsigned long index=0;

    for (GLfloat u = 0.0f; u <= (2 * M_PI); u += uStep) 
    {
        for (GLfloat v = -M_PI_2; v <= M_PI_2; v += vStep) 
        {               
            triangleIndices[index++]=points;
            triangleIndices[index++]=points+1;
            triangleIndices[index++]=points+2;
            triangleIndices[index++]=points+2;
            triangleIndices[index++]=points+3;
            triangleIndices[index++]=points;

            points++;
            spherePoints[(points - 1) * 3] = fRadius * cosf(v) * cosf(u);             // x
            spherePoints[((points - 1) * 3) + 1] = fRadius * cosf(v) * sinf(u);       // y
            spherePoints[((points - 1) * 3) + 2] = fRadius * sinf(v);                 // z

            points++;
            spherePoints[(points - 1) * 3] = fRadius * cosf(v) * cosf(u + uStep);             // x
            spherePoints[((points - 1) * 3) + 1] = fRadius * cosf(v) * sinf(u + uStep);       // y
            spherePoints[((points - 1) * 3) + 2] = fRadius * sinf(v);                         // z              

            points++;
            spherePoints[(points - 1) * 3] = fRadius * cosf(v + vStep) * cosf(u);                  // x
            spherePoints[((points - 1) * 3) + 1] = fRadius * cosf(v + vStep) * sinf(u);            // y
            spherePoints[((points - 1) * 3) + 2] = fRadius * sinf(v + vStep);                      // z

            points++;
            spherePoints[(points - 1) * 3] = fRadius * cosf(v + vStep) * cosf(u + uStep);           // x
            spherePoints[((points - 1) * 3) + 1] = fRadius * cosf(v + vStep) * sinf(u + uStep);     // y
            spherePoints[((points - 1) * 3) + 2] = fRadius * sinf(v + vStep);                       // z

        }
    }        
    return points;
}

在 SetupGL 中我有:

..
glEnable(GL_CULL_FACE);
..
..
glGenVertexArraysOES(1, &_vertexArray);
glBindVertexArrayOES(_vertexArray);

 numPoints=createSphere(sphereVerticies, triangleIndices, 30.0f, 20.0f);

    glGenBuffers(1, &_vertexBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, _vertexBuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(sphereVerticies), sphereVerticies, GL_STATIC_DRAW);

    glGenBuffers(1, &_indexBuffer);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBuffer);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(triangleIndices), triangleIndices, GL_STATIC_DRAW);

    // New lines (were previously in draw)
    glEnableVertexAttribArray(GLKVertexAttribPosition);        
    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 0, (const GLvoid *) sphereVerticies);

glBindVertexArrayOES(0);

最后在 drawInRect 中:

glBindVertexArrayOES(_vertexArray);   
    glDrawElements(GL_TRIANGLES, (int)(numPoints*1.5), GL_UNSIGNED_BYTE, 0);

现在我在这里做错了什么？我没有看到任何视觉输出。任何帮助将不胜感激。谢谢。

Answer 1

我可以看到两件事:

glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 0, (const GLvoid *) sphereVerticies);

最后一个参数应该是 0，因为顶点已经缓冲并绑定(bind)在 VBO 中。

OpenGL ES - glVertexAttribPointer documentation

If a non-zero named buffer object is bound to the GL_ARRAY_BUFFER target (see glBindBuffer) while a generic vertex attribute array is specified, pointer is treated as a byte offset into the buffer object's data store...

还有，

glDrawElements(GL_TRIANGLES, (int)(numPoints*1.5), GL_UNSIGNED_BYTE, 0);

你应该使用 GL_UNSIGNED_SHORT 因为你有超过 255 个顶点。

---

至于索引，这是你索引它们的方式:

                 (0,1,2)        (2,3,0)
 2       3      2              2       3
 o-------o      o              o-------o
 |       |      | \            |     /
 |       |  =>  |   \          |   /
 |       |      |     \        | /
 o-------o      o-------o      o
 0       1      0       1      0

所以，从这个图表我们可以看出两个问题:

1) 三角形不闭合多边形

2) 注意绕组，第一个三角形是 CCW，第二个是 CW。

试试这段代码:

        triangleIndices[index++]=points;
        triangleIndices[index++]=points+1;
        triangleIndices[index++]=points+2;
        triangleIndices[index++]=points+3;
        triangleIndices[index++]=points+2;
        triangleIndices[index++]=points+1;