William F Pokorny <ano### [at] anonymousorg> wrote:
>
> Just something I happened to see while looking into other height_field
> questions of late.
>
> The HF zero (y as image/fnct evaluated) and z HF result should cleanly
> show up no matter scaling! At best it is today noisy.
>
> Looks to be an issue back through v3.7 stable at least. I think given
> the noise it's likely some numerical and/or bounding issue rather than
> the actual HF mesh. We'll see.
>

[Running v3.8.0 beta 1 in Windows 10]

I ran a bunch of animation tests of
height_field{
      function 500,500 {0}
.....

....while changing various values. Here are some results:

If the HF is given a solid color pigment, I don't see any speckles or odd
coincident-surface problems at all.

But if I give it
       pigment{ gradient y color_map{[0 rgb 0][1 red 10]}}
I do see the speckles.

Apparently, the color_map is repeating from the top of its red color, but from
'below' the HF.

I also ran some tests while slightly varying the function value itself, and also
used min_extent/max_extent to see what values they would return. The results are
a bit odd, to say the least:

(using the gradient y color_map):

function 500,500 {0} has the speckles
MIN_EXT = <0.0000000000, -0.0000000023, 0.0000000000> note minus sign for y
MAX_EXT = <0.0000000000, -0.0000000023, 0.0000000000>

function 500,500 { 0 + .0000152} has the speckles.
The resulting height_field size:
MIN_EXT = <0.0000000000, -0.0000000023, 0.0000000000> -- same as above
MAX_EXT = <0.0000000000, -0.0000000023, 0.0000000000>

function 500,500 { 0 + .0000153} shows no speckles at all -- but with an abrupt
change in the y value:
MIN_EXT = <0.0000000000, 0.0000022865, 0.0000000000>
MAX_EXT = <0.0000000000, 0.0000022865, 0.0000000000>
---------
Now, if I change the function additions to subtractions:
function 500,500 { 0 - .0000152} has the speckles.
MIN_EXT = <0.0000000000, -0.0000000023, 0.0000000000> -- same as addition
MAX_EXT = <0.0000000000, -0.0000000023, 0.0000000000> -- ditto

function 500,500 { 0 - .0000152} -- The planar HF jumps up to y=1 (almost!) as I
kind of expected... but no speckles at all again, which was surprising.
MIN_EXT = <0.0000000000, 0.9999847412, 0.0000000000> -- y is not quite 1.0
MAX_EXT = <0.0000000000, 0.9999847412, 0.0000000000>

So from these stats and results, my *guess* is that there could be a very slight
'bias'(?) in the HF creation code-- or function-to-HF process? -- whereby the HF
is not actually from 0-1, but (0 minus a small value) to (1 minus the small
value.) OR, the same with the color_map mechanism. Although, the abrupt 'jumps'
between .0000152 and .0000153 are puzzling.

The tests were interesting at least!

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