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Need Help with Compositing Basics.

Discussion in 'Fan Films, Fan Audio & SciFi 3D' started by citizen, Sep 9, 2002.

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  1. citizen

    citizen Jedi Youngling star 2

    Registered:
    Dec 19, 2001
    I am getting into composite rendering and trying to learn the basics. I'm using three still images. 1st image is a model of a space craft behind a black BG, 2nd pic is a white mask of the craft with a black BG. The 3rd is a picture of stars for the backdrop. I'm importing all the images into PhotoShop. The layers are in the same order as the pictures above (ship pic on top, stars pic on the bottom with the white mask in the middle). I set the top layer to screen and I notice that no matter what I do with the mask layer. The stars seem to bleed through the ship. I've seen several tutorials and it seems to me I'm not making an alpha channel. I can admit I know my way around PhotoShop with the exception of channels. So in short I need someone to check my settings and if making an alpha channel is the key to my problem. Then in terms a toddler can understand. How the heck do you make an alpha channel? This example is only for still pics so I assume whatever process for Photoshop should work for After Effects too. PLEASE HELP
     
  2. tumblemoster

    tumblemoster Jedi Padawan star 4

    Registered:
    Dec 1, 2000
    It sounds like you are almost there, but not. I'll make some assumptions about what you are trying to accomplish, and we'll go from there.

    I assume that your middle layer (the one with the black and white out line) is the layer you wish to assign as an alpha channel. NOw, since this is a simple still composite, you really have no need for an alpha channel, but, since you will eventually want to be able to use one, I will show you how to make an alpha from your center layer as well.

    First things first. To get the result you want (ie a ship with a starfield background) is fairly straight forward. First, you need to select the black space of your center layer (which we will from now on call your mask layer or matte layer, because that is what it's for). Depending on how your mask layer was created, there are several ways to select it: If it is solid white inside solid black (the white should be the ships shape) either set a very low tolerance on the magic wand tool and select the black space, or choose Select>Color Range from the select menu and select by black. Basically, your goal is to get a selection from which you can then make a mask. After you have your selection, click on the ship layer and then (if you have photoshop 6 or later) click the "Add Layer Mask" button. Voila! You now have a mask layer outlining your ship, with a starfield in the background. You won't need to change the blending mode either, you can leave it set to normal. If for some reason you selected the wrong color, or if you only see stars where the ship should be, your mask is backwords. Simply invert your mask (which now shows on the same layer as your ship, just to the right of the small image in the layers palette), and the image will appear as it should. You can then also delete or simply make invisible your center layer, as you no longer need it.

    Now, in your original question you asked about alpha channels. In a still image, an alpha channel is used to store information in addition to the standard color information. The alpha channel is most often used in animation and compositing to determine the matte shape used to composite one image into another. The layer mask you created is in effect an alpha channel. For the simple composite you just made, an alpha channel is not needed. But, to make an alpha channel from your center layer is a simple matter. Do as you did above, making a selection form the center layer, but this time, instead of clicking on the ship layer and creating a layer mask, do this: Click on the ship layer as before, then click into the channel list. Now, with the selection still available, click the "create new channel button" and a new alpha channel will be created for the ship layer. You can then export as a targa and use it as a premultiplied image in a compositing from program.

    I hope some of this helps! One more suggestion I would make: If you are rendering anything for compositing (rendering from your 3d app that is) always render to a 32bit targa sequence. This a creates an RGB image with an included alpha channel. Then you don't need to create one at all, as your image is exported with a perfect alpha channel every time.

    Good luck! If you have any more problems or have additional questions, don't hesitate to ask! Or, pop into the #fanfilms IRC. I usually drop in nightly.

    -tm
     
  3. citizen

    citizen Jedi Youngling star 2

    Registered:
    Dec 19, 2001
    Hey Tumble,

    Just want to say thanks for taking the time to type all that down for me. Your step by step instructions will in no doubt help solve my problem. Can't wait to get home from work and solve this problem of mine.

    Thanks again and I hope I can return the favor one day.
     
  4. Shawn PTH

    Shawn PTH Jedi Padawan star 4

    Registered:
    Apr 22, 2000
    TARGA is your friend.
     
  5. Antilles01

    Antilles01 Jedi Youngling star 3

    Registered:
    Nov 5, 2001
    Targa IS your friend!
     
  6. Admiral_Ant

    Admiral_Ant Jedi Youngling star 1

    Registered:
    May 6, 2002
    and a very disk-space-hungry friend it is.....
     
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