Standardize how hidden geometry gets clipped in the vertex shader

[lilleyse]

There are several systems that batch features together into a single draw call and hide individual features in the vertex shader by accessing their show property in the batch table and forcing them to be clipped. All these systems have a slightly different way of doing this.

• b3dm/i3dm uses gl_Position *= show; in Cesium3DTileBatchTable.js
• PointCloud.js (for 3D Tiles) uses gl_Position.w *= float(show); (see 8479 - Fix grey screen on Other/Unclassified toggle #8538) and gl_PointSize *= float(show);
• Primitive.js distance display condition uses gl_Position *= show;
• PointPrimitiveCollection
  • Uses gl_PointSize = totalSize * show; and gl_Position *= show; and sets alpha to 0.0 (see Fixed Hidden Point Primitive Bug #8054).
  • For distance display conditions it uses positionEC.xyz = vec3(0.0, 0.0, 1.0); (see Clip point in PointPrimitiveCollection properly #8542).
  • For small eye distance scales and eye distance translucency it uses positionEC.xyz = vec3(0.0);.
• BillboardCollection uses positionEC.xyz = vec3(0.0); for small eye distance scales and eye distance translucency.
• There may be more I haven't noticed

The gl_Position.w *= show approach is probably the easiest to use everywhere and is the easiest one to implement as a built-in function. Another thing to look at is how log depth affects all this, especially anything in DerivedCommand.createLogDepthCommand.

[lilleyse]

Another option is setting one of the coordinates to a NaN value. See

• http://www.elopezr.com/a-macro-view-of-nanite/
• https://twitter.com/MittringMartin/status/1107770271119736833

From @loshjawrence

[ptrgags]

I wish I had seen this sooner, for ModelExperimental we've been doing yet a different way: positionMC *= 0.0 (see CPUStylingStageVS.glsl). This makes an assumption that WebGL won't render zero-area triangles as a single pixel. We'd have to refactor the shader a bit. If we just switched to gl_Position, the value is likely to get clobbered by later shader stages.

But also, in this forum thread I saw a use case where a user wanted to hide a specific building in a vertex shader so something else could be rendered in its place. We'd need another variable in custom shaders to hide a vertex, as positionMC doesn't have a w component.