Much cleaner edge rendering when enabling logarithmic depth buffer

[tmarti]

Descrption

Use the same gradient technique has here #1162 (comment) for much better visual quality in edge rendering when the logarithmic depth buffer is enabled.

Screenshots: Before / After

The screenshots below were taken with a far plane distance equal to 1M km!

Before (i)

After (i)

Before (ii)

After (ii)

Explanation

Sub-pixel z-fighting (explained here #1162 (comment)) makes edge drawing worse when logarithmic depth buffer is enabled, possibly because the dynamic range of the Z values get compressed (via the log function) together with much bigger far plane distances being used with logarithmic z-buffer.

As the rendering of edged surfaces happens in two stages in xeokit-sdk...

• a) first, regular surfaces are rendered, and this initializes the Z-buffer
• b) on top of it, edges are rendered, which then z-fight with the pre-initialized z-buffer in "a)"

... the reduced dynamic range affects more the case where log-depth-buffer is enabled.

This PR makes use of the gradient trick so, during the regular surface rendering, their z-values are "pushed" far away from the camera to the same distance where their immediate furthest neighbour pixel is.

This makes the z-buffer pre-initialization much more friendly with stage "b)", and fixes the sub-pixel z-fighting problem.

[Amoki]

It may fix #651, I'll check!

Thanks :D

[tmarti]

Just checked, it should indeed fix it 🎁