[daros]

Basic but for me fundamental issue.
The alpha channel in not only useful to replace completely the background.
May you want simply to blend something on it, may you want only to apply a color correction on it or add a noise or watever.
If there is a separate alpha file there is no need premultiply it over original render.
In this way you will also avoid the actual black spill at the alpha boundaries.

thank you

David Rossmann

[tom]

The alpha channel in not only useful to replace completely the background.
May you want simply to blend something on it, may you want only to apply a color correction on it or add a noise or watever.

Right and I see no conflict here.

If there is a separate alpha file there is no need premultiply it over original render.

Alpha is not the thing that's being pre-multiplied on render. Render itself if being pre-multipled over black.

In this way you will also avoid the actual black spill at the alpha boundaries.

I don't think there's a black spill at the alpha boundaries. Are you sure you do unmultiply black? Take a look at the following Photoshop action:
http://think.maxwellrender.com/maxwell_ ... n-110.html

[daros]

You are right about the spill. It works. Probably i did a stupid mistake. Thanks Tom.

However it would be great to have the original render intact. It has to many advantages.

Thanks again.

[tom]

However it would be great to have the original render intact. It has to many advantages.

Can you explain this a bit? I can't see what's missing

[daros]

The fundamental problem i have is following, and may I'm missing something.
if i make a rendering with the standard physical sky and i activate the "Alpha" tag for the rendering i obtain two separate renderings; one normal rendering on which the sky is black and the alpha channel as a separate file. May i missed some way to restore the original maxwell sky in a rendering with "alpha" tagged on?

if not, following are the problems i have:


1) Now if my client decides in the last moment that hi wants the standard maxwell sky instead of a photoshopped one the only option y have is to re-render the whole image with the alpha tag off.

2) if i want a common maxwell sky but i want wo apply a color correction on it i need the original sky intact.

3) if i want to replace the existing Maxwell sky with a different one in photoshop it is very useful to have in the background the original maxwell sky to match correctly color dominances of the replacing sky.

4)if i want to blend slightly a sky with a few clouds over the existing maxwell sky i need the original sky intact.

thanks

David

[tom]

Well, this has been discussed in the past so probably you've missed that topic. I can make you sure there's no way rendering the sky and scene elements apart at the same time and it's not an alpha generation problem. Even without DOF blur you will always have AA pixels on the edge of frontal elements which has been unavoidably blended with the sky in the background. There's simply no way to place a pixel over nothingness, so there it is. In case when you also need the original sky and instead of re-rendering, you should make another render only with the sky (without everything else in the scene). So, you can still composite the original sky back in it. As rendering the sky-alone won't last longer than re-rendering, it's the ultimate remedy you should be looking for. This is not a workaround, it's just how it works.

[daros]

thanks Tom.
but only to be sure...
thechnically it never will be possible that maxwell produce following set of images in a single pass?



instead of



with the first set of images you can produce this composite in the last minute:



May with a new tag which says "do not premultiply to black"...
I understand that the first set of images is not a true alpha because some pixels are really blended with the background but it is useful in most daylight situations.

thank you again Tom

[tom]

technically it never will be possible that maxwell produce following set of images in a single pass?

No, because it'd be a huge mistake doing that.

May with a new tag which says "do not premultiply to black"...
I understand that the first set of images is not a true alpha because some pixels are really blended with the background but it is useful in most daylight situations.

thank you again Tom

I strongly suggest you reading the explanations here,
http://www.maxwellrender.com/forum/view ... 539#234539

and visiting the following link about alpha technology:
http://www.itbusinessnet.com/articles/v ... d=135386-0

This is the only industry standard available and all other fairy tale approaches are completely out of discussion. You can do shortcuts like you've presented above but I'd strongly suggest you not to do that on a professional job and well, not even for fun.

Thanks David!

[daros]

i agree obviously with the article Tom.
But we are so used to make such a kind of small color adjustments on the backgrounds of our renderings that I'm only surprised about your surprise
It happens to us continuously that clients change their mind 20 minutes before the deadline. If you want to replace the maxwell sky in an rendering which was done without "alpha" tagged on, you have to re-render it with all objects inside which can take long time.

Almost everything you see in our web page is done in this way.

One example:

original render


our procedure for quick background replacement. (a simple 32 bit image...)



using your photoshop action.


From professional point of view the only clear difference i see between this two procedures is about how the lower left ball blends n to the sky. There is a small difference but it's not so easy to understand which one is the correct one.

thanks again and sorry for boring

[daros]

P.S.
i perfectly understand that our procedure works only if you have to replace the existing backround with a similar one.

[tom]

From professional point of view the only clear difference i see between this two procedures is about how the lower left ball blends n to the sky. There is a small difference but it's not so easy to understand which one is the correct one.

If you quote this among VFX professionals, you should run fast
And your example tells a lot, I don't need to make further explanation.

[daros]

i do not have any intention and interest to publish something at vfx professionals. i work for architects, as most people around here, and 99% for competitions with very short deadlines. no time to re-render nothing even with our huge amount of processors. i was only talking about something that could be a aceptable solution in certain situations. thats all. let's stop here if you want. thank you anyway.

[tom]

David, it's not my intention to make you angry but I must point you the fact how alpha works and will work. As your post is a "wish" it may tend to cause expectations and I need to clearly outline it will keep working like this and we're not planning to do something like suggested above. Peace.

[daros]

hi tom i relly do not want to replace the existing alpha with my last minute option. i'm asking for an additional option. i propose to delete entirely this thread because my first post is confusing. i would like to replace it with following thread: additional "do not premultiply with black" option if "alpha" is on.

[big K]

hi,

just want to join daros here.
i would love to have this additional option. (also doing last minute archvis )

michael