Shadows example looks weird

[cosw0t]

Hi there!

I'm trying to run the Python Rendering/Shadows example with vtk8.1.0, on windows 10 (through anaconda3).

The result seems a bit odd to me. I can't understand where the rectangular shaded area comes from. What is casting that long straight shadow on the floor? It can't be from the sphere as a sphere would only cast oblong shadows.

the code is just as in master, the lights are at [0, 1, 0.2], and [1.0, 1.0, 1.0]

[ajpmaclean]

If you use cow.g you get the image as on the Web site. If no image is supplied it defaults to a sphere on a plane. In the case of the sphere and plane some of the shadowing may be from the plane. Try moving the lights.

On Wed, 20 May 2020, 20:04 michele-arrival, [email protected] wrote:

Hi there!

I'm trying to run the Shadows Example with vtk8.2.0, on windows (through anaconda3)

The result seems a bit odd to me. I can't understand where the rectangular shaded area comes from. What is casting that shadow on the floor?

[image: image] https://user-images.githubusercontent.com/54891577/82433420-5118d380-9a89-11ea-8838-f9038670c656.png

Am I wrong at seeing the same thing in the image in this page? https://lorensen.github.io/VTKExamples/site/Python/Rendering/Shadows/

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Andrew Maclean

[cosw0t]

Hi Andrew, and thanks for replying to my issue.

I've place some spheres where the lights would be, and as you can see they are all above. I can't understand how the plane can cast a shadow on itself like that?

if i use the cow as you suggested (and I move both lights up by 1 unit, +[0, 1., 0.] i get this which also looks weird - and different from the image in https://lorensen.github.io/VTKExamples/site/Python/Rendering/Shadows/:

Finally, if i switch to vtk 8.2.0, the result is the following:

which is similar to the image in https://lorensen.github.io/VTKExamples/site/Python/Rendering/Shadows/ But why is the light not reaching all the way until the end of the plane? And why is the behaviour so much different between v8.1.0 and 8.2.0? is there a hidden light somewhere?

thanks, Michele

[ajpmaclean]

Look at ShadowsLightsDemo and you will see that the light is actually a frustum, hence the rectangular cross-section on the plane. I'll do a Python version of this soon. It will appear in the master but not on the web pages at this point in time.

If you use this as a plane source, you can see the illuminated rectangular areas quite clearly:

    plane = vtk.vtkPlaneSource()
    expand = 1
    x = bounds[1] - bounds[0] + (rnge[0] * expand)
    y = -0.5
    z = bounds[5] - bounds[4] + (rnge[2] * expand)

    plane.SetOrigin(-x, y, -z)
    plane.SetPoint1(x, y, -z)
    plane.SetPoint2(-x, y, z)
    plane.SetXResolution(100)
    plane.SetYResolution(100)

As to the differences between the versions of VTK this is to be expected since there is a process of continuous improvement going on. As you can see the VTK 8.2 rendering is much better than VTK 8.1. The VTK 9.0 rendering is even better.

Regards Andrew

[ajpmaclean]

Here is a Python version of ShadowsLightsDemo : ShadowsLightsDemo.zip

Regards Andrew

[cosw0t]

Hi Andrew, thanks for the explanation. Thanks also for the ShadowLightsDemo.

Yes i see the rectangular areas clearly. But why does the light frustum cover a smaller volume than the light cone?

In a similar way I notice in the ShadowLightsDemo that while the pyramid shown as frustum for white light embraces the whole expected cone volume, and does indeed reach large part of the plane, yet the plane is not illuminated entirely. Again the question is why the frustum doesn't embrace the whole cone, and why does it not match what the prop seem to show?

Thanks

[ajpmaclean]

I cam answer that, it is probably best if you ask on the VTK Discourse forum.

[cosw0t]

Thank you very much.

For reference: https://discourse.vtk.org/t/shadowlightsdemo-extents-of-shadows-light-frustum-darkened-areas/3410