Blood 'n guts 'n after fx

I haven't posted in a while so I thought I'd post a real good'n. I've never really talked about compositing and I'm often told "hey can't we just fix it in the computer later." A lot of times I say yes but always reluctantly. Compositing is a slow, tedious, detailed, mind numbing, processor crunching, and an insanely complicated, time consuming process. Or in other words, compositing is 24 or 30 photoshopped images for every second of video that needs to be fixed. Or for 5 seconds of video 150 pictures will have to be photoshopped (painted, corrected, layered, keyed, tracked, rotoscoped) while keeping their consistency and movement accurate across the scene. If you're already exhausted then you're getting the idea.

To show this and to detail what compositing is for those who still aren't grasping the idea. I'm going to tick through a few scenes from last years film "the Hitchhiker".

SCENE - DOC IS KILLED
Josh stood in front of the camera and mimed being hit by an axe.

Later we shot Matt swinging an axe across the shot.

From these two shots I was supposed to create a horrifying scene.
After about 6 hours of work this is what I came up with.

12 hours seems long for less than 3 seconds of footage but it makes sense when you break it all down:

Step 1: The original shot is exported into a RAW(Just images no compression) quicktime file that can be manipulated on a frame by frame basis. This is because HDV is a "lossy" interframe codec or by its proper name "mpeg". Mpeg only stores every 15nth frame and just change data for the other frames. This is how it compresses gigs of HD video down to a few megs. If After FX imports the HDV it'll have to constantly re-render frames just to play back - let alone composite. So RAW it is.

Step 2: The Axe footage is manipulated with the time stretch filter. It's sped up, but also slowed down when it hits, so it looks like it stopped on flesh. It's rendered with pixel accurate frame blending so it doesn't look like it's been sped up or slowed down.

Step 3: The rendered Axe footage is then rotomatted or as I usually call it rotoscoped. This is where I surround something (the axe) that I want to cut out and paste into another scene. Remember, I have to do this to every frame - so, 3 seconds = 90 frames of surround'n stuff. So, if it takes me 2 minutes per frame... well you get the idea.

Step 4: The rotoscoped axe is then layered on top of the original shot - or as it's usually said "composited" over the original shot. That's why it's called compositing.

Step 5: The axe doesn't look like it's going behind his head and into his neck - so we create another rotomatte around Matt's head that Overwrites the axe footage with the original footage, making it appear the axe is disappearing behind the head. Again, this is done to each frame individually.

Step 6: We need blood. Usually you'd film some blood spewing in front of a green screen. The green screen allows the computer to automatically remove everything that's green. Or it auto-rotomattes it for you - nice. I didn't want to spend the hours making gallons of blood and a green screen to throw in front of - so I decided to do it with particles.

After Fx has a basic particle simulator. This allows you to generate, via fractal or random numbers, hundreds of dots or paricles. You can then give these dots or particles color and other properties. You can make them bounce, float, or fall with gravity. And if you do it right, they can look like smoke, water, oil, or even blood.

Step 7: Put it all together and render. You usually tweak the layer's color, light, motion blur, etc.. at this point to make it seem seamless.