Blue glass materials alpha channel is yellowish

[kimmo_hernborg]

I have an issue with blue glass alpha channel being rendered as yellowish.

Brand new scene. One rectangular vray light, a thin box with vray material applied. Diffuse and relflection black, refraction white, fog light blue. Alpha affect color+alpha.

If I have objects behind my glas box (like a white plane) the glas look blueish. If I add a background layer in the VFB the glass look blue against the background.
If I look at the alpha channel where there isnt anything behind my glas box in the VFB the glass is yellowish. If I save my render as a png and add a white background in photoshop the glass doenst have any color, its basically gray.

If I set fog color to yellowish color the alpha is blue instead, and in photoshop the glass looks pretty gray.

Am I missing something? I want my renders to behave like they do in the VFB: if I add a white background layer the glass should look blue against that.

[deyan.hadzhiev]

Hey kimmo_hernborg, when you want colored alpha, you need to save it to a separate file, where the alpha will properly use the three color channels to represent the exact contributions per channel. Then you can use this separate file as an alpha mask layer in photoshop to achieve similar results to what you observe in the V-Ray VFB.

In order to produce a separate alpha file, you must use the V-Ray output system, as shown in this screenshot:


Also, depending on how you blend your layers, you may also need to use the option below "Don't Save Alpha Channel", to skip the alpha channel in the RGB output image. I hope this helps you achieve your desired effect.

[kimmo_hernborg]

Hi Deyan

Thanks for your help. Maybe I´m missing something or the issue is something else.

I tried following your instructions. I have rendered with "alpha channel to seperate file" and "dont save alpha channel". I took the rgb pass and applied a layer mask to that layer in photoshop, then I copy pasted the alpha channel image into the mask. The result is the same like before. The glass is gray and lack the blue tint.

I tried something similar in after effects. I used the alpha channel as a matte set to luma. Same result.

[boyan]

Hey, Kimmo,

Thank you for your post. Please send us the rendered images with alpha for internal checking. Please check what you have in VFB as layers; this might affect your image as well.

Cheers,

Please check the attachment. I guess this has happened with your mask and layers