Glass alpha channels

number12 · March 26, 2013, 04:51:46 PM

Hi Guys,

just wondering, is it possible to get some accurate alpha for my glass material so that, if I need, I can cut my model out and place it on a different background whilst retaining the realistic glass in, say, Photoshop?
I am only currently working in the demo software so want to know if this is possible please.

PhilippeV8 · March 27, 2013, 12:28:13 AM

http://blog.chipp.com/the-perfect-green-screen-key-effect-in-photoshop-2/

number12 · March 27, 2013, 07:11:23 AM

My company has written tutorials on Photoshop for quite a few well known mags so I'm aware of how to do this in PS but it would be much less hassle if I could generate a decent alpha straight from Keyshot.

Does this mean Keyshot can't do this then?...... Seriously?.....

guest84672 · March 27, 2013, 03:12:09 PM

You can create an alpha channel, but whatever is behind glass is based on what color background you had set inside KeyShot.

number12 · March 27, 2013, 05:38:26 PM

So in reality all you can do is, say, set a black BG then import your model into your image editing software and use the actually model to manually create an alpha using the values of the image?

If this is the case I think I'll have to look for an alternative software... unfortunately, as I really liked the other aspects of Keyshot...

evilmaul · March 28, 2013, 11:55:41 AM

what you are saying doesnt make any sense man!
Glass will reflect and refract whats the enviroment around it. If you change the background in Photoshop you do DON'T retain the realistic glass anymore and it will look fake end of the story

DriesV · March 28, 2013, 12:45:05 PM

Quote from: evilmaul on March 28, 2013, 11:55:41 AM
what you are saying doesnt make any sense man!
Glass will reflect and refract whats the enviroment around it. If you change the background in Photoshop you do DON'T retain the realistic glass anymore and it will look fake end of the story

I second that.
Besides, what keeps you from using a backplate in KeyShot to put behind your lamp?

Dries

number12 · April 02, 2013, 04:37:41 PM

No you're wrong, you may want to isolate your model for post processing and as such this is actually quite poor to not be able to accurately cut out glass. If you think you shouldn't be able to separate your model from BG then that is dumb, of course there are situations where this is necessary.

Quite a massive oversight in my opinion but I'm guessing anyone who has an "interest" in this software will defend it.

DriesV · April 02, 2013, 04:52:26 PM

If you really want this feature, then I suggest you drop it in the Wish List on this forum and post a clear exemple image of what you think is lacking and how it should be.
Maybe one of the devs picks it up!

Still, I think the current alpha channel approach in KeyShot is OK. Also for glass objects.
What you may be looking for is some sort of 'opacity pass' which specifies in B/W values what parts of your scene objects are transparent or not.
This could maybe be a future addition to the render pass arsenal...

Dries

PhilippeV8 · April 03, 2013, 12:14:29 AM

It would be awesome if we could get a glass alpha channel ... I too could very much so use it. However, I do understand the difference between separating the shadow from the background, compared to separating glass from the background.
Shadow does just darken what is directly behind it, while glass does 2 things. 1) it shows pixels from a different location, depending on the camera angle 2) it changes the color, depending on reflection and glass color.
You would need a very smart file format in order to take these data into a post process.

number12 >> What software do you know that can do this ??

DriesV · April 03, 2013, 12:39:50 AM

Quote from: PhilippeV8 on April 03, 2013, 12:14:29 AM
...
Shadow does just darken what is directly behind it, while glass does 2 things. 1) it shows pixels from a different location, depending on the camera angle 2) it changes the color, depending on reflection and glass color.
You would need a very smart file format in order to take these data into a post process.
...

I know of not a single render product that can do this.

Having said that, Maxwell Render can do this:

You can specify whether the alpha channel is completely opaque for your scene objects OR...
The alpha channel can also account for transparency (but just that...) that enables you to semi-view the background behind transparent objects. I think this is what the topic starter is after. However, you don't get light dispersion, refractions or any of that fancy stuff.

Dries

PhilippeV8 · April 03, 2013, 01:06:43 AM

It would be a good place to start ... me thinks ...

DriesV · April 03, 2013, 01:22:12 AM

I just tried it out in Photoshop using the RGB and alpha section of that image.
It actually looks rather OK, I think...

Dries

PhilippeV8 · April 03, 2013, 02:18:36 AM

We kinda have this already ... no ? Did a quick test but something went wrong. It's supposed to be windsheilds and I colored the back faces in Inventor so I can put different material to it. The top center one should've been "Refractive glass + clear rough plastic".

DriesV · April 03, 2013, 02:27:03 AM

Nice experiment, Philippe!

Indeed, it does work for the 'simple glass' (basic and refractive), but not for any other of the transparent material types (e.g. solid glass, advanced, plastics...). The simple glass really is an odd duck in many ways.

For architectural glass and/or thin sheets of glass that might even be good enough.

Dries

Glass alpha channels

guest84672