Revolt Game No Z Buffer Fighting

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Blog - Recent events in the Tigray region of Ethiopia have made international headlines. Read as Atlantic Council Africa Center experts react, analyzing what the conflict means for the country. Aug 22, 2016 Once a Z buffer is defined Direct. Welcome to Re-Volt Forum. But you should be able to run the game and play online. I live in vietnam and my original revolt cd is in france, so no. If I start the game Re-Volt, it shows me The message: No Z-Buffer available. From games to virtual reality, mobile phones to supercomputers.

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12.010 How do I make depth buffering work?

Your application needs to do at least thefollowing to get depth buffering to work:

  1. Ask for a depth buffer when you createyour window.
  2. Place a call to glEnable (GL_DEPTH_TEST) inyour program's initialization routine, after acontext is created and made current.
  3. Ensure that your zNear and zFarclipping planes are set correctly and in a waythat provides adequate depth buffer precision.
  4. Pass GL_DEPTH_BUFFER_BIT as a parameter toglClear, typically bitwise OR'd with other valuessuch as GL_COLOR_BUFFER_BIT.

There are a number of OpenGL exampleprograms available on the Web, which use depth buffering. Ifyou're having trouble getting depth buffering to workcorrectly, you might benefit from looking at an exampleprogram to see what is done differently. This FAQ contains links toseveral web sites that have example OpenGL code.

12.020 Depth buffering doesn't work in myperspective rendering. What's going on?

Make sure the zNear and zFar clippingplanes are specified correctly in your calls to glFrustum()or gluPerspective().

A mistake many programmers make is tospecify a zNear clipping plane value of 0.0 or anegative value which isn't allowed. Both the zNear andzFar clipping planes are positive (not zero ornegative) values that represent distances in front of the eye.

Specifying a zNear clipping planevalue of 0.0 to gluPerspective() won't generate an OpenGLerror, but it might cause depth buffering to act as if it'sdisabled. A negative zNear or zFar clippingplane value would produce undesirable results.

A zNear or zFar clippingplane value of zero or negative, when passed to glFrustum(),will produce an error that you can retrieve by callingglGetError(). The function will then act as a no-op.

12.030 How do I write a previously stored depthimage to the depth buffer?

Use the glDrawPixels() command, with theformat parameter set to GL_DEPTH_COMPONENT. You may want tomask off the color buffer when you do this, with a call toglColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE); .

12.040 Depth buffering seems to work, butpolygons seem to bleed through polygons that are in front of them.What's going on?

You may have configured your zNear andzFar clipping planes in a way that severely limitsyour depth buffer precision. Generally, this is caused by a zNearclipping plane value that's too close to 0.0. As the zNearclipping plane is set increasingly closer to 0.0, theeffective precision of the depth buffer decreasesdramatically. Moving the zFar clipping plane furtheraway from the eye always has a negative impact on depthbuffer precision, but it's not one as dramatic as moving the zNearclipping plane.

The OpenGLReference Manual descriptionfor glFrustum() relates depth precision to the zNear andzFar clipping planes by saying that roughly log2(zFar/zNear)bits of precision are lost. Clearly, as zNearapproaches zero, this equation approaches infinity.

While the blue book description is good atpointing out the relationship, it's somewhat inaccurate. Asthe ratio (zFar/zNear) increases, less precision isavailable near the back of the depth buffer and moreprecision is available close to the front of the depth buffer.So primitives are more likely to interact in Z if they arefurther from the viewer.

It's possible that you simply don't haveenough precision in your depth buffer to render your scene.See the last questionin this section for more info.

It's also possible that you are drawingcoplanar primitives. Round-off errors or differences inrasterization typically create 'Z fighting' forcoplanar primitives. Here are some options to assist youwhen rendering coplanar primitives.

12.050 Why is my depth buffer precision so poor?

The depth buffer precision in eyecoordinates is strongly affected by the ratio of zFar tozNear, the zFar clipping plane, and how faran object is from the zNear clipping plane.

You need to do whatever you can to push thezNear clipping plane out and pull the zFar planein as much as possible.

To be more specific, consider thetransformation of depth from eye coordinates

Support

xe, ye, ze,we

to window coordinates

xw, yw, zw

with a perspective projection matrixspecified 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 presentto the programmer a right-handed coordinate system beforeprojection 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)

Revolt game no z buffer fighting unblocked

The viewport transformation scales andoffsets by the depth range (Assume it to be [0, 1]) and thenscales by s = (2n-1) where n is the bit depth ofthe 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 zNearclipping 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, zwis quantized to integers. The next representable z bufferdepth 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 ateye coordinate depths from -395.9 to -1000 has to map intoeither 65534 or 65535 in the z buffer. Almost two thirds ofthe distance between the zNear and zFar clippingplanes 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

Revolt Game No Z Buffer Fighting 2.9

d (ze / we) / d zw= - f * n * (f-n) * (1/s) / ((zw / s) * (f-n)- f)2

Now evaluate it at zw = s

Revolt

d (ze / we) / d zw= - f * (f-n) * (1/s) / n

= - f * (f/n-1) / s [**]

If you want your depth buffer to be usefulnear the zFar clipping plane, you need to keep thisvalue to less than the size of your objects in eye space (formost practical uses, world space).

Revolt Game No Z Buffer Fighting

12.060 How do I turn off the zNearclipping plane?

See thisquestion in the Clipping section.

12.070 Why is there more precision at the frontof the depth buffer?

After the projection matrix transforms theclip coordinates, the XYZ-vertex values are divided by theirclip coordinate W value, which results in normalized devicecoordinates. This step is known as the perspective divide.The clip coordinate W value represents the distance from theeye. As the distance from the eye increases, 1/W approaches 0.Therefore, X/W and Y/W also approach zero, causing therendered primitives to occupy less screen space and appearsmaller. This is how computers simulate a perspective view.

As in reality, motion toward or away fromthe eye has a less profound effect for objects that arealready in the distance. For example, if you move six inchescloser to the computer screen in front of your face, it'sapparent size should increase quite dramatically. On theother hand, if the computer screen were already 20 feet awayfrom you, moving six inches closer would have littlenoticeable impact on its apparent size. The perspectivedivide takes this into account.

As part of the perspective divide, Z isalso divided by W with the same results. For objects that arealready close to the back of the view volume, a change indistance of one coordinate unit has less impact on Z/W thanif the object is near the front of the view volume. To put itanother way, an object coordinate Z unit occupies a largerslice of NDC-depth space close to the front of the viewvolume than it does near the back of the view volume.

Revolt Game No Z Buffer Fighting Simulator

In summary, the perspective divide, by itsnature, causes more Z precision close to the front of theview volume than near the back.

Revolt Game No Z Buffer Fighting Games

A previous question in thissection contains related information.

Z Buffer Beta Mercaptoethanol

12.080 There is no way that a standard-sizeddepth buffer will have enough precision for my astronomicallylarge scene. What are my options?

Z-buffer Algorithm

The typical approach is to use a multipasstechnique. The application might divide the geometry databaseinto regions that don't interfere with each other in Z. Thegeometry in each region is then rendered, starting at thefurthest region, with a clear of the depth buffer before eachregion is rendered. This way the precision of the entiredepth buffer is made available to each region.