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gforget |
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function [ow]=gcmfaces_quadmap(px,py,ox,oy); |
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%[ow]=gcmfaces_quadmap(px,py,x,y); |
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%object: compute bilinear interpolation coefficients for x(i,:),y(i,:) |
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% in px(i,:),py(i,:) by remapping x(i,:),y(i,:) along with the |
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% px(i,:),py(i,:) quadrilateral to the 0-1,0-1 square. |
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%inputs: px,py are Mx4 matrices where each line specifies one quad |
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%(optional) ox,oy are MxP position matrices |
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%outputs: pw are the MxPx4 bilinear interpolation weights |
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doDisplay=0; |
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if nargin==0; |
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%the following test case is based upon https://www.particleincell.com/2012/quad-interpolation/ |
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%(see end of routine for alternatives) |
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px = [-1, 8, 13, -4]; |
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py = [-1, 3, 11, 8]; |
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ox=0; oy=6; |
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doDisplay=1; |
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end; |
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if nargin==2; ox=[]; oy=[]; end; |
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%solve linear problem for a,b vectors (knowing px,py) |
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% logical (l,m) to physical (x,y) mapping is then |
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% x=a(1)+a(2)*l+a(3)*m+a(2)*l*m; |
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% y=b(1)+b(2)*l+b(3)*m+b(2)*l*m; |
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% A=[1 0 0 0;1 1 0 0;1 1 1 1;1 0 1 0]; AI = inv(A); |
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% AI=[1 0 0 0;-1 1 0 0;-1 0 0 1; 1 -1 1 -1]; |
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% a = AI*px'; |
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% b = AI*py'; |
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tmp1=px(:,1); |
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tmp2=-px(:,1)+px(:,2); |
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tmp3=-px(:,1)+px(:,4); |
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tmp4=px(:,1)-px(:,2)+px(:,3)-px(:,4); |
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a=[tmp1 tmp2 tmp3 tmp4]; |
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% |
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tmp1=py(:,1); |
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tmp2=-py(:,1)+py(:,2); |
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tmp3=-py(:,1)+py(:,4); |
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tmp4=py(:,1)-py(:,2)+py(:,3)-py(:,4); |
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b=[tmp1 tmp2 tmp3 tmp4]; |
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%chose between the two mapping solutions dep. on sum of interior angles |
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[angsum]=gcmfaces_polygonangle(px,py); |
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sgn=NaN*px(:,1); |
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ii=find(abs(angsum-360)<1e-3); sgn(ii)=1; |
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ii=find(abs(angsum+360)<1e-3); sgn(ii)=-1; |
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ii=find(isnan(angsum)); |
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if length(ii)>0; |
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warning('edge point was found'); |
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keyboard; |
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end; |
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%solve non-linear problem for pl,pm (knowing px,py,a,b) |
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% physical (x,y) to logical (l,m) mapping |
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% |
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% quadratic equation coeffs, aa*mm^2+bb*m+cc=0 |
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if ~isempty(ox); |
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x=[px ox]; y=[py oy]; |
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else; |
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x=px; y=py; |
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end; |
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a=reshape(a,[size(a,1) 1 size(a,2)]); a=repmat(a,[1 size(x,2) 1]); |
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b=reshape(b,[size(b,1) 1 size(b,2)]); b=repmat(b,[1 size(x,2) 1]); |
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sgn=repmat(sgn,[1 size(x,2)]); |
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% |
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aa = a(:,:,4).*b(:,:,3) - a(:,:,3).*b(:,:,4); |
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bb = a(:,:,4).*b(:,:,1) -a(:,:,1).*b(:,:,4) + a(:,:,2).*b(:,:,3) ... |
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- a(:,:,3).*b(:,:,2) + x.*b(:,:,4) - y.*a(:,:,4); |
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cc = a(:,:,2).*b(:,:,1) -a(:,:,1).*b(:,:,2) + x.*b(:,:,2) - y.*a(:,:,2); |
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%compute m = (-b+sqrt(b^2-4ac))/(2a) |
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det = sqrt(bb.*bb - 4.*aa.*cc); |
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pm = (-bb+sgn.*det)./(2.*aa); |
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%compute l by substitution in equation system |
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pl = (x-a(:,:,1)-a(:,:,3).*pm)./(a(:,:,2)+a(:,:,4).*pm); |
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ow=[]; |
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if ~isempty(ox); |
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tmp1=(1-pl(:,5:end)).*(1-pm(:,5:end)); |
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tmp2=pl(:,5:end).*(1-pm(:,5:end)); |
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tmp3=pl(:,5:end).*pm(:,5:end); |
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tmp4=(1-pl(:,5:end)).*pm(:,5:end); |
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ow=cat(3,tmp1,tmp2,tmp3,tmp4); |
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end; |
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if doDisplay; |
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cols='brgm'; |
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% |
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figureL; |
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%plot original quad and obs |
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subplot(1,2,1); |
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plot([px px(1)],[py py(1)],'k-','LineWidth',2); hold on; |
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for pp=1:4; |
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plot(px(pp),py(pp),[cols(pp) '.'],'MarkerSize',64); |
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end; |
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aa=axis; aa(1)=aa(1)-1; aa(2)=aa(2)+1; aa(3)=aa(3)-1; aa(4)=aa(4)+1; axis(aa); |
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grid on; plot(ox,oy,'k.','MarkerSize',64); |
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% |
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subplot(1,2,2); |
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plot([pl(1:4) pl(1)],[pm(1:4) pm(1)],'k-','LineWidth',2); hold on; |
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for pp=1:4; plot(pl(pp),pm(pp),[cols(pp) '.'],'MarkerSize',64); end; |
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aa=axis; aa(1)=aa(1)-1; aa(2)=aa(2)+1; aa(3)=aa(3)-1; aa(4)=aa(4)+1; axis(aa); |
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grid on; plot(pl(5:end),pm(5:end),'k.','MarkerSize',64); |
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end; |
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%swap px and py: |
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% ppx=px; ppy=py; |
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% px=ppy; py=ppx; |
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% |
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%shift quad corners: |
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% nn=0; |
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% px=circshift(px,[0 nn]); |
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% py=circshift(py,[0 nn]); |
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% |
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%flip quad corners (see sum of interior angles): |
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% ppx=flipdim(px,2); ppy=flipdim(py,2); |
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% px=ppx; py=ppy; |
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% |
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%test cases for x,y locations: |
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% ox=0; oy=6;%inside point : sum(ang2)=+-360 |
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% ox=(px(1)+px(2))/2; oy=(py(1)+py(2))/2;%edge limit case : sum(ang2)=+-180 |
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% ox=(px(1)+px(4))/2; oy=(py(1)+py(4))/2;%edge limit case : sum(ang2)=-180 |
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% ox=px(1); oy=py(1);%corner limit case : sum(ang2)=NaN |
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% ox=0; oy=12;%outside point : sum(ang2)=0 |
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