Enable MeshShape to override alpha value

[jslee02]

This PR fixes that it was not able to change alpha value when the color mode of MeshShape is MATERIAL_COLOR.

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[mxgrey]

I'm not sure how I feel about adding an AlphaMode concept to the MeshShape. I suspect that would make it harder for users to alter transparency to their meshes, because they'd have to iterate through their shapes and find which shapes are of the type MeshShape and then toggle the value for that shape.

What if instead we always multiply the mesh material's alpha value by the VisualAspect's alpha value? That way if the VisualAspect has an alpha value of 1.0, then the rendering will just have the normal material alpha value (so materials like glass can still be transparent), and the closer the VisualAspect's alpha value gets to 0.0, the more transparent the mesh will be, until it's invisible.

[codecov]

Codecov Report

Merging #1315 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1315      +/-   ##
==========================================
+ Coverage   57.12%   57.12%   +<.01%     
==========================================
  Files         366      366              
  Lines       27055    27052       -3     
==========================================
  Hits        15454    15454              
+ Misses      11601    11598       -3

Impacted Files	Coverage Δ
dart/dynamics/MeshShape.cpp	66.12% <0%> (+1.56%)	⬆️

[jslee02]

I see your point. I think this is because setting alpha (and the color) of MeshShape is already tricky. Users could have different expectations, and it seems your proposal is another behavior (maybe I call it blending mode?). One corner case I can think of is that it cannot handle the case that the user wants to set 1.0 alpha when the embedded alpha is lower than 1.0. So I wanted to make it explicit the behavior.

I would go with any better way as long as it can handle all the cases. My bottom line is that we should be able to set any value of the mesh alpha regardless of the color mode.

If we go with adding alpha mode, then I'm open to the default mode.

This is a little off topic, but I would like to add another color mode to MeshShape: tinting (or blending) the embedded mesh color (and texture) with the shape color. I haven't tested it myself recently but according to #739 we already allow it unintentionally. It would be good to make it explicit in that mode so that we can also disable it. One good use case of tinting is that tinting a skeleton or a mesh that is in a collision.

[mxgrey]

If we go with adding alpha mode, then I'm open to the default mode.

That sounds like a reasonable compromise. We could have the "alpha blend" mode be the default while still having a mode selection available from the MeshShape interface.

My only concern with loading so many "modes" into the MeshShape is that (by design) the specific shape type of a Shape instance is usually opaque to the user without manually inspecting the types, and I think that tends to make those APIs less accessible to users, to the point that a user might not realize that such functionality is available.

My ideal preference would be to have a way to manage these settings from the base Shape type, but I admit that I don't have a good suggestion for how to do that, since these modes are so specific to MeshShape.

I would like to add another color mode to MeshShape: tinting

I can definitely see the value of that 👍

[jslee02]

Updated based on the comments. Hope the changes agrees with the compromise.

My ideal preference would be to have a way to manage these settings from the base Shape type

Agree. I also couldn't come up with a better solution. Let's revisit this once we have more neat solution.

I saved the tinting option for future PRs.

[mxgrey]

Thanks! Hopefully this makes transparency a lot easier for users 👍