OpenGL 3/4 glVertexAttribPointer stride and offset miscalculation

Question

I am having a problem getting my vertex array pointed to properly:

const float vertices[] = {
/* position */ 0.75f, 0.75f, 0.0f, 1.0f, /* color */ 1.0f, 0.0f, 0.0f, 1.0f,
/* position */ 0.75f, -0.75f, 0.0f, 1.0f, /* color */ 0.0f, 1.0f, 0.0f, 1.0f,
/* position */ -0.75f, -0.75f, 0.0f, 1.0f, /* color */ 0.0f, 0.0f, 1.0f, 1.0f, };

...

glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObject);
glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);
glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, (void*)16);

glDrawArrays(GL_TRIANGLES, 0, 3);

glDisableVertexAttribArray(0);
glDisableVertexAttribArray(1);

I do not understand how stride and offset work. What is the correct way of going about using glVertexAttribPointer() in my situation?

Answer 1

Since glVertexAttribPointer often make troubles, I try to further explain it here.

The formula to calculate the start pos of the i-th attribute in attributes array is :

startPos(i) = offset + i * stride (From derhass' another answer)

And explained in the graph below :

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If you need code example, continue reading.

From Formatting VBO Data, we know we can manage our vertex data in three format. Make a example of drawing a triangle , with vert color and texture color mixed, here is the way to prepare vert attribute data:

---

#way1 Each attribute a VBO.

this format like : (xyzxyz...)(rgbrgb...)(stst....), and we can let both sride = 0 and offset = 0.

void prepareVertData_moreVBO(GLuint& VAOId, std::vector<GLuint>& VBOIdVec)
{
    GLfloat vertPos[] = {
        -0.5f, 0.0f, 0.0f, 0.5f, 0.0f, 0.0f, 0.0f, 0.5f, 0.0f,  // positon
    };
    GLfloat vertColor[] = {
      1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f,   // color
    };
   GLfloat vertTextCoord[] = {
      0.0f, 0.0f, 1.0f, 0.0f, 1.0f, 1.0f     // texture coordinate
   };

    GLuint VBOId[3];

    glGenVertexArrays(1, &VAOId);
    glBindVertexArray(VAOId);
    glGenBuffers(3, VBOId);
    // specify position attribute
    glBindBuffer(GL_ARRAY_BUFFER, VBOId[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertPos), vertPos, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
    glEnableVertexAttribArray(0);
    // specify color attribute
    glBindBuffer(GL_ARRAY_BUFFER, VBOId[1]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertColor), vertColor, GL_STATIC_DRAW);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, NULL);
    glEnableVertexAttribArray(1);
    // specify texture coordinate attribute
    glBindBuffer(GL_ARRAY_BUFFER, VBOId[2]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertTextCoord), vertTextCoord, GL_STATIC_DRAW);
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 0, NULL);
    glEnableVertexAttribArray(2);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    VBOIdVec.push_back(VBOId[0]);
    VBOIdVec.push_back(VBOId[1]);
    VBOIdVec.push_back(VBOId[2]);
}

---

#way2: Each attribute is sequential, batched in a single VBO.

this format is like: (xyzxyzxyz... rgbrgb... ststst...), we can let stride=0 but offset should be specified.

void prepareVertData_seqBatchVBO(GLuint& VAOId, std::vector<GLuint>& VBOIdVec)
{

    GLfloat vertices[] = {
     -0.5f, 0.0f, 0.0f, 0.5f, 0.0f, 0.0f, 0.0f, 0.5f, 0.0f,  // position
     1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f,   // color
     0.0f, 0.0f, 1.0f, 0.0f, 1.0f, 1.0f     // texture coordinate
    };

    GLuint VBOId;
    glGenVertexArrays(1, &VAOId);
    glBindVertexArray(VAOId);

    glGenBuffers(1, &VBOId);
    glBindBuffer(GL_ARRAY_BUFFER, VBOId);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    // specifiy position attribute
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (GLvoid*)0); // stride can aslo be 3 * sizeof(GL_FLOAT)
    glEnableVertexAttribArray(0);
    // specify color attribute
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, (GLvoid*)(9 * sizeof(GL_FLOAT)));
    glEnableVertexAttribArray(1);
    // specify texture coordinate
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 0, (GLvoid*)(18 * sizeof(GL_FLOAT)));
    glEnableVertexAttribArray(2);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    VBOIdVec.push_back(VBOId);
}

---

#way3: Interleaved attribute in a single VBO

this format is like: (xyzrgbstxyzrgbst...),we have to manually specify the offset and stride .

void prepareVertData_interleavedBatchVBO(GLuint& VAOId, std::vector<GLuint>& VBOIdVec)
{
    // interleaved data
    GLfloat vertices[] = {
       -0.5f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f,  // 0
       0.5f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f,  // 1
       0.0f, 0.5f, 0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 1.0f,  // 2
    };

    GLuint VBOId;
    glGenVertexArrays(1, &VAOId);
    glBindVertexArray(VAOId);
    glGenBuffers(1, &VBOId);
    glBindBuffer(GL_ARRAY_BUFFER, VBOId);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    // specify position attribute
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE,8 * sizeof(GL_FLOAT), (GLvoid*)0);
    glEnableVertexAttribArray(0);
    // specify color attribute
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE,
        8 * sizeof(GL_FLOAT),(GLvoid*)(3 * sizeof(GL_FLOAT)));
    glEnableVertexAttribArray(1);
    // specify texture coordinate
    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 
        8 * sizeof(GL_FLOAT), (GLvoid*)(6 * sizeof(GL_FLOAT)));
    glEnableVertexAttribArray(2);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    VBOIdVec.push_back(VBOId);
}

And thanks for derhass's answer.