Modo color gamma node connection stops working after refresh in this scene

[1funk]

Modo color gamma node connection stops working after refresh in this scene

MODO 10.2v2 131034
V-Ray 3.40.03 27160

1. Load the attached scene into the vray render layout
2. Start RT CPU
NOTE: The meshes render in a blue color
3. Go to the schematic and connect the Color Gamma Output color, to the "Trans Col degamma" color input
NOTE: You will see the color change in RT (red/orange). This is correct so far
4. Restart RT

RESULT: The material now renders black
EXPECTED: The color should be red/orange

VIDEO: https://www.dropbox.com/s/9r7i2jx4zk...start.mp4?dl=0

[vladimir.nedev]

I don't think the refresh is causing this.
When you connect it with RT running it doesn't update properly and works correctly by chance.

I will take a look what's wrong.

Greetings,
Vladimir Nedev

[vladimir.nedev]

This looks like a MODO bug to me, I will try to work around it, but it will be nice if you report it to The Foundry as well.

So, here is what happens, when you have the Color Gamma Output color connected to Trans Col degamma color input.
Since V-Ray supports the Color Gamma node it exports it as an internal texture and tries to fill the parameters of this texture using the Color Gamma channels.
Since, Input Color is connected to something that is not an output of a texture/channel modifier, its value is taken directly.
So, V-Ray tries to read the value of the Color Gamma -> Input Color channel.

It seems to me that MODO has a bug, where input channels of channel modifiers, which are driven, do not return correct values.
Instead they return the value stored in the channel (black in this case).

This can be checked with a simple Python script modified from here : http://modo.sdk.thefoundry.co.uk/wik...Driven_Channel

Execute the following code in your scene (doesn't matter if you made the connection to "Trans Col degamma"

import lxu.select

scene = lxu.select.SceneSelection().current()
scnsvc = lx.service.Scene()
 
gammaNode = scene.ItemLookup('Color Gamma')
chan = scene.Channels(None, 0.0)
chan_idx = gammaNode.ChannelLookup("inputColor.R")
print "The channel value is : " + str(chan.Double(gammaNode, chan_idx))

Here is the result :

Greetings,
Vladimir Nedev

[vladimir.nedev]

Note that the item.channel command returns the correct value in this case.
It seems the bug is present only when using the ChannelRead API.

I cannot switch to the item.channel command because
- it might be slower
- it might break other stuff
- it doesn't support reading at arbitrary times, which I need for the animation export and motion blur export, so using item.channel will require modifications to that code as well.

I plan to work around the issue by reading the values from the texture feeding "Input Color", but I am not sure this will be enough to fix all possible scenarios.

Greetings,
Vladimir Nedev

[vladimir.nedev]

I've added the work around for this issue in today's nightly build ( 2017.01.25 ).
Give it a try when you can.

Greetings,
Vladimir Nedev

[1funk]

Thanks for the info Vladimir

[1funk]

Vladimir, I tried something else and am getting a crash in V-Ray.

Modo lets you do crazy stuff like this: The color value goes to the gamma node, then back into the color value, then modo uses the gamma corrected value in the render. You'd think there would be a dependency loop but it works.

Preview renders the gamma corrected color, but starting RT gives me a crash

[boyan]

Aloha Funky,

Well done, I can reproduce the crash with V-Ray build # 27160 & MODO 10.2.1

BR.Boyan

[vladimir.nedev]

Vladimir, I tried something else and am getting a crash in V-Ray.

Modo lets you do crazy stuff like this: The color value goes to the gamma node, then back into the color value, then modo uses the gamma corrected value in the render. You'd think there would be a dependency loop but it works.

Preview renders the gamma corrected color, but starting RT gives me a crash

I am working on fixing the crash, but I don't think the result of this is well defined.
It depends on where in the loop you "start".

Also MODO seems to handle this inconsistently in some cases.
In the attached scene, I have connected a constant value equal to 100% to the mask slot.
In theory, this should have the same result as not connecting anything to the mask slot.
But instead the result is as if the gamma was applied twice.

So, I will try to fix the crash, but I can't guarantee that the results in V-Ray will match MODO.
The results may change if you connect some of the intermediate nodes of the loop to other textures, thus changing where the loop "begins" when V-Ray exports it.

color_gamma_and_constant_loop_with_mask.zip

Greetings,
Vladimir Nedev

[1funk]

Thanks Vladimir. I don't expect the results to always match, especially if the rules don't seem well defined.

The important thing is preventing the crash.

[vladimir.nedev]

So I tried to make a longer loop to test my code.
I just tried to add a "Color Invert" in the loop, but MODO doesn't allow it.
I wonder whether a loop with 2 items is a special case on purpose or just a bug in the MODO code that detects those loops.

Greetings,
Vladimir Nedev

[vladimir.nedev]

The crash should be fixed in tomorrow's nightly build ( 2017.01.27 ).

Greetings,
Vladimir Nedev

[1funk]

Thank you.