Mismatch between VFB and Saved file

[ene.xis]

yep...theres definetely something going wrong there...
i am with sv on this, every day s workflow here and not a single problem!

[bakbek]

SV, I did a test like your scene with the tepots... and it looked ok. but, i keep getting this mismatch in other scenes even went starting from scratch and using the same setup ?!?!?

here is an example... left is VFB + sRGB right is MAX Clone and also the way it looks in the saved EXR file



Can't nail the problem yet... first i thought it's just on materials with an image in the textures slots. but it happens in pure color too.

[bakbek]

I took a scene from archinteriors and did this test, gradually from the original into LWF using SV's suggestions and this is the output



in the end the on screen VFB looks better then the actual saved EXR file... this scene makes use of standard direct light and camera and no material was changed other the lighting adjustments to make it exposure close to the original none LWF render.

VLADO - Any chance you can pinpoint the reason for this kind of visible mismatch, the LUT settings and color mapping settings are just as SV suggested up above.

[percydaman]

I have the same 'issue' Ive always just performed an inverse gamma function on the 32bit exr and moved on. Wasn't aware it was supposed to be a problem.

[sv]

Ive always just performed an inverse gamma function on the 32bit exr and moved on.

Percy; do you do this in Photoshop? I'd be interested in knowing how you do this as i'm not quite sure - i know how to add or remove a 2.2 gamma, but am not sure about proper sRGB.

[percydaman]

Ya I don't really care about any of that. Whats most important to me is that I can get it looking in photoshop what it looked like in the vfb. The real work starts when I get it into photoshop anyways.

[vlado]

All differences are typically because image processing programs have weird notions of how a given image format should be displayed.

For example, I had the following case recently, where an image saved from the V-Ray VFB to a .vrimg file and then converted to an .exr, failed to match the same image if saved from 3ds Max directly as an .sgi file (or .exr too for that matter). It turned out that this was because of the following reasons:

1) The display program performed different display color correction to the different image formats: for the .exr file, it applied a sRGB curve, but for the .sgi file, it used a gamma 2.2 curve. Now, sRGB is not the same as gamma 2.2, particularly in the dark regions (although they are very similar), which caused visible differences.

2) When we tried to save to an .exr directly from 3ds Max, rather than an .sgi file, 3ds Max automatically burned a gamma correction into the file (the 3ds Max output gamma was enabled). Thus it failed to match the .exr file produced by vrimg2exr, which by default does not include gamma correction. What 3ds Max did was also completely wrong - imagine that the image was a z-depth render element - applying any color correction to this is certainly not what you want!

From this and other experiences, I am lead into the firm belief that the way to handle this is to:

1) never, ever, keep a gamma- or sRGB- corrected image on your harddrive, which automatically means that
2) never, ever, fiddle with input/output gamma- or sRGB- settings in any program. Instead, gamma- or sRGB-correction should be used only for display purposes by whatever program is displaying the image (like for example the way the V-Ray VFB does it, or 3ds Max display gamma, without the input/output bitmap settings).

That means that, whenever you take an image from an outer source (e.g. photo camera, a texture library etc), it is a good idea to remove any gamma- or sRGB- correction from this image on the spot, and keep the de-gamma-ed (de-sRGB-ed) image in linear space on your harddrive.

Output gamma correction can be burned into the images only whenever you output for a CRT viewing device, and only after you have done all the rendering, compositing, post-processing or whatever.

In all other cases, keeping the images in non-linear space on your hard-drive is likely to cause a lot of headaches, even for people who really understand what is going on. For example, while 3ds Max can remove gamma-correction from input bitmaps, it cannot remove sRGB color correction (and the two are not the same). Also, 3ds Max can burn the gamma correction into output images, but there is no way to make it burn a sRGB color correction. And in general, you are kept wondering all the time, "Is this image gamma-corrected? Do I need to remove the gamma, or I can use it as is?" and so on. Some file formats support gamma correction information, but others don't - so you always have to remember if this particular texture is gamma-corrected or not...

Anyway, this is my personal view based on some observations Officially, we do not recommend any particular gamma workflow, as there are many different ways to do this.

Best regards,
Vlado

[Aldaryn]

2) never, ever, fiddle with input/output gamma- or sRGB- settings in any program. Instead, gamma- or sRGB-correction should be used only for display purposes by whatever program is displaying the image (like for example the way the V-Ray VFB does it, or 3ds Max display gamma, without the input/output bitmap settings).

I *will* print this out, and send it to all our clients!

Thank you for the insight!

Best regards,

A.

[DaForce]

Excellent info Vlado!!

I think what catches alot of people out is they think/assume that sRGB is the same as 2.2 Gamma.

[bakbek]

Thanks for the detail Vlado.

In my personal scenario working pure linear or burring the gamma into the image is secondary to the fact the VFB looks Different then the saved file - i think this might have to do with color profiles? could that be the case.

I have a Samsung SuncMaster 226bw and it has it's own ICM profile, perhaps this is why on screen colors + captured images from screen look differently (and better) the the actual saved file from max that might use a different color profile?

[stevesideas]

Well thats confused me. I thought sRGB was the same as 2.2 gamma. In fact, if i dont use sRGB and put 2.2 gamma in the vray gamma it produces the same image...well, what thought was the same image..obviously not.