What we need to be doing is ignore subtraction of slopes of A.stl, which are too steep...

we are subtracting (3D Subtraction) A.stl from B.stl. What we need to be doing is ignore subtraction of slopes of A.stl, which are too steep. PS: we can detect which points have greater than entered slope in A.stl

[DGM]

It's not clear what A and B are, how they relate geometrically, or what "subtraction" means in this context -- specifically what subtraction can mean if it's a slope-dependent process. It's also unclear how slope is being measured.

I suspect that A and B may be something like simple height maps, in which case I suspect that "ignore subtraction" is probably supposed to actually mean "clamp the minimum result". Otherwise, you're asking for a bunch of step discontinuities.

Answer 1

[DGM]

If I'm gonna guess, I might as well invest in it.

unzip saddles.zip

% these two files have directly correlated vertex lists
% this is not typical, so don't expect to meaningfully subtract random
% objects like this.
B = stlread('saddletop.stl');
A = stlread('saddletop90.stl');
isequal(A.Points(:,1:2),B.Points(:,1:2)) % x,y locations match

ans = logical
   1

subplot(1,2,1); trisurf(B)
subplot(1,2,2); trisurf(A)

You could ignore cases where some angle criteria is exceeded.

% get some sort of "angle"
fn = vertexNormal(A);
th = acosd(fn(:,3)./vecnorm(fn,2,2)); % elevation angle
ignorethese = th > 45;
V = B.Points;
V(:,3) = V(:,3) - A.Points(:,3);
V(ignorethese,3) = B.Points(ignorethese,3);
T = triangulation(B.ConnectivityList,V);

% display it using patch()
figure
patch('faces',T.ConnectivityList,'vertices',T.Points, ...
    'facecolor','w','edgecolor','k');
view(3); camlight; view(10,33)
axis equal; grid on
xlabel('X'); ylabel('Y'); zlabel('Z')

... or you could do something else.

% just do some sort of constrained difference
V = B.Points;
V(:,3) = max(V(:,3) - A.Points(:,3),-0.5); % clamp at some level
T = triangulation(B.ConnectivityList,V);

% display it using patch()
figure
patch('faces',T.ConnectivityList,'vertices',T.Points, ...
    'facecolor','w','edgecolor','k');
view(3); camlight; view(10,33)
axis equal; grid on
xlabel('X'); ylabel('Y'); zlabel('Z')

... or something entirely different.