VFB colour problem

[re:FORM]

I've been having this strange problem with the Vray frame buffer. No matter what colour or gamma correction I apply, I cannot get my saved files to match the colours in the viewport.

The difference is quite subtle most of the time, so I've been able to ignore it. But, this time I need the renders to save the same as they appear in the VFB. Basically, the saved file appears slightly less saturated than the VFB. I've saved as EXR, JPG, PNG, in linear, gamma corrected and 8, 16, 32bit and the problem remains. Here's a screen grab: max vfb / photoshop on the left, vrayFB on the right.



I hope I'm just doing something stupid

[dlparisi]

I posted (http://www.chaosgroup.com/forums/vbu...ad.php?t=40528) something a while ago about this same problem (I think) and never really got an answer although I did have some luck getting a good match. Maybe it'll help or spark some discussion...

[re:FORM]

Here's an animated GIF for clarity.

[re:FORM]

The annoying thing is that you can't correct the colour shift very easily. Increasing the contrast gets it close, but not quite the same. Adjusting gamma is no use.

<edit>
in 32-bit mode, after loading the EXR image, adjusting the levels so that the "input levels" read 1, 0.98, 255 rather than 0, 1.00, 255 gets the result almost the same as the VFB.... not exact, but its alot closer than previous attempts. This gives you some idea how subtle the colour shift is.

Or applying an exposure "offset" of -0.0059 seems to produce an "almost" perfect result. The only colour difference appears to be where the alpha channel is antialiased.

Such a strange thing... why does the colour get shifted from what I can see on the screen when it gets written to a file?!
</edit>

[sv]

I'd say it's because the colour profile in Photoshop is displaying your colours differently when you load a file.
Best thing to do would be to calibrate your monitor, then use the calibrated profile in PotatoChop...colours should look the same then.

[peakyfreak80]

Not sure what workflow you're using, but check to make sure your Gamma/Lut settings are all at 1.0. Even if they are disabled, re-enable them, turn them to 1.0, and then turn them off. Hope that is your solution. Looks like an annoying little problem.

peakyfreak

[re:FORM]

Well that's the thing, I've tried virtually every workflow to try to work out where the problem lies. If the problem is gamma related, then you would expect that it could be "corrected" by adjusting the gamma in photoshop, just as you do when you load linear images....

With my discovery that a exposure offset of -0.0059 seems to fix the problem, I have to ask, where could a colour shift come from as a result of my gamma settings (rendering to linear, 1/1/1, max gamma 2.2, inpout 2.2, output 1.0)

I rendered with max gamma/LUT disabled, and the result was the same

[re:FORM]

I'd say it's because the colour profile in Photoshop is displaying your colours differently when you load a file.
Best thing to do would be to calibrate your monitor, then use the calibrated profile in PotatoChop...colours should look the same then.

Thanks for the suggestion, but unfortunately I don't think that follows when :-
a) screen grabbing the VFB and pasting into photoshop retains the colours as they appear in the VFB.
b) opening the jpg's in windows image view also shows the same colour shift.
c) copying the VFB to max's FB also shows the same colour shift.

from the above statement, I think its safe to say that photoshop's colour settings have no bearing on this problem.

[vlado]

Are you using the sRGB button in the VFB?

Best regards,
Vlado

[re:FORM]

Thanks for the quick response Vlado,
Hah! That's it!
I was using the sRGB button because I was rendering to with Color Correction set to Linear 1/1/1.

So, does this mean there is a problem with the linear workflow? or more likely, a problem with MY workflow!

<edit>
So with :-

CC set to Linear - 1/1/2.2, and all option checkboxes ticked, there is a colour difference between the VFB and the saved file (with sRGB button ticked).

CC set to Linear - 1/1/2.2, and all option checkboxes ticked except "don't affect colours", there is no colour difference between the VFB and the saved file (with sRGB button not ticked).

CC set to Linear - 1/1/1, and all option checkboxes ticked, there is a colour difference between the VFB and the saved file (with sRGB button ticked).

To summarise, with sRGB enabled in the VFB it is not possible to save the image as it appears in the VFB without doing a screengrab. Is that right?

Please excuse me if I'm going over old ground here... is this expected VFB behaviour?

[vlado]

The sRGB button just transforms the way the image is displayed on the screen (similar to the 3ds Max Gamma setting). That transformation is not applied to the image if it is saved or copied into a 3ds Max frame buffer. In these cases, the normal 3ds Max gamma rules will apply to the image. The discrepancy here may arise from the fact that the VFB uses sRGB transformation, while 3ds Max uses simple gamma curve.

So in short, this is as designed.

Best regards,
Vlado

[RE:FORM_STUDIOS]

Thanks Vlado, I had a feeling the likely problem was my lack of understanding of the software!

Is there any way to replicate the correct sRGB correction that vRay applies when I load the image into photoshop? I tried converting to different colourspaces but none looked like the VFB. Only applying the exposure offset mentioned in a previous post got me close.

Perhaps my workflow is inappropriate; I want to render in linear colourspace(linear ColorCorrection, 1/1/1), so I usually work with the sRGB button enabled so I can see how the image should look in sRGB colourspace. If the vFB doesn't represent what I'll end up with when I open the EXR in photoshop then I may have to return to rendering with linear colour correction with gamma set to 2.2, which I thought meant that the gamma was baked into the image (perhaps this doesn't degrade the gamut or other qualities of the EXR like I am assuming)
I'm probably just being stupid as usual and misunderstanding the correct settings for working the lWF, but from what I've read I thought to get clean Linear output I had to set gamma in the colour correction tab to "1".

Thanks again for your patience!
Patrick.

[jacksc02]

patrick, in photoshop you can specify the color space you want to work in, from memory its under preferences.

[re:FORM]

patrick, in photoshop you can specify the color space you want to work in, from memory its under preferences.

Thanks Jacks, but as I said in my previous post, I have tried the different colourspaces in Photoshop without any success.

My photoshop colourspace is sRGB. VFB is sRGB. I don't understand why the results don't match.

Vlado says that max applies a "simple gamma curve" when I copy the VFB to the MFB. This appears to be the same adjustment that photoshop and windows applies to the image when viewed outside the VFB. How do I change photoshop or windows itself to open the file with the same colourspace as the VFB?

At the moment, the ONLY way I can get the VFB to match the EXR is to have Colour correction set to 1/1/2.2 and "don't affect colours" unchecked. To my understanding, this is not the correct setup for working in linear.