strange output from Cycles Vector Curves Node #46915

New Issue

bassam kurdali · 2015-12-02T20:35:33+01:00

bassam kurdali commented

2015-12-02 20:35:33 +01:00

System Information
GNULinux (fedora 22) intel cpu, nvidia gpu

Blender Version
Broken: (example: 2.69.7 e5e1010
Worked: ?

Short description of error
when Vector Curves are not linear, and we have a point at 0,0 feeding in zero doesn't give zero but some non zero output. I'm unable to predict or understand what the math going on here is supposed to be...

Exact steps for others to reproduce the error

download the attached .blend{F262008}
open it in blender
turn on viewport rendering
try plugging in R then G then B into the Emission Strength, you get a black cube
the same for Y and Z, but if you plug in X you get some greyness
plug in Y, you should get a black cube again
now create a point in the middle of the Y curve and move it away from the linear curve
You now get a grey cube!

what I expect is the cube to stay black since the input is zero/zero/zero and all the curves have a point at zero, zero

**System Information** GNULinux (fedora 22) intel cpu, nvidia gpu **Blender Version** Broken: (example: 2.69.7 e5e1010 Worked: ? **Short description of error** when Vector Curves are not linear, and we have a point at 0,0 feeding in zero doesn't give zero but some non zero output. I'm unable to predict or understand what the math going on here is supposed to be... **Exact steps for others to reproduce the error** - download the attached .blend{[F262008](https://archive.blender.org/developer/F262008/vector.blend)} - open it in blender - turn on viewport rendering - try plugging in R then G then B into the Emission Strength, you get a black cube - the same for Y and Z, but if you plug in X you get some greyness - plug in Y, you should get a black cube again - now create a point in the middle of the Y curve and move it away from the linear curve - You now get a grey cube! what I expect is the cube to stay black since the input is zero/zero/zero and all the curves have a point at zero, zero

bassam kurdali commented

2015-12-02 20:35:33 +01:00

Changed status to: 'Open'

Sergey Sharybin was assigned by bassam kurdali

2015-12-02 20:35:33 +01:00

bassam kurdali commented

2015-12-02 20:35:33 +01:00

Added subscriber: @BassamKurdali

Sergey Sharybin commented

2015-12-03 11:46:51 +01:00

Added subscriber: @brecht

Sergey Sharybin commented

2015-12-03 11:46:51 +01:00

This isn't really a bug, just some specific of how curve mapping works in Cycles in general and how Vector Curve works in particular.

The thing here is -- any curve mapping in Cycles is expected to be Use Cllipping-ed, because internally the curve is getting rasterized in [0..1] range.

This makes color mapping working as expected (mainly), because colors are not negative. Only issue here is that Cycles will clip the input (solving this is in my TODO).

But vectors are usually negative, so what Curve Mapping node is doing is it maps input values from [-0.5..0.5] range to [0..1], applies the mapping and then goes back to [-0.5..0.5] range. This makes it so curve point corresponding to curve's x=0.5 is actually defines mapping for the input value of 0.

it's not really a bug, but quite confusing logic here and not sure how we can solve this without intruding regressions.

What we can do, is to solve the clipping limitation, make it so Curve Mapping is default to -0.5..0.5 range, do some do-versions code to preserve existing renders. IMO, this is most friendly approach.

@brecht, do you have any inputs here from your side?

This isn't really a bug, just some specific of how curve mapping works in Cycles in general and how Vector Curve works in particular. The thing here is -- any curve mapping in Cycles is expected to be `Use Cllipping`-ed, because internally the curve is getting rasterized in `[0..1]` range. This makes color mapping working as expected (mainly), because colors are not negative. Only issue here is that Cycles will clip the input (solving this is in my TODO). But vectors are usually negative, so what Curve Mapping node is doing is it maps input values from `[-0.5..0.5]` range to `[0..1]`, applies the mapping and then goes back to `[-0.5..0.5]` range. This makes it so curve point corresponding to curve's `x=0.5` is actually defines mapping for the input value of `0`. it's not really a bug, but quite confusing logic here and not sure how we can solve this without intruding regressions. What we can do, is to solve the clipping limitation, make it so Curve Mapping is default to `-0.5..0.5` range, do some do-versions code to preserve existing renders. IMO, this is most friendly approach. @brecht, do you have any inputs here from your side?

Sergey Sharybin commented

2015-12-15 12:34:39 +01:00

Submitted a patch which implements the logic above, D1672.

@BassamKurdali, mind giving it a whirl and see if it's indeed behavior you was dreaming about?

Submitted a patch which implements the logic above, [D1672](https://archive.blender.org/developer/D1672). @BassamKurdali, mind giving it a whirl and see if it's indeed behavior you was dreaming about?

blender-admin commented

2015-12-31 16:40:22 +01:00

This issue was referenced by c81e6ffdf9

This issue was referenced by c81e6ffdf942a7e8ac58ef3a48ca1bcab78f01fd

Sergey Sharybin commented

2015-12-31 16:43:13 +01:00

Changed status from 'Open' to: 'Resolved'

Sergey Sharybin closed this issue

2015-12-31 16:43:13 +01:00

blender-admin commented

2016-01-24 12:48:49 +01:00

This issue was referenced by blender/cycles@2260ddcd2d

This issue was referenced by blender/cycles@2260ddcd2d91b4ade10e7ef097f08358cb994bb9

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strange output from Cycles Vector Curves Node #46915