Graphics 犹他茶壶中的贝塞尔贴片是如何工作的?
我过早地发布了一个代码高尔夫挑战,以使用()绘制。()但当我深入研究这些数据以制作一个小例子时,我意识到我不知道这些数据是怎么回事。我对二维贝塞尔曲线有很好的理解。但是对于3D,它的工作原理是一样的吗 对!!是的 数据包含面片,每个面片包含16个顶点。这些布局是否有标准的顺序?如果它们对应于二维曲线,那么四个角点实际上接触到曲面,剩下的12个是控件,对吗 对! 我的“原始计划”是将形状简化为矩形,将它们投影到画布上,然后以灰色绘制填充形状,该灰色由垂直于光向量的面片点积的大小计算。如果我把它简化那么远,它会看起来像茶壶吗?是否必须使用光线跟踪来获得可识别的图像 那是主观的-( 虽然这看起来像几个问题,但它们都是这个问题的一个方面:“请,仁慈的大师,教我一些贝塞尔补丁?我需要知道什么来画茶壶?”Graphics 犹他茶壶中的贝塞尔贴片是如何工作的?,graphics,3d,postscript,bezier,bspline,Graphics,3d,Postscript,Bezier,Bspline,我过早地发布了一个代码高尔夫挑战,以使用()绘制。()但当我深入研究这些数据以制作一个小例子时,我意识到我不知道这些数据是怎么回事。我对二维贝塞尔曲线有很好的理解。但是对于3D,它的工作原理是一样的吗 对!!是的 数据包含面片,每个面片包含16个顶点。这些布局是否有标准的顺序?如果它们对应于二维曲线,那么四个角点实际上接触到曲面,剩下的12个是控件,对吗 对! 我的“原始计划”是将形状简化为矩形,将它们投影到画布上,然后以灰色绘制填充形状,该灰色由垂直于光向量的面片点积的大小计算。如果我把它简化
以下是我迄今为止编写的代码。(使用此矩阵库:) 这是我的建议 这就是我糟糕的形象:
更新:修正错误。也许它们毕竟是“正常”布置的。选择正确的角点至少会给出一个对称的形状:
更新:边界曲线看起来更好。
双三次贝塞尔曲面片是一个4x4的3D点阵列。是的,四个角接触曲面;行是贝塞尔曲线,列也是贝塞尔曲线。但deCasteljau算法基于计算两点之间的中值,在3D和2D中同样有意义 完成上述代码的下一步是细分面片以覆盖较小的部分。然后,上面简单的边界曲线提取成为合适的多边形网格 首先展平面片,直接插入顶点数据,而不是使用单独的缓存。此代码在面片中迭代,在顶点数组中查找点,并构造一个新的面片数组,然后用相同的名称重新定义
/patch[ patch{ [exch { 1 sub vert exch get }forall ] }forall ]def
然后我们需要deCasteljau算法来分割Bezier曲线。vop
来自矩阵库,对向量的相应元素应用二进制运算,并生成一个新的向量作为结果
/median { % [x0 y0 z0] [x1 y1 z1]
{add 2 div} vop % [ (x0+x1)/2 (y0+y1)/2 (z0+z1)/2 ]
} def
/decasteljau { % [P0] P1 P2 P3 . P0 P1' P2' P3' P3' P4' P5' P3
{p3 p2 p1 p0}{exch def}forall
/p01 p0 p1 median def
/p12 p1 p2 median def
/p23 p2 p3 median def
/p012 p01 p12 median def
/p123 p12 p23 median def
/p0123 p012 p123 median def
p0 p01 p012 p0123 % first half-curve
p0123 p123 p23 p3 % second half-curve
} def
然后进行一些堆栈操作,以应用于面片的每一行,并将结果组合成两个新面片
/splitrows { % [b0 .. b15] . [c0 .. c15] [d0 .. d15]
aload pop % b0 .. b15
4 { % on each of 4 rows
16 12 roll decasteljau % roll the first 4 to the top
8 4 roll % exch left and right halves (probably unnecessary)
20 4 roll % roll new curve to below the patch (pushing earlier ones lower)
} repeat
16 array astore % pack the left patch
17 1 roll 16 array astore % roll, pack the right patch
} def
一个丑陋的实用程序允许我们对列重复使用行代码。堆栈注释是编写此过程所必需的,因此它们可能是读取此过程所必需的。nj roll
rolls n个元素(向左),j次;==第n个元素上方的顶部j个元素(从1开始计数)因此,n稳步减小,选择元素放置的位置,j选择元素放置的位置(拖动所有其他元素)。如果应用了bind
,此过程将比基于字典的过程快得多
% [ 0 1 2 3
% 4 5 6 7
% 8 9 10 11
% 12 13 14 15 ]
/xpose {
aload pop % 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
15 12 roll % 0 4 5 6 7 8 9 10 11 12 13 14 15 1 2 3
14 11 roll % 0 4 8 9 10 11 12 13 14 15 1 2 3 5 6 7
13 10 roll % 0 4 8 12 13 14 15 1 2 3 5 6 7 9 10 11
12 9 roll % 0 4 8 12 1 2 3 5 6 7 9 10 11 13 14 15
11 9 roll % 0 4 8 12 1 5 6 7 9 10 11 13 14 15 2 3
10 8 roll % 0 4 8 12 1 5 9 10 11 13 14 15 2 3 6 7
9 7 roll % 0 4 8 12 1 5 9 13 14 15 2 3 6 7 10 11
8 6 roll % 0 4 8 12 1 5 9 13 2 3 6 7 10 11 14 15
7 6 roll % 0 4 8 12 1 5 9 13 2 6 7 10 11 14 15 3
6 5 roll % 0 4 8 12 1 5 9 13 2 6 10 11 14 15 3 7
5 4 roll % 0 4 8 12 1 5 9 13 2 6 10 14 15 3 7 11
4 3 roll % 0 4 8 12 1 5 9 13 2 6 10 14 3 7 11 15
16 array astore
} def
% [ 0 4 8 12
% 1 5 9 13
% 2 6 10 14
% 3 7 11 15 ]
/splitcols {
xpose
splitrows
xpose
} def
然后将这些函数应用于补丁数据。同样,每次都重新定义补丁
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitcols }forall ]def
/patch[ patch{ splitcols }forall ]def
这样就可以处理较小的碎片。添加可见性测试
/visible { % patch . patch boolean
dup % p p
dup 3 get exch dup 0 get exch 12 get % p p3 p0 p12
1 index {sub} vop % p p3 p0 v0->12
3 1 roll {sub} vop % p v0->12 v0->3
cross /normal exch def
dup
[ exch dup 0 get exch dup 3 get exch dup 12 get exch 15 get ]
{ Cam {sub} vop normal dot 0 ge } forall
%add add add 4 div 0 lt
or or or
} def
生产更新:测试是反向的。
更新:测试是无用的!你可以从图像中看到底部的部分不是向外的,当然,背面剔除并不能阻止把手从锅中显示出来。这需要去除隐藏的表面。而且因为Postscript不支持Z缓冲区,我猜它必须是一个二进制空间分区。所以它是ba帮我把书翻过来 更新:添加模型->世界变换以使物体直立
/Model -90 rotx def % model->world transform
/proj {
Model matmul 0 get % perform model->world transform
Cam {sub} vop % translate to camera coords
...
生产此产品。至此已完成程序。(使用矩阵库:)在ghostscript中,您可以通过按住
[enter]
来查看动画旋转
%!
%%BoundingBox: 109 246 492 487
%-109 -246 translate
(mat.ps)run %include matrix library
(det.ps)run %supplementary determinant function
/tok{ token pop exch pop }def
/s{(,){search{ tok 3 1 roll }{ tok exit }ifelse }loop }def
/f(teapot)(r)file def
/patch[ f token pop { [ f 100 string readline pop s ] } repeat ]def
/vert[ f token pop { [ f 100 string readline pop s ] } repeat ]def
/patch[ patch{ [exch { 1 sub vert exch get }forall ] }forall ]def
%vert == patch == %test data input flush quit
/I3 3 ident def % 3D identity matrix
/Cam [ 0 0 10 ] def % world coords of camera center viewpoint
/Theta [ 0 0 0 ] def % y-rotation x-rotation z-rotation
/Eye [ 0 0 15 ] def % eye relative to camera vp
/Rot I3 def % initial rotation seq
/Model -90 rotx def % model->world transform
/makerot {
Theta 0 get roty % pan
Theta 1 get rotx matmul % tilt
Theta 2 get rotz matmul % twist
} def
/proj {
Model matmul 0 get % perform model->world transform
Cam {sub} vop % translate to camera coords
Rot matmul % perform camera rotation
0 get aload pop Eye aload pop % extract dot x,y,z and eye xyz
4 3 roll div exch neg % perform perspective projection
4 3 roll add 1 index mul
4 1 roll 3 1 roll sub mul exch % (ez/dz)(dx-ex) (ez/dz)(dy-ey)
} def
/median { % [x0 y0 z0] [x1 y1 z1]
{add 2 div} vop % [ (x0+x1)/2 (y0+y1)/2 (z0+z1)/2 ]
} def
/decasteljau { % [P0] P1 P2 P3 . P0 P1' P2' P3' P3' P4' P5' P3
{p3 p2 p1 p0}{exch def}forall
/p01 p0 p1 median def
/p12 p1 p2 median def
/p23 p2 p3 median def
/p012 p01 p12 median def
/p123 p12 p23 median def
/p0123 p012 p123 median def
p0 p01 p012 p0123
p0123 p123 p23 p3
} def
/splitrows { % [b0 .. b15] . [c0 .. c15] [d0 .. d15]
aload pop % b0 .. b15
4 {
16 12 roll decasteljau
%8 4 roll
20 4 roll
} repeat
16 array astore
17 1 roll 16 array astore
} def
/xpose {
aload pop % 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
15 12 roll % 0 4 5 6 7 8 9 10 11 12 13 14 15 1 2 3
14 11 roll % 0 4 8 9 10 11 12 13 14 15 1 2 3 5 6 7
13 10 roll % 0 4 8 12 13 14 15 1 2 3 5 6 7 9 10 11
12 9 roll % 0 4 8 12 1 2 3 5 6 7 9 10 11 13 14 15
11 9 roll % 0 4 8 12 1 5 6 7 9 10 11 13 14 15 2 3
10 8 roll % 0 4 8 12 1 5 9 10 11 13 14 15 2 3 6 7
9 7 roll % 0 4 8 12 1 5 9 13 14 15 2 3 6 7 10 11
8 6 roll % 0 4 8 12 1 5 9 13 2 3 6 7 10 11 14 15
7 6 roll % 0 4 8 12 1 5 9 13 2 6 7 10 11 14 15 3
6 5 roll % 0 4 8 12 1 5 9 13 2 6 10 11 14 15 3 7
5 4 roll % 0 4 8 12 1 5 9 13 2 6 10 14 15 3 7 11
4 3 roll % 0 4 8 12 1 5 9 13 2 6 10 14 3 7 11 14
16 array astore
} def
/splitcols {
xpose
splitrows
xpose exch xpose
} def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitcols }forall ]def
/patch[ patch{ splitcols }forall ]def
/color {normal light dot 1 add 4 div
%1 exch sub
setgray} def
/visible { % patch . patch boolean
dup % p p
dup 3 get exch dup 0 get exch 12 get % p p3 p0 p12
1 index {sub} vop % p p3 p0 v0->12
3 1 roll {sub} vop % p v0->12 v0->3
cross /normal exch def
dup
[ exch dup 0 get exch dup 3 get exch dup 12 get exch 15 get ]
{ Cam {sub} vop normal dot 0 ge } forall
%add add add 4 div 0 lt
or or or
} def
/drawpatch {
% Four corners
%[ exch dup 0 get exch dup 3 get exch dup 12 get exch 15 get ]
visible {
[ exch
% control rows
%dup 4 get exch dup 5 get exch dup 6 get exch dup 7 get exch
%dup 11 get exch dup 10 get exch dup 9 get exch dup 8 get exch
% control columns
%dup 1 get exch dup 5 get exch dup 9 get exch dup 13 get exch
%dup 14 get exch dup 10 get exch dup 6 get exch dup 2 get exch
% Boundary curves
dup 8 get exch dup 4 get exch dup 0 get exch %curveto4
dup 14 get exch dup 13 get exch dup 12 get exch %curveto3
dup 7 get exch dup 11 get exch dup 15 get exch %curveto2
dup 1 get exch dup 2 get exch dup 3 get exch %curveto1
dup 0 get exch %moveto
pop ]
{ proj } forall
moveto curveto curveto curveto curveto
%moveto lineto lineto lineto lineto lineto lineto lineto closepath
%moveto lineto lineto lineto lineto lineto lineto lineto closepath
stroke
%flushpage flush (%lineedit)(r)file pop
}{
pop
}ifelse
} def
/R 20 def
/H -3 def
/ang 10 def
{
300 700 translate
1 70 dup dup scale div setlinewidth
% camera revolves around Y axis at height H, dist R
/Cam [ ang sin R mul H ang cos R mul ] def
/Theta [ ang H R atan 0 ] def % rotate camera back to origin
/Rot makerot def % squash rotation sequence into a matrix
patch {
drawpatch
} forall
pstack
showpage
%exit
/ang ang 10 add def
} loop
基于上的帮助,我被引导到一个子目标,即用一个计算行列式的函数来补充矩阵库
因此,这段代码通过了一些笨拙的初始测试,但我必须承认,它非常丑陋:
基本上,你是对的:它们有4个控制点(4个角)。同样的原则适用于3D bezier面片:如果你需要它们“平滑”连接,你必须对齐控制点。一旦你“平滑”连接了足够多的面片,你就可以代表你想要的每个曲面(具有合理的精度)很久以前,我编写了一个bezier补丁编辑器:请随意看看。我们用它制作了可乐罐、轮船、飞机;因此,对犹他茶壶进行一个很好的描述应该不难。哦,当我们编写它时,我们真的很欣赏这些gamasutra教程:这是一个很好的链接。谢谢。但是,哦,他们在底部提到了Foley。我有Foley。我可能应该看看。我是个笨蛋。:(我也有Foley,IIRC解释太短,无法完全掌握问题的解决方案(例如,编写工作代码)。我喜欢gamasutra文章的实用性。Foley Intro非常纤薄,但Foley&vanDam Fundamentals有很多内容,包括生成网格的伪代码。一些BSP链接:要求帮助生成BSP。注意:/splitcols
中的第二个xpose
应该是xpose exch xpose
,以应用传输到两个新补丁(以及另一个exch
,如果保留顺序很重要)。上述错误可能是“鸡丝”的原因在上一幅图像中的效果。通过使用斜接而不是斜接。在该函数期间,如何将矩阵存储在某个名称下,并使用一个函数提取矩阵的一个成员?然后您可以编写0 memb
而不是dup 0 get 0 get
。然后您可以编写内容
%!
%%BoundingBox: 109 246 492 487
%-109 -246 translate
(mat.ps)run %include matrix library
(det.ps)run %supplementary determinant function
/tok{ token pop exch pop }def
/s{(,){search{ tok 3 1 roll }{ tok exit }ifelse }loop }def
/f(teapot)(r)file def
/patch[ f token pop { [ f 100 string readline pop s ] } repeat ]def
/vert[ f token pop { [ f 100 string readline pop s ] } repeat ]def
/patch[ patch{ [exch { 1 sub vert exch get }forall ] }forall ]def
%vert == patch == %test data input flush quit
/I3 3 ident def % 3D identity matrix
/Cam [ 0 0 10 ] def % world coords of camera center viewpoint
/Theta [ 0 0 0 ] def % y-rotation x-rotation z-rotation
/Eye [ 0 0 15 ] def % eye relative to camera vp
/Rot I3 def % initial rotation seq
/Model -90 rotx def % model->world transform
/makerot {
Theta 0 get roty % pan
Theta 1 get rotx matmul % tilt
Theta 2 get rotz matmul % twist
} def
/proj {
Model matmul 0 get % perform model->world transform
Cam {sub} vop % translate to camera coords
Rot matmul % perform camera rotation
0 get aload pop Eye aload pop % extract dot x,y,z and eye xyz
4 3 roll div exch neg % perform perspective projection
4 3 roll add 1 index mul
4 1 roll 3 1 roll sub mul exch % (ez/dz)(dx-ex) (ez/dz)(dy-ey)
} def
/median { % [x0 y0 z0] [x1 y1 z1]
{add 2 div} vop % [ (x0+x1)/2 (y0+y1)/2 (z0+z1)/2 ]
} def
/decasteljau { % [P0] P1 P2 P3 . P0 P1' P2' P3' P3' P4' P5' P3
{p3 p2 p1 p0}{exch def}forall
/p01 p0 p1 median def
/p12 p1 p2 median def
/p23 p2 p3 median def
/p012 p01 p12 median def
/p123 p12 p23 median def
/p0123 p012 p123 median def
p0 p01 p012 p0123
p0123 p123 p23 p3
} def
/splitrows { % [b0 .. b15] . [c0 .. c15] [d0 .. d15]
aload pop % b0 .. b15
4 {
16 12 roll decasteljau
%8 4 roll
20 4 roll
} repeat
16 array astore
17 1 roll 16 array astore
} def
/xpose {
aload pop % 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
15 12 roll % 0 4 5 6 7 8 9 10 11 12 13 14 15 1 2 3
14 11 roll % 0 4 8 9 10 11 12 13 14 15 1 2 3 5 6 7
13 10 roll % 0 4 8 12 13 14 15 1 2 3 5 6 7 9 10 11
12 9 roll % 0 4 8 12 1 2 3 5 6 7 9 10 11 13 14 15
11 9 roll % 0 4 8 12 1 5 6 7 9 10 11 13 14 15 2 3
10 8 roll % 0 4 8 12 1 5 9 10 11 13 14 15 2 3 6 7
9 7 roll % 0 4 8 12 1 5 9 13 14 15 2 3 6 7 10 11
8 6 roll % 0 4 8 12 1 5 9 13 2 3 6 7 10 11 14 15
7 6 roll % 0 4 8 12 1 5 9 13 2 6 7 10 11 14 15 3
6 5 roll % 0 4 8 12 1 5 9 13 2 6 10 11 14 15 3 7
5 4 roll % 0 4 8 12 1 5 9 13 2 6 10 14 15 3 7 11
4 3 roll % 0 4 8 12 1 5 9 13 2 6 10 14 3 7 11 14
16 array astore
} def
/splitcols {
xpose
splitrows
xpose exch xpose
} def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitrows }forall ]def
/patch[ patch{ splitcols }forall ]def
/patch[ patch{ splitcols }forall ]def
/color {normal light dot 1 add 4 div
%1 exch sub
setgray} def
/visible { % patch . patch boolean
dup % p p
dup 3 get exch dup 0 get exch 12 get % p p3 p0 p12
1 index {sub} vop % p p3 p0 v0->12
3 1 roll {sub} vop % p v0->12 v0->3
cross /normal exch def
dup
[ exch dup 0 get exch dup 3 get exch dup 12 get exch 15 get ]
{ Cam {sub} vop normal dot 0 ge } forall
%add add add 4 div 0 lt
or or or
} def
/drawpatch {
% Four corners
%[ exch dup 0 get exch dup 3 get exch dup 12 get exch 15 get ]
visible {
[ exch
% control rows
%dup 4 get exch dup 5 get exch dup 6 get exch dup 7 get exch
%dup 11 get exch dup 10 get exch dup 9 get exch dup 8 get exch
% control columns
%dup 1 get exch dup 5 get exch dup 9 get exch dup 13 get exch
%dup 14 get exch dup 10 get exch dup 6 get exch dup 2 get exch
% Boundary curves
dup 8 get exch dup 4 get exch dup 0 get exch %curveto4
dup 14 get exch dup 13 get exch dup 12 get exch %curveto3
dup 7 get exch dup 11 get exch dup 15 get exch %curveto2
dup 1 get exch dup 2 get exch dup 3 get exch %curveto1
dup 0 get exch %moveto
pop ]
{ proj } forall
moveto curveto curveto curveto curveto
%moveto lineto lineto lineto lineto lineto lineto lineto closepath
%moveto lineto lineto lineto lineto lineto lineto lineto closepath
stroke
%flushpage flush (%lineedit)(r)file pop
}{
pop
}ifelse
} def
/R 20 def
/H -3 def
/ang 10 def
{
300 700 translate
1 70 dup dup scale div setlinewidth
% camera revolves around Y axis at height H, dist R
/Cam [ ang sin R mul H ang cos R mul ] def
/Theta [ ang H R atan 0 ] def % rotate camera back to origin
/Rot makerot def % squash rotation sequence into a matrix
patch {
drawpatch
} forall
pstack
showpage
%exit
/ang ang 10 add def
} loop
GS>[[1 0][0 1]] det
GS<1>=
1
GS>[[0 1][1 0]] det =
-1
GS>(mat.ps) run
GS>3 ident
GS<1>det =
1
GS>[[1 2 3][4 5 6][7 8 9]] det =
0
GS>
(mat.ps) run % use dot and cross from matrix library
/elem { % M i j
3 1 roll get exch get % M_i_j
} def
/det {
dup length 1 index 0 get length ne { /det cvx /typecheck signalerror } if
1 dict begin /M exch def
M length 2 eq {
M 0 0 elem
M 1 1 elem mul
M 0 1 elem
M 1 0 elem mul sub
}{
M length 3 eq {
M aload pop cross dot
}{ /det cvx /rangecheck signalerror } ifelse
} ifelse
end
} def