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Exposure discrepency in VRay Camera

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  • #46
    I'll try and explain better why i use "such dark" materials...
    In normal conditions, without a physcam, or HDR lightsources (so say, standard max omni light with multiplier at 1, and standard camera), i always used the whole spectrum of whites.

    A half step back might be required: the final pixel color can be rougly approximated, for a perfectly diffuse surface, as the surface brightness*the light brightness (do not stone me if i trash like so the shading algorithms, it's for figure of speech... :P). So in traditional rendering, a value of 255 white for the material and a value of 1.0 for the light would return me an average 255 white.

    Now, with these new tools we have, the incoming Lights are EXTREMELY bright, very often (sun is a light with 1000.0 as multiplier, or thereabout, i gather).
    Of course, in a perfect world, VRay would take care of everything (and given enough time it might well do), but as far as my experience goes, lowering the material brightness to 120 for a white material leaves me ROOM for specularity, on top of helping the light decrease its bounced intensity more rapidly.
    I speak of having room for specularity, and here the other "strange" thing: rarely, if ever, my specular levels exceed 120, as well
    120 for the surface X a value of 3.0 in some pixels of the hdri would still give me an over-the-white reflection, and modulate correctly around the hotpoints.
    Notice how 120 for diffuse and 120 for specular sums up 240, which is LOWER than 255 pure white (i'm also fairly conservative).
    This material, to me, would be a good candidate for white enamel, for instance:It would obey the energy conservation laws, and through exposure and lights i can easily make it look as shaded or as lit as i want, and the same i can do to reflections...

    We now have the VRay COLOR map, and i am using it a lot, as it is fairly flexible, and most of all, it expresses RGB colours as normalised to 1, rather than with the old 8 bit method.
    Still, the energy preservation law would dictate the material diffuse+specular (the two light bouncing components) HAVE to be lower than 1.0.

    Change it to 250 white with 250 reflections, and bang: energy preservation is broken, material will likely look burned out (it would in LDR, go figure with HDR lighting), and GI methods have to work double time to try and make any sense of outgoing light being stronger than the incoming.

    I hope to have made the concept more of a messy affair

    Lele

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    • #47
      Small sample of the above.




      Here are two images rendered with both a standard cam (first sample) and an exposed physcam (second one).
      The sphere material is 122 grey for diffuse AND speculars(i actually used a VRay Colour map to set them to RGB 0.48 )
      The walls materials are 34 grey diffuse only(RGB 0.1333 in the vray colour map).
      Light is a vray AL with intensity of 1.
      Reflection environment is Kitchen.hdr with multiplier of 1.
      Only difference between the renderings is the camera.


      Lele

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      • #48
        This may look screwy, but do some experimentation before you write it off.

        We are used to hearing from a client that they want color XYZ for house siding. When we make a material with that color, it doesn't render correctly at all. But think about it. You are specifying the color in the diffuse slot - NOT in the "full burn" slot, or in the "GI lit" slot, or the reflection or specular. You are specifying diffuse.

        I just used my little Colorzilla picker on Lele's images. And guess what? The lighter sphere, which intuitively looks far too bright, actually reads close to 122 way down near the base, where it is not receiving any direct sun or sky light. Makes sense now, eh? (BTW - the dark sphere is still not quite 122 - even at the very top where it shouold be brightest.)

        I have to keep reminding my clients - and myself. Color is not absolute - it is more of an emotional perception than it is a scientific measurement. Look at these pics. What color is each hydrant?

        But alas, I have nearly given up, since many clients don't care. Even though I explain that the sun is yellowy-orange at dawn/dusk (and they know this!!), they *still* want the siding to look like the swatch - regardless any color influences. Oh well.










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        • #49
          Thanks for good exemple, but when i try to do this with my scene i have else a much dark shadows (Vray sun) or my scene become "grey". Render seems with a grey layer on with oppacity of 25%. In fact what i reely need is the Vray Sun with Vray sky and good color, no need physical camera. But it's impossible or i am in error ?
          http://www.a-3d.fr

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          • #50
            Well, look at the hydrants pictures above.
            In reality, some shots would be with pitch black shadows.
            Only some others exhibit transparent shadows.
            Interiors and exteriors differ a lot as well, as far as light behavior, and camera preferred usage go.
            It's on a scene by scene basis, and there is no definite answer (or we'd mostly be out of work ).
            If you want to use a standard camera, what i suggest you to do is to lower the sun/sky intensity to 0.01, and go on from there.

            The phisical method is a PREFERENCE, not THE way of doing things.
            As long as it looks good, it IS good.

            Lele

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            • #51
              Ok i will try to setup 0.01.

              It's right for exterior/interior scene, but still this morning i'm looking shadows on building, and i'm sorry but the shadows aren't in majority black. I think (like i do with standard cam) the sky affect more the shadows. When i try with sky/sun vray, i can't do this. Vray sky not seems to illuminate my scene.
              http://www.a-3d.fr

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              • #52
                vray sky ONLY illuminates with GI turned on.
                Make sure you do that.
                It seems to me you're describing a direct lighting situation here.

                Lele

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                • #53
                  Re: Small sample of the above.

                  Lele,
                  i m still a little confused here (sory i m a little slow )
                  wadd u mean by:

                  Originally posted by studioDIM
                  The sphere material is 122 grey for diffuse AND speculars
                  ?
                  Nuno de Castro

                  www.ene-digital.com
                  nuno@ene-digital.com
                  00351 917593145

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                  • #54
                    Of course my GI is turned ON....

                    Exact, it's a direct light i would like to do with a vray sun/sky like i do with a direct light and a 360 dome with map of sky.

                    Here two exemples. All made with VrayphyCam.

                    Color Correction-> Linear default settings (1.0 1.0 1.0)
                    Here colors are fine but shadows so blacks

                    Color Correction-> Reihnard: Burn 0.8 - Gamma 2.2
                    Here shadows are fine but color are "greyed".
                    http://www.a-3d.fr

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                    • #55
                      i d say ur 2nd image is double gamma corrected...
                      Nuno de Castro

                      www.ene-digital.com
                      nuno@ene-digital.com
                      00351 917593145

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                      • #56
                        ene.xis I have took Vlado settings from scene on this topic.

                        2.2 Gamma correction in max settings and 2.2 Gamma correction in color correction in vray.
                        http://www.a-3d.fr

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                        • #57
                          Originally posted by inf3rno
                          ene.xis I have took Vlado settings from scene on this topic.

                          2.2 Gamma correction in max settings and 2.2 Gamma correction in color correction in vray.
                          If you're rendering to the Vray VFB, which Vlado did, then he is correct (as always) to enable the 2.2 gamma in the color mapping rollout. If you're rendering to the regular max frame buffer setting this to 2.2 will double gamma correct the image.
                          www.dpict3d.com - "That's a very nice rendering, Dave. I think you've improved a great deal." - HAL9000... At least I have one fan.

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                          • #58
                            i see...ok so its ur materials...right about what Lele was talking about...
                            ur textures have also to be color corrected!
                            Nuno de Castro

                            www.ene-digital.com
                            nuno@ene-digital.com
                            00351 917593145

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                            • #59
                              Originally posted by dlparisi
                              Originally posted by inf3rno
                              ene.xis I have took Vlado settings from scene on this topic.

                              2.2 Gamma correction in max settings and 2.2 Gamma correction in color correction in vray.
                              If you're rendering to the Vray VFB, which Vlado did, then he is correct (as always) to enable the 2.2 gamma in the color mapping rollout. If you're rendering to the regular max frame buffer setting this to 2.2 will double gamma correct the image.
                              All renderings made in VFB...
                              http://www.a-3d.fr

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                              • #60
                                Originally posted by ene.xis
                                i see...ok so its ur materials...right about what Lele was talking about...
                                ur textures have also to be color corrected!
                                I changed my materials too.... but the sky is "greyed" too. So i think my material are fine. I try to respect the different settings from this topic.
                                http://www.a-3d.fr

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