VRay Blend - bug ?

[NicoC]

Hello,

A long time I regularly face this bug, and I finally take some time to report it
When you use std 3dsmax Blend material, you can use the same file (let's say a TIFF for example) as RGB and mask, as long as it includes an alpha channel. Then the only thing to change is to switch Mono Channel Output from RGB intensity to Alpha. The same thing perfectly works for a VRay material for which you need some opacity: duplicate diffuse map to opacity slot, then Mono Channel Output to Alpha, and here you go, you've got diffuse and opacity with only one map. So far so good.
The problem I get is that Mono Channel Output switching doesn't work when it comes to use in a VRay Blend material, as blend amount map ! from my various tests, it's mandatory to create a separated B&W map as separated mask file ... really not convenient !
Oh and btw, we're on 3dsmax 2014, VRay 3.00.07, but this behavior was also true with previous 3dsmax and VRay versions.

Thanks in advance !

Regards.

[vlado]

This is because the blend amount is a color value, not a mono value - I don't see why it should be restricted to grayscale only.

Best regards,
Vlado

[NicoC]

Hello Vlado,

Hmmm ok, seen like that I wasn't aware of this, certainly because I use VRay Blend like I used 3dsmax Blend: using Mask and Blend amount the same way, with a map meant to be applied like a "simple" mask ... could you please elaborate a bit ?
That being said, the VRay 3.0 manual presents Blend amount that way:
"Blend amount - this color specifies how much of the final result is contributed by the corresponding coating material, and the rest of the materials below it. If the Blend amount is white, the final result is comprised of the coat material only, and other materials below it are blocked. If the Blend amount is black, a coat material has no effect on the final result. This parameter can also be controlled by a texture map."
Does that concern only the color slot ? Am I misunderstanding something there ?

Thanks

[Svetlozar Draganov]

The same rule about Blend amount applies for non-gray-scaled masks as well.

Here how the Final Color is calculated with grayscale color:
FinalColor = CoatColor*BlendAmount + BaseColor*(1-BlendAmount)

So if the BlendAmount color is white:
FinalColor = CoatColor*1 + BaseColor*(1-1)=CoatColor

If it is black:
FinalColor = CoatColor*0 + BaseColor*(1-0)=BaseColor

Now the same formula but this time applied for R/G/B channels separately:

FinalColor.Red = (CoatColor.Red)*(BlendAmount.Red) + (BaseColor.Red)*(1-(BlendAmount.Red))
FinalColor.Green= (CoatColor.Green)*(BlendAmount.Green) + (BaseColor.Green)*(1-(BlendAmount.Green))
FinalColor.Blue= (CoatColor.Blue)*(BlendAmount.Blue) + (BaseColor.Blue)*(1-(BlendAmount.Blue))

[NicoC]

The same rule about Blend amount applies for non-gray-scaled masks as well.

Here how the Final Color is calculated with grayscale color:
FinalColor = CoatColor*BlendAmount + BaseColor*(1-BlendAmount)

So if the BlendAmount color is white:
FinalColor = CoatColor*1 + BaseColor*(1-1)=CoatColor

If it is black:
FinalColor = CoatColor*0 + BaseColor*(1-0)=BaseColor

Now the same formula but this time applied for R/G/B channels separately:

FinalColor.Red = (CoatColor.Red)*(BlendAmount.Red) + (BaseColor.Red)*(1-(BlendAmount.Red))
FinalColor.Green= (CoatColor.Green)*(BlendAmount.Green) + (BaseColor.Green)*(1-(BlendAmount.Green))
FinalColor.Blue= (CoatColor.Blue)*(BlendAmount.Blue) + (BaseColor.Blue)*(1-(BlendAmount.Blue))

Hello

Thanks a lot for this reminder and the detailed explanation, that's pretty clear ! well, even if I don't really see for now where I would need to put an RGB map to handle blend amount, but that's ok
However, sorry to insist, that doesn't explain why switching Mono Channel Output from RGB intensity to Alpha doesn't work ... I mean, it "simply" force the RGB map to be considered as black & white map, using only its alpha channel info as output ?
It's ok that blend amount isn't limited to a mono value, but I don't get why it doesn't let me choose either I want to use the color value or the grayscale (alpha) only. Not convenient ...

Thanks !

[vlado]

I don't really see for now where I would need to put an RGB map to handle blend amount

It is sometimes used to fake dispersion where you blend three different refractive materials with different color blend amounts (f.e. green/blue). This is not possible to do if the blend was only allowed to be grayscale.

Best regards,
Vlado

[Svetlozar Draganov]

It seems that VrayBlendMtl doesn't take into account Mono Channel Output but you can get the same effect with the help of Color Correction map:

[NicoC]

It is sometimes used to fake dispersion where you blend three different refractive materials with different color blend amounts (f.e. green/blue). This is not possible to do if the blend was only allowed to be grayscale.

Best regards,
Vlado

Ah ok, thanks for the example ! we don't need such a fake, that's probably why we never tested

Cheers.

[NicoC]

It seems that VrayBlendMtl doesn't take into account Mono Channel Output but you can get the same effect with the help of Color Correction map:
[ATTACH=CONFIG]21726[/ATTACH]

Clever workaround indeed, thanks for the help ! still less convenient than using Mono Channel Output, but better than nothing ...

Thanks a lot !