1 glPixelStore
http://www.zwqxin.com/archives/opengl/opengl-api-memorandum-2.html
2 depth clamping
glEnable(GL_DEPTH_CLAMP);
A more reasonable mechanism is depth clamping. What this does is turn off camera near/far plane clipping altogether. Instead, the depth for these fragments are clamped to the [-1, 1] range in NDC space.
What depth clamping does; it makes fragment depth values outside of the range be clamped to within the range. So depth values smaller than depth zNear become depth zNear, and values larger than depth zFar
become depth zFar.
Note
We defined depth clamping as, in part, turning off clipping against the camera near and far planes. If you're wondering what happens when you have depth clamping, which turns off clipping, and a clip-space W <= 0, it's simple. In camera space, near and far clipping is represented as turning a pyramid into a frustum: cutting off the top and bottom. If near/far clipping is not active, then the frustum becomes a pyramid. The other 4 clipping planes are still fully in effect. Clip-space vertices with a W of less than 0 are all outside of the boundary of any of the other four clipping planes.
The only clip-space point with a W of 0 that is within this volume is the homogeneous origin point: (0, 0, 0, 0); everything else will be clipped. And a triangle made from three positions that all are at the same position would have no area; it would therefore generate no fragments anyway. It can be safely eliminated before the perspective divide.