Rounding error in ColorRamp node #74198
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Reference: blender/blender#74198
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System Information
Operating system: Windows 10
Graphics card: GeForce GTX 1080 8K
Blender Version
Broken: 2.83.4,
a322b43e3d, 2020-02-24Short description of error
In the Cycles implementation of the ColorRamp node, marker positions seem to be rounded to the next higher multiple of 1/255, leading to unexpected output, especially when set to 'Constant' interpolation.
EDIT: Happens in EEVEE too, but only when there are two or more markers set on the ColorRamp, and with multiples of 1/256.
Exact steps for others to reproduce the error
rounding error.blend
0.2197(> 56/255)0.2235(< 57/255)The node will output the red color from the left of the spectrum, even though the input value is greater than the yellow marker's position.
This only occurs in Cycles, EEVEE correctly outputs yellow.As a side note, 0.2335 should be rounded up to 0.234 when displayed with three decimals, but isn't.
EDIT: To reproduce in EEVEE:
0.50010.5039(As with Cycles)
Added subscriber: @DustInCompetent
#101205 was marked as duplicate of this issue
#100694 was marked as duplicate of this issue
#91769 was marked as duplicate of this issue
Added subscriber: @TheOldBen
Changed status from 'Needs Triage' to: 'Confirmed'
Can reproduce with sample file and from scratch.
Changed status from 'Confirmed' to: 'Needs Triage'
Rounding error in Cycles ColorRamp nodeto Rounding error in ColorRamp nodeChanged status from 'Needs Triage' to: 'Confirmed'
Can confirm, using 2.83.4
Not sure if this is a "known issue", either. I could not find it in the tracker at any rate.
Added subscriber: @CharlieJolly
Color Ramps are converted to a texture and then sampled from a 256 px wide image, this low resolution is prone to sampling errors and is not exact. There is an optimisation in Eevee when there are only two stops which explains why it breaks after adding another stop. See the following thread from twitter: https://twitter.com/BartekMoniewski/status/1229063557330984960
Added subscribers: @RMK-1, @PratikPB2123, @SteffenD
Added subscriber: @jgilhutton
Is there any updates on this one? Or should we asume this is how the node will work for ever? I'm asking with scripting purposes in mind.
If we inspect the color ramp visually, we won't take long before we realise there's something wrong; but problem comes when we deal with the colorramp node factor input via scripting.
The real issue here is that the user/programmer will expect and trust that the input factor on the colorramp won't be rounded o the nearest 1/256 multiple. At least we could add an item to Blender's known limitations or something like that because this deviates from what's natural on the software.
Unfortunately in my case, I didn't realise of this issue until it was too late :(
Added subscribers: @Fluffies, @OmarEmaraDev, @deadpin, @scurest
Added subscriber: @akirabeta
Removed subscriber: @scurest
A question for those still watching this: It's obvious the 255 vs 256 discrepancy between Cycles and Eevee should be fixed and of course documented. But what should the number be fixed to?
Using 255 or 256 for both would result in the least amount of visual change from prior versions. Going down to 250 would provide a more even/natural 0.004 step between each stop rather than 0.003906 but has lower resolution than before and might produce visually different results. The next highest resolution that would produce natural stops would be to use a resolution of 400. However this has downsides in both Cycles and Eevee with increased ramp calculation times, memory usage, and (maybe) sampling times in Cycles though it's hard to measure.
256, if anything. Increasing or decreasing it to some other "round" number doesn't solve the underlying issue, there would always be some case where it's not perfectly divisible, even if you set it to 240 or something.
Since the workaround is having an ugly staircase of Greater Than and RGB Mix nodes, I'd like to have a checkbox option in the Color Ramp node to explicitly enable stepless markers at the cost of reduced performance.
As an afterthought, maybe the width of the ramp texture could be made configurable too, if that'd be easier to implement and make less of a performance impact.