matlab实现图像四点投影变换

%% MATLAB实现投影变换
%% 2015-09-05 create by fengyun
%% 在屏幕上依次点四个点(左上->右上->左下->右下)，再点击回车键即可
clear all;
close all;
clc;

img1=imread('2.3.jpg');
img1=rgb2gray(img1);
[h1 w1]=size(img1);
mask=uint8(ones(h1,w1));    %二值模板，方便最后的合成

img2=imread('pai.jpg');
[h2 w2]=size(img2);

imshow(img1);
figure;imshow(img2);

p1=[1,1;w1,1;1,h1;w1,h1];
p2=ginput();        %依次点击公告牌左上、右上、左下、右下

T=calc_homography(p1,p2);   %计算单应性矩阵
T=maketform('projective',T);   %投影矩阵

[imgn X Y]=imtransform(img1,T);     %投影
mask=imtransform(mask,T);

T2=eye(3);
if X(1)>0, T2(3,1)= X(1); end
if Y(1)>0, T2(3,2)= Y(1); end
T2=maketform('affine',T2);      %仿射矩阵

imgn=imtransform(imgn,T2,'XData',[1 w2],'YData',[1 h2]);    %仿射
mask=imtransform(mask,T2,'XData',[1 w2],'YData',[1 h2]);

img=img2.*(1-mask)+imgn.*mask;  %合成
figure;imshow(img,[])

%% 透视投影，把lena贴到billboard上
%% 参考资料：http://www.cnblogs.com/tiandsp/p/4033071.html
function T = calc_homography(points1, points2)

xaxb = points2(:,1) .* points1(:,1);
xayb = points2(:,1) .* points1(:,2);
yaxb = points2(:,2) .* points1(:,1);
yayb = points2(:,2) .* points1(:,2);

A = zeros(size(points1, 1)*2, 9);
A(1:2:end,3) = 1;
A(2:2:end,6) = 1;
A(1:2:end,1:2) = points1;
A(2:2:end,4:5) = points1;
A(1:2:end,7) = -xaxb;
A(1:2:end,8) = -xayb;
A(2:2:end,7) = -yaxb;
A(2:2:end,8) = -yayb;
A(1:2:end,9) = -points2(:,1);
A(2:2:end,9) = -points2(:,2);

[junk1,junk2,V] = svd(A);
h = V(:,9) ./ V(9,9);
T= reshape(h,3,3);
end