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Search the help files for Animating Masks. Search the Web for Animating masks.
Here's my quick tip. Draw the mask on your red solid so that the pool of blood looks like you want it to look when the shot ends. Move the CTI to the time in the timeline when you want the blood to stop flowing. Reveal the mask properties by pressing MM and set a keyframe by clicking on the keyframe icon in the timeline.
Now move to the start of the shot and start grabbing vertices with the direct selection tool and moving them into the starting position for your pool of blood. Vertices always move in straight lines so make sure that you think about that when you are moving your vertices. Don't delete any of them. If you've got rotobezier masks available (I don't remember when they came out) they will be easier to manipulate.
Now scrub through your timeline and observe the mask. As it changes pick points to move the keyframes around a bit and change the shape.
There you go. Your animated mask.
Here's a thought though. If you have a video camera, any kind of video camera, get some water and some corn starch and some black ink or maybe even some food coloring. Mix the corn starch with the water until you get a liquid that runs like blood. Now set up your camera so that it is approximately at the same angle that the floor is in your shot. Set down some plastic wrap or a show card or some poster board and then let the mixed up blood run through a tube or just slowly pour it into the shot. When you get the consistency right add ink or food coloring to the mixture and shoot it for real. Bring that into AE and you'll have a shot that you can easily key (using chanel effects) that will look like real flowing liquid because it will be real flowing liquid. The whole process will probably take half the time that it would to animate a mask so that it looks like a real flowing pool of blood. You'll be able to colorize it later.
Thank you - knowing what to search ALWAYS makes it easier to search.
No, it wouldn't be faster to shoot it as I would never be able to get it to look like what I need it to look like. I need control over it. I need to start with a black screen, and then blood slowly begins to flow down into the frame, and spreads out in a pool. Sounds like this will be a real ***** to animate - guess I go with the crappy version of just having a picture and doing a wipe. Dang. I didn't think this would be so insane - it seemed simple.
Thanks though. Now I know.
Is there no way to do this with the expansion? The only reason that wasn't working is because the animation starts in the center of the blob - can I not somehow make it start at the top?
Try drawing your mask, then adjust the anchor point of the mask to where you want the ooze to start, then animate scale and expansion. Combine that with animating the shape of the mask and you'll be good to go. Use this for a track matte for another solid and apply your edge effects to that.
I would approach this with multiple masks. One for the final shape (set to add) and another one or more (set to intersect). This wayyou have perfect controll over the final shape and create individual streams. Use layer styles/bevel for some bevel and highlights. Another way to fill your final shape would be the Mr. Mercury plugin applied to your precomped shape.
One item that has not been broached yet: do you intend this trickle of blood to flow upon a planar surface or an irregular surface? One is easy, one is not quite as easy....
Anchor point - that's it - yes. I just can't see one, so I didn't know how. I'll try a search for that. Thanks!
Thank you, Klaus - I'll give those a try! I was using "subtract" on the second layer, but I'll try these methods and the plugin.
Dave: the projection will be on a flat floor - nothing fancy. Technically it would be completely wrong with how the blood would pool, but it's a new media art piece, so there's a certain look I am going for.
Mr. Mercury seems to be working the best so far, so THANK YOU.
I've managed to get it to look right, and I can position it so that it flows like I want, but there's one thing I can't seem to fix:
I have made the effect create a nice blob that moves very nicely, but there's a line that appears on the outside of the blob.
I'm pretty sure this is because you applied mr. mercury to a masked layer. Use Mercury on a seperate layer to roughly reveal the shape and either use your final shape as a matte or precom mr.mercury (move all attributes...) and then aply the mask.
I have no idea what any of that means. I can only get Mr. Mercury into a blob. When I put it on its own layer, it's a very large blob. There's no extra lines, but if I just cut a whole in a mask on a layer above it, then I don't get the blood sides and shading.
I guess I'll try a frame by fram cover mask over the lines cause at least it's working.
Thanks for the help though everyone!