comp doesn't work ?

[bernhard]

Hello friends,

this is really crazy.. I do have a simple scene file setup in which my render passes are not working for the compositing.

I made several comps before and had never such a problem.

But in that specific scene file must be something that makes it impossible to composite just two simple passes...

after rendering, I put just two layers together

- reflection (linear dodge - add)
- gi (normal)

after comping it should give you the final RGB... just for that specifc file... because there is not Lighting, no specular, etc.

but guess what, if you try to composite just the gi and the refelction, it will NOT match with the final RGB.

for some reason the composition is brighter in the reflections and does not match with the final rgb ... grr.....

the weired thing is, if I load a different render preset it works.

but I want to find out what it is, what's causing that problem.

so, here is this crazy file: max2009
http://www.bernhardrieder.com/ask/test_comp.max


thx for every littel help,
bernhard

[DanielBrew]

Your gamma is set to 1.4, have you tried with it set to 1?

[bernhard]

yep

I tried already almost every gamma as well..

I compared all the settings to just one other.. can't find anything that makes the difference..

just wanted to find it... well, I guess I will load my other preset, and make my settings again.. but it's just weired.. wanted to find what it is...

[Sushidelic]

Hi!

It's definitely the gamma that produces the error. Uncheck "color mapping" on the reflection and gi render element, add them together and apply a 1.4 gamma after that, then you will get the desired, identical result.

Although i would always recommend to work with linear elements, i would note that as kind of a bug, as you would expect it to work identical both ways.

Best regards,
Michael

[Sushidelic]

I quickly dived into a mathematical approach of this problem, and it is quite obvious that gamma burned into render elements is likely to cause problems.
I will try to explain what i mean:

In a rendering, you have a pixel that is exactly 1.0, consisting for example of 0.5 gi and 0.5 reflection.
This pixel will always be 1.0 with whatever gamma as 1^anything = always 1.
now for your elements, the color mapping is applied individually to each element.
With a gamma of 1, there's no problem, as
0.5^1 + 0.5^1 = 0.5 + 0.5 = 1.
But what about a gamma of 1.4 or else?
0.5^(1/1.4) + 0.5^(1/1.4) = 0.5^0.714 + 0.5^0.714 = 0.61 + 0.61 = 1.22 (!)
There's another way of dealing with pre - color mapped elements, as you could also apply the inverse gamma to each element, adding them, and then in the end adding the rendered gamma on top of your comp.

Hope this explains it a bit better,
Michael