Render background to alpha...how?

Hi 3dpeeps :slight_smile:

I’m wondering how and IF it’s possible to render
background transparent when we render with Blender.

Uses: Eg. when I create a cartoon character and
want to use it in a paint-program…so that I might
put this cartoon character say…into a forest…

Of course I can use the Magic wand…AND I can use
“make-white-transparent”…but it’s far from optimal
and it takes hours to nibble-away the little “grey-scales”
that gets left over from Blenders anti-aliasing.

I’d like to render to Transparent background…so the
object itself are either on a different layer or that
the background is completely transparent (non-existant).

Any help appreciated.

Sure. Turn on RGBA (RGB+ Alpha) on the Render panel, render and save the resulting image in a format that supports an alpha channel (PNG, TGA).

also you have to click on ‘premul’ or ‘key’ in render buttons

arangel & Modron:

Thanks for that information! Excellent!

:smiley:

Of the choices: Sky, Premul, Key … why is “Premul” correct? And why does Blender say that it “assumes ‘Premul’” in certain cases?

What, can someone explain to me, were each of these choices “intended” for?

In a quote from another thread:

… nevermind %| … I found this thread: http://www.elYsiun.com/forum/viewtopic.php?t=135&highlight=premul

premul is if you want antialiasing on the edges. key leaves a definate edge. key is probably better for making sprites.

Key antialias the alpha layer but leaves the color as it is. (IIRC)

Martin

Quick compositing tests show that premul is the appropriate setting in my case.

  • Sky, the default, seemed to cause unacceptable “darkening” of the shot as the composites were added, especially when shadows were used. I presume that the world-color was “polluting” those areas. - Key created noticeably rough edges between the composited layers. - Premul avoids the darkening and creates edges on the layered objects which blend well in compositing.
    Most of my foreground and midground slices include shadows (created by using the OnlyShadow setting on the materials which receive shadows from visible objects in that slice, you do need to temporarily set it to Sky so that you can see where those shadows will lie, then switch to Premul for the render.

(I’m using the word “slice” to refer to a layer of ‘film’ that will be composited with others. I don’t know the film industry term.)