Why nearby triangles tend to disappear?

Question

I've just enabled back face culling and I'm noticing a weird behavior: when all vertices of my triangle is outside the view and 2 of them is behind me (I think) the triangle disappears.

So to see it, here is a GIF.

I suspect the projection matrix reverses the order of the two vertices when they fall behind me, and changes the winding of my triangle.

But it's unclear why does the triangles disappear only if all vertices out of view...

How can I work around this problem, if possible?

I develop on Linux if that matters.

UPDATE:

It's pointed out it might not be due to the back face culling. I disabled it and I can indeed reproduce it. The cubes are 20×20 and the vertical field view is 90°. Its vertical apparent size roughly fills the window.

UPDATE 2:

Ok I'll post the relevant part of the code, projection and view matrixes are set up using my own functions:

void createViewMatrix(
    GLfloat matrix[16],
    const Vector3 *forward,
    const Vector3 *up,
    const Vector3 *pos
)
{
    /* Setting up perpendicular axes */
    Vector3 rright;
    Vector3 rup = *up;
    Vector3 rforward = *forward;

    vbonorm(&rright, &rup, &rforward); /* Orthonormalization (right is computed from scratch) */

    /* Filling the matrix */
    matrix[0] = rright.x;
    matrix[1] = rup.x;
    matrix[2] = -rforward.x;
    matrix[3] = 0;

    matrix[4] = rright.y;
    matrix[5] = rup.y;
    matrix[6] = -rforward.y;
    matrix[7] = 0;

    matrix[8] = rright.z;
    matrix[9] = rup.z;
    matrix[10] = -rforward.z;
    matrix[11] = 0;

    matrix[12] = -vdp(pos, &rright);
    matrix[13] = -vdp(pos, &rup);
    matrix[14] = vdp(pos, &rforward);
    matrix[15] = 1;
}

void createProjectionMatrix(
    GLfloat matrix[16],
    GLfloat vfov,
    GLfloat aspect,
    GLfloat near,
    GLfloat far
)
{
    GLfloat vfovtan = 1 / tan(RAD(vfov * 0.5));

    memset(matrix, 0, sizeof(*matrix) * 16);
    matrix[0] = vfovtan / aspect;
    matrix[5] = vfovtan;
    matrix[10] = (near+far)/(near-far);
    matrix[11] = -1;
    matrix[14] = (2*near*far)/(near-far);
}

Projection matrix set up with this call:

createProjectionMatrix(projMatrix, VERTICAL_FOV, ASPECT_RATIO, Z_NEAR, 10000);

(VERTICAL_FOV = 90, ASPECT_RATIO = 4.0/3, Z_NEAR = 1)

Level drawing is simply:

void drawStuff()
{
    GLfloat projectView[16];

    glClearColor(0, 0, 0, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    createViewMatrix(viewMatrix, &camera.forward, &camera.up, &camera.pos);

    multiplyMatrix(projectView, viewMatrix, projMatrix); /*< Row mayor multiplication. */

    glUniformMatrix4fv(renderingMatrixId, 1, GL_FALSE, projectView);
    bailOnGlError(__FILE__, __LINE__);

    renderLevel(&testLevel);
}

Cubes are rendered wall by wall (optimizing this will be another story):

    for (j = 0; j < 6; j++)
    {
        glBindTexture(GL_TEXTURE_2D, cube->wallTextureIds[j]);
        bailOnGlError(__FILE__, __LINE__);

        glDrawElements(GL_TRIANGLE_FAN, 4, GL_UNSIGNED_INT, (void*)(sizeof(GLuint) * 4 * j));
        bailOnGlError(__FILE__, __LINE__);
        glUniform4f(extraColorId, 1, 1, 1, 1);
        bailOnGlError(__FILE__, __LINE__);
    }

Vertex shader:

#version 110

attribute vec3 position;
attribute vec3 color;
attribute vec2 texCoord;

varying vec4 f_color;
varying vec2 f_texCoord;

uniform mat4 renderingMatrix;

void main()
{
    gl_Position =  renderingMatrix * vec4(position, 1);
    f_color = vec4(color, 1);
    f_texCoord = texCoord;
}

Fragment shader:

#version 110

varying vec4 f_color;
varying vec2 f_texCoord;

uniform sampler2D tex;

uniform vec4 extraColor;

void main()
{
    gl_FragColor = texture2D(tex, f_texCoord) * vec4(f_color) * extraColor;
}

The depth buffer simply set up by enabling it.

Answer 1

Though similar issues are often caused by clipping, the near plane is not the issue here. If it would be, the disappearance would be per pixel and not per triangle.

In your animation the triangles exactly disappear at the moment all of its three vertices get outside of the screen. Your algorithm may be based on the false assumption that triangles are hidden when all their vertices are hidden.

Here is an article talking about a good frustum culling implementation.

Answer 2

So far it seems it is an OpenGL driver issue. I don't have other computer to test this on to confirm.

If I force software rendering by

$ export LIBGL_ALWAYS_SOFTWARE=1

The problem goes away. Probably I need to look around bugs with Mesa.

Answer 3

its purely related to the near clipping plane for camera and has nothing related to backface culling. U need to reduce the near clipping plane and the clipping will go off.

Answer 4

The triangles are disappearing because of the camera frustrum's near plane. Everything that is closer to the camera than the near plane is automatically clipped. You just need to put your near plane even more close of the the camera location. How you can configure the near plane depends on your graphical library or engine. In OpenGL, for example you should set the zNear parameter of gluPerspective:

void gluPerspective(GLdouble fovy, GLdouble aspect, GLdouble zNear, GLdouble zFar);