12 The Depth Buffer
Your application needs to do at least the following to get depth buffering to work:
- Ask for a depth buffer when you create your window.
- Place a call to glEnable (GL_DEPTH_TEST) in your program's initialization routine, after a context is created and made current.
- Ensure that your zNear and zFar clipping planes are set correctly and in a way that provides adequate depth buffer precision.
- Pass GL_DEPTH_BUFFER_BIT as a parameter to glClear, typically bitwise OR'd with other values such as GL_COLOR_BUFFER_BIT.
There are a number of OpenGL example programs available on the Web, which use depth buffering. If you're having trouble getting depth buffering to work correctly, you might benefit from looking at an example program to see what is done differently. This FAQ contains links to several web sites that have example OpenGL code.
Make sure the zNear and zFar clipping planes are specified correctly in your calls to glFrustum() or gluPerspective().
A mistake many programmers make is to specify a zNear clipping plane value of 0.0 or a negative value which isn't allowed. Both the zNear and zFar clipping planes are positive (not zero or negative) values that represent distances in front of the eye.
Specifying a zNear clipping plane value of 0.0 to gluPerspective() won't generate an OpenGL error, but it might cause depth buffering to act as if it's disabled. A negative zNear or zFar clipping plane value would produce undesirable results.
A zNear or zFar clipping plane value of zero or negative, when passed to glFrustum(), will produce an error that you can retrieve by calling glGetError(). The function will then act as a no-op.
Use the glDrawPixels() command, with the format parameter set to GL_DEPTH_COMPONENT. You may want to mask off the color buffer when you do this, with a call to glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE); .
You may have configured your zNear and zFar clipping planes in a way that severely limits your depth buffer precision. Generally, this is caused by a zNear clipping plane value that's too close to 0.0. As the zNear clipping plane is set increasingly closer to 0.0, the effective precision of the depth buffer decreases dramatically. Moving the zFar clipping plane further away from the eye always has a negative impact on depth buffer precision, but it's not one as dramatic as moving the zNear clipping plane.
The OpenGL Reference Manual description for glFrustum() relates depth precision to the zNear and zFar clipping planes by saying that roughly log2(zFar/zNear) bits of precision are lost. Clearly, as zNear approaches zero, this equation approaches infinity.
While the blue book description is good at pointing out the relationship, it's somewhat inaccurate. As the ratio (zFar/zNear) increases, less precision is available near the back of the depth buffer and more precision is available close to the front of the depth buffer. So primitives are more likely to interact in Z if they are further from the viewer.
It's possible that you simply don't have enough precision in your depth buffer to render your scene. See the last question in this section for more info.
It's also possible that you are drawing coplanar primitives. Round-off errors or differences in rasterization typically create "Z fighting" for coplanar primitives. Here are some options to assist you when rendering coplanar primitives.
The depth buffer precision in eye coordinates is strongly affected by the ratio of zFar to zNear, the zFar clipping plane, and how far an object is from the zNear clipping plane.
You need to do whatever you can to push the zNear clipping plane out and pull the zFar plane in as much as possible.
To be more specific, consider the transformation of depth from eye coordinates
xe, ye, ze, we
to window coordinates
xw, yw, zw
with a perspective projection matrix specified by
glFrustum(l, r, b, t, n, f);
and assume the default viewport transform. The clip coordinates of zc and wc are
zc = -ze* (f+n)/(f-n) - we* 2*f*n/(f-n)
wc = -ze
Why the negations? OpenGL wants to present to the programmer a right-handed coordinate system before projection and left-handed coordinate system after projection.
and the ndc coordinate:
zndc = zc / wc = [ -ze * (f+n)/(f-n) - we * 2*f*n/(f-n) ] / -ze
= (f+n)/(f-n) + (we / ze) * 2*f*n/(f-n)
The viewport transformation scales and offsets by the depth range (Assume it to be [0, 1]) and then scales by s = (2n-1) where n is the bit depth of the depth buffer:
zw = s * [ (we / ze) * f*n/(f-n) + 0.5 * (f+n)/(f-n) + 0.5 ]
Let's rearrange this equation to express ze / we as a function of zw
ze / we = f*n/(f-n) / ((zw / s) - 0.5 * (f+n)/(f-n) - 0.5)
= f * n / ((zw / s) * (f-n) - 0.5 * (f+n) - 0.5 * (f-n))
= f * n / ((zw / s) * (f-n) - f) [*]
Now let's look at two points, the zNear clipping plane and the zFar clipping plane:
zw = 0 => ze / we = f * n / (-f) = -n
zw = s => ze / we = f * n / ((f-n) - f) = -f
In a fixed-point depth buffer, zw is quantized to integers. The next representable z buffer depth away from the clip planes are 1 and s-1:
zw = 1 => ze / we = f * n / ((1/s) * (f-n) - f)
zw = s-1 => ze / we = f * n / (((s-1)/s) * (f-n) - f)
Now let's plug in some numbers, for example, n = 0.01, f = 1000 and s = 65535 (i.e., a 16-bit depth buffer)
zw = 1 => ze / we = -0.01000015
zw = s-1 => ze / we = -395.90054
Think about this last line. Everything at eye coordinate depths from -395.9 to -1000 has to map into either 65534 or 65535 in the z buffer. Almost two thirds of the distance between the zNear and zFar clipping planes will have one of two z-buffer values!
To further analyze the z-buffer resolution, let's take the derivative of [*] with respect to zw
d (ze / we) / d zw = - f * n * (f-n) * (1/s) / ((zw / s) * (f-n) - f)2
Now evaluate it at zw = s
d (ze / we) / d zw = - f * (f-n) * (1/s) / n
= - f * (f/n-1) / s [**]
If you want your depth buffer to be useful near the zFar clipping plane, you need to keep this value to less than the size of your objects in eye space (for most practical uses, world space).
After the projection matrix transforms the clip coordinates, the XYZ-vertex values are divided by their clip coordinate W value, which results in normalized device coordinates. This step is known as the perspective divide. The clip coordinate W value represents the distance from the eye. As the distance from the eye increases, 1/W approaches 0. Therefore, X/W and Y/W also approach zero, causing the rendered primitives to occupy less screen space and appear smaller. This is how computers simulate a perspective view.
As in reality, motion toward or away from the eye has a less profound effect for objects that are already in the distance. For example, if you move six inches closer to the computer screen in front of your face, it's apparent size should increase quite dramatically. On the other hand, if the computer screen were already 20 feet away from you, moving six inches closer would have little noticeable impact on its apparent size. The perspective divide takes this into account.
As part of the perspective divide, Z is also divided by W with the same results. For objects that are already close to the back of the view volume, a change in distance of one coordinate unit has less impact on Z/W than if the object is near the front of the view volume. To put it another way, an object coordinate Z unit occupies a larger slice of NDC-depth space close to the front of the view volume than it does near the back of the view volume.
In summary, the perspective divide, by its nature, causes more Z precision close to the front of the view volume than near the back.
The typical approach is to use a multipass technique. The application might divide the geometry database into regions that don't interfere with each other in Z. The geometry in each region is then rendered, starting at the furthest region, with a clear of the depth buffer before each region is rendered. This way the precision of the entire depth buffer is made available to each region.