Why does EdgeLighting on patches not work?

Question

Theo 2011-10-21

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/18983-why-does-edgelighting-on-patches-not-work

编辑： Will 2019-7-29

I'm trying to create a line on a surface with the EdgeLighting property set to phong (for example). An example code would be

[x,y,z] = sphere(40);
S = surf(x,y,z);
P = patch(x(32,:), y(32,:), z(32,:), 'k');
set(S, 'FaceLighting', 'phong', 'EdgeColor', 'none', 'FaceColor', 'inter');
set(P, 'EdgeLighting', 'phong', 'EdgeColor', 'k', 'FaceColor', 'none');
light

However, the line (patch) is not lit. In other words, replacing the ('EdgeLighting', 'phong') option to 'none' does not affect the figure. Does anybody spot the problem?

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Theo 2011-10-26

Bumping this up, as it seems to be a fairly elementary issue. Any comments?

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Patrick Kalita 2011-10-26

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/18983-why-does-edgelighting-on-patches-not-work#answer_25843

在 MATLAB Online 中打开

It is being lit, just not the way you're expecting based on how the patch was created.

For example, after doing this...

[x,y,z] = sphere(40);
S = surf(x,y,z);
P = patch(x(32,:), y(32,:), z(32,:), 'k');
set(S, 'FaceLighting', 'phong', 'EdgeColor', 'none', 'FaceColor', 'inter');
set(P, 'EdgeLighting', 'phong', 'EdgeColor', 'k', 'FaceColor', 'none');
light

... view it from this angle ...

view(0, 50)

... now from this angle...

view(-90, 50)

The patch's edge certainly does change color based on its orientation relative to the light. I'm guessing you were expecting the patch to be lit similar to the surface, which obviously is not what's happening now. Right now, the light seems to affect the entire edge uniformly. That's because the patch is using automatically calculated normals:

get( P, 'VertexNormals' )

You can see all the normals are pointing the same direction, namely down. Hence the light has the same effect on each vertex. Using the one-dimensional X, Y, and Z data that you gave to the patch, the normal calculator couldn't have possibly know that the data represented part of a sphere -- so it did the best it could. But you can tell the patch to use normals more appropriate for this situation. The normals of a spherical surface are trivial:

N = [ get(P, 'XData') get(P, 'YData') get(P, 'ZData') ]; % Caution! this only works for a sphere!!!
set( P, 'VertexNormals', N )
view(180, 40)

Now the patch edge is lit as I think you were expecting.

2 个评论
显示无隐藏无

Theo 2011-10-26

Perfect! Thanks. I also add that the surfnorm command will allow the normals to be computed from a given surface data. So that can be combined to define the lighting on the patches.

If anybody is wondering why we should bother with this, we can now modify the wireframe code by Ennis on the file exchange to use patches. This allows us to plot surfaces with dense meshes, but a less dense distribution of edges. However, the wireframe code previously used lines instead of patches, so one was unable to light the edges correctly.

Will 2019-7-29

编辑：Will 2019-7-29

在 MATLAB Online 中打开

For the record, the approach using surfnorm for any surface, not just the spherical example case, would be:

[nx, ny, nz] = surfnorm(x, y, z);
N = [nx(32,:); ny(32,:); nz(32,:)]';
set(P, 'VertexNormals', N)

I'm interested in how to apply this for a line across a surface that is not defined by the mesh of the surface. I've asked this in a separate question here.

请先登录，再进行评论。