Rendering Alpha channel image only

[GIJOE17]

Hi is it possible to render an Alpha channel only after the fact? e.g. I rendered an image which did not create an alpha channel image because the site plan was on. I have since turned it off and only which to render the alpha to cut out the house from the previously rendered image and was hoping this could be done and that it would reduce render time....

Regards

Joseph

[bocomofo]

I wouild say it is, in the VFb rollout just uncheck RGB colour, and leave only the alpha highlighted. should go pretty quick too.

[andybot_cg]

(edit - yes, you can select just that channel, my mistake). You could re-render with GI turned off and just do the raytrace, and reduced samples. Might get you there reasonably quickly. I've had that happen too, such a pain. Though if anyone has a magic solution, I would love to hear. I'd also like to see the vfb history come back...

[bocomofo]

but you can just render the alpha channel?, and if you reduce your setting right down, i.e. you dont need any lights, or GI, or secondary engine settings, or material settings.
Once you've switched almost everything off save the settings as a visopt file, that way if you ever do it again you can just load these settigns in, you just need to adjust the output file size to match your original render.
Not a magic one button solution, but it does ease the pain slightly. Oh it wont help with the the time it takes vray to process the scene though.

second that motion for a triumphant return of the VFB history

[GIJOE17]

Hi Guys,

I have lost the option to choose Alpha in this file. When I look under the RGB pull down Alpha is missing? any advice? I have one scene with a white background and another with a transparent background (for Alpha). Please help me!