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Is it possible to avoid rendering twice?
I want to render and have alpha channel but I also like to get the "correct" tones of the physical sky background ...
(since alpha makes the background black)
thanx !

You can do a quick (we're talking seconds) render of the sky by itself to composite in. But I don't understand why you would need to cut out the sky only to put it back in.

I don't put it in back, I want to be able to modify my post-work background sky to match the correct color the physical sky has after playing with shutter and iso...but it appears that I have to use your idea.
still I didn't understand why rendering with alpha means the bg has to be black and cannot be kept.

just render an object channel and invert

choo-chee wrote: still I didn't understand why rendering with alpha means the bg has to be black and cannot be kept.

Because the alpha channel is useless unless the background is a solid color (preferably white or black). Here is a render which includes the sky and its alpha channel:




Try to use the alpha channel to replace the sky with this background:



You will get a halo around the sphere because the edge pixels are antialiased against the sky. There's no way to remove the sky color from the edge, because each pixel is blended with a different color:



Here's the sphere rendered against a black background:



If you apply the alpha channel to this image and remove the black matte (Layer -> Matting -> Remove black matting in Photoshop), you can apply it on the new background:



This is exactly the same image you would get if you rendered the sphere against the pier background in the first place (with the pier HDR in the background IBL channel and the reflection, refraction and illumination channels set to "use sky").

I think tom has explained this a couple of times before. He even made a Photoshop action which applies alpha and removes the black matte in one go, it's somewhere on the think! site if I recall correctly.

all these I know yet I don't want HDRI in the typical render I do, just need the correct tones of the sky when I finish tweaking the ISO etc....
nevermind I guess I'll just render twice - second time a quick render to get the correct tones.

choo-chee wrote:still I didn't understand why rendering with alpha means the bg has to be black and cannot be kept.

choo-chee wrote:all these I know

I meant that I know about the halo and that, back to my question - I think a nice feature would be like in artlantis - you get a separate channel for the background (like you get alpha and material and that) so the render will be anti-aliased correctly but still there's a reference for the right tones of the sky...

+1 But bear in mind its a dirty (allbeit convenient) workaround, because you'll be tinting the edge pixels of your skyline too.

For minor adjustments, like adding a little haze or minor tinting I would like such an option.

ppl, you don't get me, I'll try to explain better... check out the way this software-artlantis-gives you the bg.
it delivers a 4 layer psd that has the render+alpha mask, bg layer, and material and object layer.
So you always have a full bg not connected to the render itself.
That's my idea- the bg should be rendered as a seperate channel, the alpha should stay the same - with transparent (-->black) bg but
there will be a channel -let's call it background ID - that will be rendered with the sky factors in it.

What would you do with the alpha then?

the alpha is needed since I want to put my own sky. as I said, my problem is to get the correct tones of my sky, so I want a reference how they should look if it was maxwell's own sky.

Ive got a nice compromise idea here, how about maxwell saves the image as it does now, with a black sky, but on screen it is displayed with the sky/hdr showing which would help for tweaking mulilight, and generally give better feedback to the user...

Is this do-able?

I'm with choo on this one. I actually like the fact that Maxwell forces you to get a perfect alpha, however I wish it didn't do this at the expense of the background sky. Something like what choo suggests artlantis does would be nice. I've always felt the answer from NL isn't that 'we can't do it' but rather 'you don't need it' which is a bit off putting given the number of times this has come up. It can be nice to see the sky for reference, maybe to match tones, maybe so I can put another sky over it with 50% opacity to blend them, maybe just so when my boss walks by he can see a better representation of the building rather than seeing it with a big black background as if it's a night shot.

-Brodie

yes that's what I meant.... I think that a small circle with "p" inside for "physical sky" will be nice between all the other small circles of render, alpha, etc....