向量化代码以从坐标向量创建由坐标窗口组成的 3 维数组

Question

我有一个坐标向量comp_points 在每一行中保存一个图像坐标对。现在我想创建一个数组comp_windows，在comp_points 的坐标周围保存一个图像的nxm-windows。这些窗口应沿comp_windows 的第三维对齐。 我解决了这样的任务：

I2=randi([0 255],[500 500]);
comp_points=randi([10 490],[20 2]);
delta_u_window=5;
delta_v_window=5;
for ii=1:size(comp_points,1)
    comp_windows(:,:,ii)=I2(...
        comp_points(ii,1)-delta_u_window:...
        comp_points(ii,1)+delta_u_window,...
        comp_points(ii,2)-delta_v_window:...
        comp_points(ii,2)+delta_v_window);
end

现在我觉得我可以在没有 for-loop 的情况下使用连接或索引表达式或其他东西来做到这一点，但我想不通。

Answer 1

您已经拥有slicing 的操作，无需任何计算。所以，我不确定是否值得对其进行矢量化，但无论如何让我们在bsxfun 的大力帮助下将其推出 -

% Get range arrays
r1 = [-delta_u_window : delta_u_window];
r2 = [-delta_v_window : delta_v_window];

% Get row and column indices for all points in comp_points
r = bsxfun(@plus,r1(:),comp_points(:,1).');
c = bsxfun(@plus,r2(:),comp_points(:,2).');

% Next up, the work is to combine those row and column indices in a meshed-way

% Get 3D version of r and c - Keeping their last dim aligned and "spreading
% out" their first dims against each others. Then, perform elementwise
% summations to give us a summed up array of indices, indexing into which
% would give us the desired output.
r3D = reshape(r,size(r,1),1,[]);
c3D = reshape((c-1)*size(I2,1),1,size(c,1),[]);
out = I2(bsxfun(@plus, r3D, c3D));

对于permute爱好者，我们可以将最后三个步骤替换为一个步骤，像这样-

I2(bsxfun(@plus, permute(r,[1,3,2]), permute((c-1)* size(I2,1),[3,1,2])))