Cycles and Eevee don't agree, and I think Cycles' implementation of the bump node is wrong. Where do I go to report this?

This is why I think so:

I made a tool that "carves" a bump map I want to use. The shape is like a V, it's a linear gradient from black to white. Imagine it like a tool bit. See example:


This works well and it looks correct in Blender (I make sure to use the "Non-Color" setting): nodes and render

But now I try to use nodes to soften this shape, to ease into the transition, and give it rounded edges, like so:

rounded carve

To do so, I transform the bump map with a curve (I've tried vector and RGB curves, they behave about the same). The result is hard to achieve and shows aliasing, but looks OK in Eevee, see here:

enter image description here

Now compare this to Cycles, where it doesn't work:

enter image description here

No matter what I do, I can't get the peaks and valleys to curve smoothly like I can in Eevee.


Here with a much lower bump value, the sides are still much more straight than expected:

unexpected linear sides

Interestingly, at lower bump values the difference between Eevee and Cycles disappears.

If you want to try this yourself, here's the 16-bit grayscale Tiff: out-cone511d.tif

  • 3
    $\begingroup$ It'd be really surprising if your model was scaled such that a 1.0 distance on the bump node was appropriate. Take a look at using the bump map as displacement on a subdivided model instead-- the scale appropriate for displacement should be the distance you use for the bump. It's measured in Blender units. I suspect you're just giving crazy high levels of bump and Eevee's limited bump mapping is covering up the problems more than Cycles. $\endgroup$
    – Nathan
    Apr 17, 2021 at 14:50
  • 2
    $\begingroup$ @Nathan is right :). The bump Distance is in meters. To get useful result, try lowering the distance value (0.001 for milimeters) $\endgroup$ Apr 17, 2021 at 15:36
  • $\begingroup$ 1.0 is excessive, but for this application I want the extremes to be handled correctly. I'll edit my post and show a lesser bump, showing the sides are barely curved, whereas my curve looks like it would apply more distortion. The curve isn't softening the bump enough. $\endgroup$ Apr 17, 2021 at 17:27


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