composition of different layers from different scenes to get an amazing effect..help

Support Compositing and Post Processing
1

hi everyone!

alright here is my set up
scene 1 - background, nodes
scene 2 - a nice bike inside a dome
scene 3 - bike, no dome, plane where shadow is rendered to as alpha values

what i want to have:
a nice composition of the three, so that i have the background, then the bike where the alpha has been set from scene 3. this way i can get the nice reflectiveness fromt he dome render in that background

the problem:
around the bike, or the lower section, a white line appears (most likely fromt he dome image that the alpha channel from scene 3 hasnt masked properly???

there is no contact shadow, altough i want the shadow from the dome using the alpha to map correctly intot he road,

alright

so any idea, clues, or hints as to what i could do???
i would really appreciate any help regarding these nodes:S

well thanks in advance
-BlendMaster-

2

did you click PreMultiply Alpha button on the AlphaOver node?

3

yup i did,

without it, its a huge thick line, with it its fairly faint, but noticable!

any clues?

4

Be sure you turn off the sky option in all scenes except for your background scene. 99% of the time that is where these halos are coming from. The sky is ALWAYS the last element that any object’s alpha channel multiplies against, there fore the alpha channel of said object takes on the color of the sky. Using the sky is really a bad option in version 2.43 for this verry reason (and others). I suggest rendering an image of the sky and importing that image into an image node, then using a set alpha node to bring the alpha strength up to 100% opaque.

There is no need to use seperate scenes for your project though. Save that technique for when things begin to get more complex. See this post for how to properly manipulate render layers to get cool effects:
http://blenderartists.org/forum/showthread.php?t=84751
simply creaating all objects on their own seperate layers and picking which ones should be rendered on a per renderlayer basis will allow you more freedom of expression than just aqbout any other technique that Blender has to offer.

5

1st

how do i turn sky colour off??? i looked and looked, but couldnt find it

2nd, i cant have it all in a single scene, then my shiny parts dont look the way i want them to look, what i wanted is my 3 scene setup, that gets everything perfect,
except that i dont have a shadow, and that thin line from alpha over

…how would i add the shadow?

thanks btw:)

6

you can set it to black by Material World settings. or click the X next to the name of the world settings to delete the link to the datablock.

7

You can have it all in a single scene and get it exactly the way that you want it to be! You simply need to be aware of the manner in which Blender’s render pipeline works. The link to the post that I gave you clearly outlines the manner in which you need to configure your scene layers and render layers. If you try the technique in the link you will likely be astonished, though this is Blender functioning on it’s most basic level!

To render without a sky simply enable the “Premul” button on the render tab.