OpenGL: Using vertex array to draw a cube causes graphical glitch

Question

When trying to draw the following quads in OpenGL using a vertex array (instead of using immediate mode rendering), I get the graphical glitch (line segment) shown in the picture, which can be found in the second link below. The line seems to extend upwards to infinity.

GLdouble vertices[] = {
    // back
    0.0, 0.0, 0.0,
    si,  0.0, 0.0,
    si,  -si, 0.0,
    0.0, -si, 0.0,

    // front
    0.0, 0.0, si,
    0.0, -si, si,
    si,  -si, si,
    si,  0.0, si,

    // left
    0.0, 0.0, 0.0,
    0.0, -si, 0.0,
    0.0, -si, si,
    0.0, 0.0, si,

    // right
    si, 0.0, 0.0,
    si, 0.0, si,
    si, -si, si,
    si, -si, 0.0,

    // top
    0.0, 0.0, 0.0,
    0.0, 0.0, si,
    si, 0.0, si,
    si, 0.0, 0.0,

    // bottom
    0.0, -si, 0.0,
    si, -si, 0.0,
    si, -si, si,
    0.0, -si, si,
};

Immediate drawing:

 glBegin(GL_QUADS);
   for (int i = 0; i < sizeof(vertices)/sizeof(*vertices)/3; i++)
     glVertex3d(vertices[i * 3], vertices[i * 3 + 1], vertices[i * 3 + 2]);
 glEnd();

Drawing with vertex array:

glVertexPointer(3, GL_DOUBLE, 0, vertices);
glDrawArrays(GL_QUADS, 0, sizeof(vertices)/sizeof(*vertices));

Images:

Correct cube drawn in immediate mode

Glitchy cube drawn with vertex array

What am I doing wrong?

Andrew Rasmussen · Accepted Answer · 2011-04-16T16:19:31.207

5

Try changing your draw to

glVertexPointer(3, GL_DOUBLE, 0, vertices);
glDrawArrays(GL_QUADS, 0, 24);

sizeof() doesn't work the way you think it does. It doesn't return the number of elements in an array. You need to somehow keep track of the number of vertices in that array, and then use that counter in your glDrawArrays call.

edit - more information:

The last parameter in glDrawArrays is the number of vertices you are rendering. sizeof(vertices) should return the number of bytes in a GLdouble* (4 bytes), and sizeof(vertices*) should return the number of bytes in a GLdouble (8 bytes). Then 4 / 8 should technically round to zero since they are integers, so I'm not actually sure why it's rendering anything.

However, I'm pretty sure that (assuming your vertices are done correctly), if you change that number to 24, it will work.

edited Apr 16 '11 at 16:19

answered Apr 16 '11 at 16:13

Andrew Rasmussen

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They way he uses it, it will give him the number of elements in the array. Just not the number of vertices, since the elements are single floats. The fix is correct tho. – ltjax Apr 16 '11 at 16:16
1

Ohh I see. So it's returning the number of floats, but not the number of vertices. So since there are 3 floats per vertex, it's returning 72 rather than 24. – Andrew Rasmussen Apr 16 '11 at 16:20
That means my understanding of sizeof is a little off, how does `sizeof(pointer-to-double)/sizeof(*pointer-to-double)` return number of array elements? – Andrew Rasmussen Apr 16 '11 at 16:23
That's right. I seem to have misread the documetation for glDrawArrays. I tought the last parameter was the number of elements (GLdoubles). Thanks! :-) – meow Apr 16 '11 at 17:17
3

arasmussen: Since the array is statically defined in the same scope, necessary type/size information is available for the compiler. In this context, sizeof(array) gives the total size of the array in bytes. sizeof(*array) is the same as sizeof(array[0]) which is the same as sizeof(GLdouble). – meow Apr 16 '11 at 17:22
Thank you sir (+1). Learn something new every day :) – Andrew Rasmussen Apr 16 '11 at 17:27
@arasmussen: `vertices` has type "array of double", not "pointer to double". `sizeof` is calculated very differently for the two cases. – Ben Voigt Apr 16 '11 at 17:54
Aren't `double array[]` and `double *array` the same thing..? A pointer to a double? Since in C, an array is represented by a pointer which contains the memory address of the first element, and a certain number (size) of elements are allocated after that first element. Could have sworn `double array[]` is declaring pointer to double. – Andrew Rasmussen Apr 16 '11 at 18:10

OpenGL: Using vertex array to draw a cube causes graphical glitch

1 Answers1