270 lines
13 KiB
Text
270 lines
13 KiB
Text
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From fatcity!root@news.cts.com Thu Mar 26 17:57:48 1998
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X-VM-v5-Data: ([nil nil nil nil nil nil nil nil nil]
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["8204" "Thu" "26" "March" "1998" "15:32:55" "-0800" "akin@pobox.com" "akin@pobox.com" nil "162" "Re: poly offset " "^From:" nil nil "3" nil nil nil nil nil]
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(Smail3.1.29.1 #5) id m0yIMSJ-0000NEa; Thu, 26 Mar 98 15:53 PST
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Message-ID: <F001.00016F4B.19980326153255@fatcity.com>
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X-Comment: OpenGL Game Developers Mailing List
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Reply-To: OPENGL-GAMEDEV-L@fatcity.com
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Organization: Fat City Network Services, San Diego, California
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X-ListServer: v1.0f, build 64; ListGuru (c) 1996-1998 Bruce A. Bergman
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From: akin@pobox.com
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Sender: root@fatcity.com
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To: Multiple recipients of list OPENGL-GAMEDEV-L <OPENGL-GAMEDEV-L@fatcity.com>
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Subject: Re: poly offset
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Date: Thu, 26 Mar 1998 15:32:55 -0800
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Bryan Gibson-Winge wrote:
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| I was curious to see if anyone would describe a method of using poly offset
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| that didn't eventually conclude with "and mess with the numbers 'till it
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| works".
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There is such a method, but since it's not in any of the usual OpenGL
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literature it's not widely discussed. I'll try to summarize it; maybe
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the result could go into the FAQ.
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The purpose of polygon offset is to separate two or more primitives
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by just enough distance in Z that they can be depth-buffered without
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artifacts caused by depth computation roundoff errors, differences in
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sampling algorithms for different types of primitives, etc.
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The ``factor'' argument provides separation in the case where the
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primitives are not parallel. The factor value to use depends on the
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screen sizes of the primitives involved.
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The canonical example is highlighting the edge of a triangle
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by drawing a line between the two associated triangle vertices.
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The depth value for each pixel on the line depends only on the
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depth values at the two vertices of the triangle edge. However,
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the depth value for each pixel in the triangle depends on all
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three of the triangle's vertices. Since lines and triangles are
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sampled differently (e.g. diamond-exit rule for lines vs. point
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sampling for triangles), at a given pixel the depth computed
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for the edge line usually differs from the depth computed
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for the underlying triangle. This can cause ``stitching'' or
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other unpleasant artifacts in the image, because depth buffering
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sometimes places the line in front of the triangle and sometimes
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behind it.
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How much separation is needed to ensure that the line is always
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in front of the triangle? If you look at the rasterization
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arithmetic or play with a few diagrams, you can see that at some
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pixels the depth value for the triangle is computed at a point
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almost one pixel away from the ideal edge (which produces the
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depth values for the line). Since the triangle can have an almost
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arbitrarily large depth slope, there is no fixed offset that will
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guarantee separation between the two primitives in all cases.
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However, a (variable) separation of 1.0 times the depth slope of
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the triangle is always sufficient. (For the purposes of this
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discussion, the depth slope is max(|dz/dx|,|dz/dy|). The spec
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has additional information.) It's sometimes more than you need,
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but it's always large enough to work.
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If the line were two pixels wide rather than one, then 1.0
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times the depth slope wouldn't be enough separation, because
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some pixels on the line might have depth values derived from an
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edge point more than one pixel away from the underlying point
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on the triangle. 2.0 would be sufficient, though.
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So now you understand the factor argument. It should be
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nonzero when you're attempting to separate two primitives that
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aren't parallel, and its value should be roughly the size of
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the screen-space overlap (measured in pixels) between the two
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primitives. For any given triangle, the factor is multiplied
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by the triangle's depth slope to compute the amount of separation.
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The ``units'' or ``bias'' argument provides separation in the case
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where the primitives are roughly parallel. The value to use
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depends on how many primitives you're trying to stack up.
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Consider the edged-triangle case again. What happens when the
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depth slope of the triangle is zero? Since the depth slope is
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zero, no matter what the factor argument is, you won't get any
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separation between the triangle and the edge line. However,
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you still want some separation between the two primitives,
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so that you get a consistent result (i.e., one that doesn't
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depend on the order in which you draw things or on whether the
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depth-comparison function tests for equality).
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The bias argument guarantees a little separation even in the case
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where the depth slope is zero. Furthermore, it generalizes to
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the case where the depth slopes of the primitives are nonzero,
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but the primitives are roughly parallel (for example, when using
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one polygon as a decal on another).
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The original version of polygon offset allowed you to specify
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a bias that was simply added to the depth value at each pixel.
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However, it was quite difficult to choose an appropriate value,
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because the choice depends not only on the number of bits in the
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depth buffer (offsets smaller than the depth buffer precision
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don't do you much good!) but also on the precision of various
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computations inside the OpenGL driver and the hardware.
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Perspective projection can also make a difference, since
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resolution is better at the near clipping plane than it is at
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the far clipping plane.
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The current version of polygon offset specifies the bias in
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multiples of a ``minimum resolvable difference,'' which is some
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value determined by the driver developer. The minimum resolvable
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difference is sufficient to separate two parallel primitives,
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taking into account the perspective projection, size of the
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depth buffer, hardware precision, etc.
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Since one unit is sufficient to separate two primitives, you
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can use more than one unit to separate additional primitives.
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For example, by using bias values of 1, 2, 3, ... you can stack
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up an arbitrary set of polygons on a base polygon.
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Since some OpenGL implementations add the bias before the
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perspective divide, a bias of 1.0 unit might be much larger
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than is needed for primitives close to the near clipping plane.
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If your app is smart enough to know exactly how the driver
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behaves, and roughly where in the view volume the primitives
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will fall, then it can use a bias of less than 1.0 to separate
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primitives close to the near clipping plane. Most of the time
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this is *not* worth the effort, though.
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Some practical examples:
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Drawing lines of width 1.0 pixel on the edge of a triangle
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should use a factor of 1.0 (to guarantee separation when the
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triangle depth slope is nonzero) and a bias of 1.0 (to guarantee
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separation when the triangle depth slope is zero).
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Drawing wider lines requires larger factors. A good rule of
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thumb might be to use a factor equal to ceil(lineWidth/2 + 0.5).
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Drawing decal polygons needs a zero factor and a nonzero bias,
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provided that you can guarantee that all of the vertices of
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the decal polygons are on the same side of the plane of the
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base polygon, and not on that plane. (If intersections occur,
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you might need a nonzero factor, because the depth slopes of
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the primitives might be different.) Use a bias of 1.0 for the
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lowest-level decal (the one closest to the base), 2.0 for the
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next-highest, and so on.
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Positive factors and biases push primitives in one direction;
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negative factors and biases push primitives in the other
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direction. Sometimes it makes a difference (for example, if you
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want to use the values in the depth buffer for a subsequent pass).
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If you run into trouble, the most likely cause is that the driver
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developer underestimated the value of the minimum resolvable difference.
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(I know of no way to predict this value; I think you just have to
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measure it. The basic idea is to binary-search across the range of depth
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values, drawing two parallel polygons at each stage and determining if
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depth-buffering artifacts occur. Do this once at the near plane, and
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once at the far plane, and take the largest result. Repeat for extreme
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depthrange values.) Tweaking the bias argument is probably the only
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way to resolve this. :-)
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Finally, as Steve Baker and I have discussed in the past, the
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reference_plane extension is often a cleaner solution than polygon offset,
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if your OpenGL implementation supports it.
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Allen
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--
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Author:
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INET: akin@pobox.com
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Fat City Network Services -- (619) 538-5051 FAX: (619) 538-5051
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San Diego, California -- Public Internet access / Mailing Lists
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--------------------------------------------------------------------
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To REMOVE yourself from this mailing list, send an E-Mail message
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to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and in
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the message BODY, include a line containing: UNSUB OPENGL-GAMEDEV-L
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(or the name of mailing list you want to be removed from). You may
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also send the HELP command for other information (like subscribing).
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From fatcity!root@news.cts.com Fri Jan 16 04:45:15 1998
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X-VM-v5-Data: ([nil nil nil nil nil nil nil nil nil]
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["1385" "Fri" "16" "January" "1998" "02:06:11" "-0800" "Mark Kilgard" "mjk@fangio.engr.sgi.com" nil "43" "Re: glPolygonOffset" "^From:" nil nil "1" nil nil nil nil nil]
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nil)
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Received: from mh2.cts.com (root@mh2.cts.com [205.163.24.68])
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by meserv.me.umn.edu (8.8.8/8.8.6) with ESMTP id EAA19075
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for <curt@me.umn.edu>; Fri, 16 Jan 1998 04:45:14 -0600 (CST)
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Received: from king.cts.com (root@king.cts.com [198.68.168.21]) by mh2.cts.com (8.8.7/8.8.5) with ESMTP id CAA19264; Fri, 16 Jan 1998 02:39:16 -0800 (PST)
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Received: from donews.cts.com (root@donews.cts.com [192.188.72.21])
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by king.cts.com (8.8.7/8.8.7) with SMTP id CAA07365;
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Fri, 16 Jan 1998 02:39:07 -0800 (PST)
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Received: from fatcity by donews.cts.com with uucp
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(Smail3.1.29.1 #5) id m0xt8sd-00008Sa; Fri, 16 Jan 98 02:20 PST
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Received: by fatcity.com (02-Jan-98/v1.0f-b63/bab) via UUCP id 0C92BDFF; Fri, 16 Jan 1998 02:06:11 -0800
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Message-ID: <F001.0C92BDFF.19980116020611@fatcity.com>
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X-Comment: OpenGL Game Developers Mailing List
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X-Sender: mjk@fangio.engr.sgi.com (Mark Kilgard)
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Reply-To: OPENGL-GAMEDEV-L@fatcity.com
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Errors-To: ML-ERRORS@fatcity.com
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Organization: Fat City Network Services, San Diego, California
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X-ListServer: v1.0f, build 63; ListGuru (c) 1996-1998 Bruce A. Bergman
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Precedence: bulk
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Mime-Version: 1.0
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Content-Type: text/plain; charset=US-ASCII
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Content-Transfer-Encoding: 7bit
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From: mjk@fangio.engr.sgi.com (Mark Kilgard)
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Sender: root@fatcity.com
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To: Multiple recipients of list OPENGL-GAMEDEV-L <OPENGL-GAMEDEV-L@fatcity.com>
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Subject: Re: glPolygonOffset
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Date: Fri, 16 Jan 1998 02:06:11 -0800
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opengl-gamedev,
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> At one time I came across web site with example for glPolygonOffset, but I
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> can not find it again. Does anybody know web site.
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Polygon offset is used to "lift" the projected planar shadows off of
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the floor to avoid Z fighting in my dinoshade.c example. See:
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http://reality.sgi.com/mjk/tips/TexShadowReflectLight.html
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Other cool OpenGL rendering techniques are shown at:
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http://reality.sgi.com/mjk/tips/
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There are also several polygon offset examples in the GLUT 3.6 source
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code distribution. See:
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http://reality.sgi.com/mjk/glut3/glut3.html
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In particular, look at:
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progs/examples/origami.c
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progs/examples/surfgrid.c
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progs/examples/dinoshade.c
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progs/examples/halomagic.c
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progs/redbook/polyoff.c
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progs/advanced/haloed.c
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I hope this helps.
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- Mark
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--
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Author: Mark Kilgard
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INET: mjk@fangio.engr.sgi.com
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Fat City Network Services -- (619) 538-5030 FAX: (619) 538-5051
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San Diego, California -- Public Internet Access
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--------------------------------------------------------------------
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To REMOVE yourself from this mailing list, send an E-Mail message
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to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and in
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the message BODY, include a line containing: UNSUB OPENGL-GAMEDEV-L
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(or the name of mailing list you want to be removed from). You may
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also send the HELP command for other information (like subscribing).
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