Saturday, January 14, 2012

Z-buffer precision, camera pivot and optimal zNear

Everyone who has done any non-trivial 3D programming knows that sooner or later you run into problems with z-buffer precision. Especially a first-person situation you can have objects very near to the camera, which should not clip against the near-plane. But you also want to draw objects far away (say hills to the horizon). So you need a small zNear and a large zFar which runs into problems with precision, because of the non-linear screen-space z.

There are well-known solutions from depth partitioning to distorting the z values, but sometimes you just need a "bit more precision" and the above solutions just cause more trouble than they solve. If only you could move the zNear a bit further away.

Let's look at a first-person situation. The necessary (and sufficient) condition for avoiding any near-plane clipping (with any colliding geometry at least) is to place the near-plane completely inside the collision sphere (for rotational symmetry) of the camera. This can be a collision sphere of a player, or it can be the largest sphere that will fit inside the AABB or whatever else we collide with.

The question is, what is the largest possible value of the zNear (for a given field-of-view) that will keep the near-plane completely inside the collision sphere? Well, it depends on where you put the "eye" point.

As should be obvious in the image, the near-plane (and hence the zNear distance) is larger in the right-image. The largest possible near-plane that will fit inside the sphere will always go through the sphere origin. It is important to realize that it doesn't matter (at all) whether the "eye" is inside the collision sphere: anything behind the near-plane will get clipped anyway and the eye is purely virtual. Both placements will avoid any near-plane clipping as long as no geometry gets inside the sphere.

In practice first-person cameras (and other cameras that you turn directly) usually use the example on the left (it's quite obvious when you zoom: if you don't see any "parallax" movement when turning, then it's using the "left" version). This is what any text-book view/projection matrices will give you (eg D3DX helpers and whatever) unless you explicitly move the camera somewhere else.

Now, I would like to argue that the placement on the right is better even if you don't have precision issues, because it ends up feeling much more natural (on top of giving you larger optimal zNear which is easier to calculate). To understand why, we need to think of the "near-plane" as a lens. Then for a narrow field-of-view (eg high zoom) the "virtual" eye should end up behind the "real eye." The pivot point then should be much closer to the lens than the virtual eye position.

When I actually tried this in practice, I immediately realized why zooming in most games has always felt "wrong" as far as turning goes. It's because the pivot is in totally wrong place with regards to the view. By placing the pivot at the near-plane I ended up reducing the feeling of "tunnel vision" significantly and high-zoom no longer felt "shaky" at all. Turning actually gives you a sense of depth too (because of the slight parallax movement). You also see slightly more to your sides, which reduces the pressure to use high field-of-view for observing your surroundings (and you no longer need high field-of-view to combat the weird pivot either). All that on top of getting 4-8 times (well, it's FoV dependent; with more "zoom" you get larger zNear and hence more precision out in the distance) as large zNear without any near-plane clipping. I can't name a single negative really (go ahead and post of comment if you disagree).

So given a collision sphere radius and field of view, how do we calculate the optimal zNear when the near-plane goes through the camera origin? It turns out to be quite simple. Something like the following:

// fovY is vertical field of view in radians
// aspect is screenWidth / screenHeight
// solve half near-plane diagonal for nominal zNear = 1
float diag = tan(fovY / 2.f) * sqrt(1.f + aspect*aspect);

// solve zNear for diagonal to equal collision radius
float zNear = collisionRadius / diag;

Now you can calculate view matrix as usual, except move the camera backwards by zNear, because "camera" is really the "virtual eye" and we want near-plane through origin of the "real" camera.

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