vray material as glass and background color problem

[j.galang]

I have noticed that when you use the vray material as glass (having refract all the way to white) and adding a very slight reflection (its glass anyway) and using the exponential or HSV color mapping algorithm, the background environmnet color gets affected a LOT. for example try making a simple box, put the vray materail glass on the box, set the environment color to sky blue and render, use exponential and hsv color mapping and render. you will see a VERY big difference in the color viewed from behind the glass. when using linear everything looks fine (and correct) but why does this happen when using exponential? this makes it very hard to use exponential and HSV when using vray material for glass like when you have a scene when you can see the background from behind the glass and not behind the glass since you will see the very big color difference in the background (i.e. looking out a partially open glass window or walking into a glass structure from the outside). any ideas on this?

thanks.

JG

[flipside]

check the 'affect background' option in the color mapping rollout. There's a difference because vray doesn't color map affect the bg with default settings. The bg viewed through glass is not considered bg anymore, it's the refraction in the glass so it gets affected by the color mapping.

[j.galang]

thanks for the tip. although i do understand that checking that affect background box does make the background the same, but my nagging question is why does this happen? particularly with the vray material, since if you use standard material and vraymap (for reflection) and not use affect background, you can achieve a good even result (no huge changes in background color behind or in front of glass). problem is its much slower. and by using this affect background, the background color (lets say a texture of a city) becomes washed out EVEN with HSV. so its so hard to get the right background color using anything other than Linear because BOTH exponential and HSV affect the background color (when affect bg is on) even if HSV is supposed to NOT affect texture colors anymore.


JG

[flipside]

so you say the bg seen in reflections or refractions with a vray map stays the same as when not using color mapping? Never tried this.

But it is indeed a problem... I solve it by adjusting the output parameters of the bg map, to compensate for the change when using color mapping... Trial and error.

[vlado]

Color mapping is applied over the final calculated image pixels. There is no way VRay can apply color mapping just to the glass and not to the background seen through it. This is possible for transparent Standard materials, since VRay recognizes transparency as a special case, but is difficult to do so for arbitrary reflections and refractions.

Best regards,
Vlado

[j.galang]

yes, this is true that it is hard to do this with vray materials, but i was wondering why even using HSV does not help solve this problem. the main difference between linear and exponential (aside from the GI algo) is how they treat color saturation right? linear treats colors better i think, retaining most of the original RGB values. using exponential however most of the RGB values of the textures change, for example a wood texture viewed in photoshop is much closer to the rendered image using linear than exponential. exponential tends to gray out the RGB values. this was addressed by the new HSV algorithm. which solves the problem quite nicely. you have the nice GI of exponential (fewer burnt out areas) while retaining the good color of linear algorithm. NOW.... my only wish is that for HSV to treat the background the same way as it treats textures. that i think will solve my problem since if i check affect background, what is seen through the vray glass will be almost the same as the one withour glass. of course flipsides suggestion is also valid but a bit cumbersome since its really hard to try and get the texture to look like you want it to since the difference in photoshop and exponential/HSV backgrounnds are pretty big. ( i hope i'm makinf sense here)



JG

[flipside]

Yes you make sense, but a wooden texture on your model is lit by your lights and bounced light etc..., while your bg image is only a bg image and doesn't get lit in any way by your lights (or skylight or whatever).

I think the problem lies there.