Compositing

[trickyo25]

I need to composite two avi's together in video post. How do I set the alpha channel for the background, so I can layer the two renders together. Does anyone no how to set the viewport background to render transparent. orangeno

 

[ January 27, 2004, 02:23 AM: Message edited by: trickyo25 ]

[Jennifer]

Hi,

 

AVIs don't have an alpha-channel, so that would be problematic if you don't have a 'key color' to work from. The Psuedo-Alpha compositor in the video post works on the color of the upper-left pixel in an image. All pixels in the image that use that color are then completely transparent. This type of transparency isn't the best looking, as the edges of transparent objects are not blended with the other layer.

 

The pseudo-alpha is a Layer event in VPost.

 

Ideally you would render-out the overlay images to 32-bit TGA or PNG images that have alpha-channel capability, and use the Alpha Compositor in video post. Then the background would be transparent in the TGA or PNG, and objects with varying transparency would also be handled correctly (glass, etc).

 

If you don't have that luxury or re-rendering, then you can use VPost's pseuso-alpha if the upper-left pixel color is useable, or look into Combustion that has a professional chroma-key compositor, or Premiere Pro or After Effects. With these other options you would still need a background color that was consistent so that it can use for the transparency, such as using 'Superblack' when rendering. Then, every rendered pixel will be a color greater then 15. Of your background is black, then you could safely use that for compositing. You would want to turn OFF antialiasing to the background color, otherwise your rendering will have a weird black trim.

 

For several reasons I never render to AVI unless it is a tiny test render. First, only one machine can render the job at a time, and that is not efficient if you have more than one machine. Second, if the render crashes, you've lost all of your rendering. If you render to individual files and create the AVI later, then a machine crash only affects the one frame it is working on, and everything before that is preserved. Multiple machines can network render, also.

 

Have fun! :ebiggrin:

 

Jenni

www.4dartists.com

 

[ January 27, 2004, 10:53 AM: Message edited by: Jennifer ]

[trickyo25]

Thanks very much for the suggestion. What would be the desired format to render for Combustion? I tried rendering a targa sequence, rpf sequence, however it comes into combustion all black. Can you think of anything that I am doing wrong and why the objects in my scene don't appear in combustion once I import the sequence. Thanks again Jen for your previous explaination it was very helpful and saved me a lot of time.

[Jennifer]

You are welcome.

 

Make sure you change the TGA parameters to '32 bit' so you have alpha information. PNG has other options for how many colors to save, including a color depth greater than TGA. TGA is a 'loss-less' compression format, whereas JPG is a 'lossy' format, and quality settings less than 80 start to show a noticeable loss of image. PNG does not have a Quality setting, and is lossless. It also can save Gamma information with the image, but I'm not certain max does this.

 

RPF lets you save additional scene and render information, which is useful if you need to do 3d compositing or effects later. PNG should be fine for compositing. Only the layer that needs transparency needs to be in PNG; JPG at 100% quality is fine for video without alpha.

 

Create a new Composite in combustion. Import the two image sequences. Make sure that the layer you want on-top of the other is listed first under the Composite operator in the Workspace. The background should be just below it.

 

 

Have fun! :ebiggrin:

 

Jenni

 

[ January 28, 2004, 09:38 AM: Message edited by: Jennifer ]

[STRAT]

looks like the latest indeo encoder allows an alpha channel -

 

[Jennifer]

Hi!

 

That's good to know. I'll have to try that and see if other programs support that within an AVI file. Either way, spending a lot of time rendering out to AVI is risky business! :angeuhoh:

 

Jenni