save background through window as alpha

i want to save a scene with an alpha channel, so that i can open my image and can do some postpro on the background. i tried the “premul” option and set rgba. now i get the background as alpha, but where i look through transparent objects like a window there is no partial alpha (i still see the image opaque). is there a way to get all areas where i can see the backgound as alpha gradiant channel?

i’m using blender 2.36 internal raytracing.

key instead of premul?

no, doesn’t work :frowning: do i have make additional settings in my materials?

It does work for me. I used PNG and Premul. Maybe turn on ZTransp? (In the Mirror Transp tab.)


did you set ray in rendering settings? as soon as i do so and i set mirror or transp to ray it doesn’t work anymore. am i missing something?

is there another way to simulate the mirror effect of a glas window without using ray?

As you have noticed, you can either choose ZTransp or Ray Transp. What do you mean by simulating a mirror effect? Ray Mirror and ZTransp do work together, don’t they?

As an answer to your question: yes, there is another way of simulating reflections: envmaps. Raytraced reflections are easier to set up and aren’t “faked”, so they’re more commonly used. Envmaps still work though:

hi egg,
thanx for your reply. the problem is, that even if i set ztransp, the windows aren’t stored anymore in the alpha channel as soon as i use raymirror.

could you check if it’s the same for you? i tested it in the standard cube scene, so i think i can’t do a lot things wrong - but maybe :wink:

meanwhile i will test the envmap.

You’re absolutely right; ZTransp and Ray Mirror don’t work together. Did you get envmaps to work?

hi egg,

i solved the problem in a different way :wink: thanx again

you can see the result here:

i just found, that the problem will be solved in the next release - as far as i understand the technical description on -> changes since 2.36 :smiley: