mesh3d: Different lighting results with different hardware

[jackparmer]

Consider this codepen:

http://codepen.io/monfera/full/ggByRW

On my 12 in Macbook, I see this:

@etpinard 's Thinkpad and @monfera 's Macbook show this:

Another example - consider this codepen:

http://codepen.io/monfera/pen/XjxZjZ

Result on my 12 in Macbook:

Result on larger Macbooks and Thinkpad:

---

I tried this in Chrome, Safari, and Firefox with the same result.

Computer specs:

Chrome specs:

[rreusser]

Hmm… looks like possible flipped normals to me. I'd suggest maybe gl_FrontFacing isn't properly supported, but 2016 seems pretty recent to fail on that, and reading the stack overflow issue more closely… not sure that's it.

See: https://github.com/gl-vis/gl-surface3d/blob/master/shaders/fragment.glsl#L28-L30

http://stackoverflow.com/questions/24375171/is-there-a-reliable-alternative-to-gl-frontfacing-in-a-fragment-shader

[etpinard]

cc @dfcreative which have insight about this.

[monfera]

I don't have access to this machine but guessing is always fun:

• perhaps the gl floats are lowp - I think the standard is quite permissive about precision
• or we're using an extension that silently causes an issue

One thing to try: visiting a bunch of WebGL content on the web to see if those work properly.

[jackparmer]

Quick update: It doesn’t seem to matter what value I set for diffuse or specular. I can set them to 0.1 or 1 or 1000 and nothing changes.

[monfera]

Do these work fine? http://webglsamples.org/

[rreusser]

Gotchas like pow(-abs(x), 2.0) being inconsistent across platforms bit me a couple times, but I didn't happen to catch anything obvious with a quick glance.

[jackparmer]

Do these work fine? http://webglsamples.org/

Yeah, nothing looks "off" with those. I can also change the diffuse and specular settings here:

http://multivis.net/lecture/phong.html

[monfera]

Not that it should inherently make it fail, but mesh3d uses the Cook-Torrance model. E.g. here one can choose the reflection model in a dropdown: http://www.calebjohnston.com/webgl.html

[jackparmer]

Seems to work too - I see this?

[rreusser]

Oops, sorry. I had the wrong module.

Maybe………(though this is necessarily positive, isn't it?)? https://github.com/stackgl/glsl-specular-cook-torrance/blob/master/index.glsl#L26

[monfera]

One more thing - possibly relating to the above possibility (lower precision floats): I needed to tweak the EPSILON value in the normals calculation. The brain mesh is made up of very small triangles. With the wrong EPSILON value, whole swaths of lighting just don't happen. This is my current best guess.

Way to test it: load some coarser mesh3d content from eg. plot.ly and see if that works fine.

Also, my old commits made EPSILON configurable from within Plotly: mikolalysenko/normals@a519d8a#diff-f02129f250a223ce2d22f0b6e4fa7297

[jackparmer]

Hm, yeah, larger triangles seem OK:

https://plot.ly/~empet/13481/delaunay-triangulation-and-alpha-shapes-for-a-set-of-3d-points-1-delaunay-triang/

https://plot.ly/~RPlotBot/3008/moebius-band-triangulation/

(I found thesemesh3d plot.ly creations via: https://plot.ly/feed/?q=plottype:mesh3d)

[jackparmer]

Changing diffuse and specular still doesn't seem to have an effect though:

I can actually see visible changes with diffuse when ambient is turned down, I don't see any changes with specular.

[monfera]

I added the epsilon attribs to the codepen, with their respective default values: http://codepen.io/monfera/pen/XjxZjZ

Could you test if lower values make it work? E.g. 1e-9 instead of 1e-6, or 1e-15 instead of 1e-12. Or some much more ambitious change.