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Stefan Wills_90
Known Participant
March 10, 2024
Question

Blood Splatter Asset on a Moving Object With Occlusion

  • March 10, 2024
  • 2 replies
  • 1101 views

Hello guys,

 

I'm looking for some more advanced help on a tack I'm trying to do. It's a chase scene from a short horror movie that I've edited. There's a moment when the villain gets to the victim and slashes her twice with an axe. I though about adding a blood splatter onto his clothing right after the first time he swings at the girl. The shot has a lot of movement, motion blur, and an occlusion, because he raises his hands with the axe, and rotates his torso. So after spending the whole yesterday trying different ways to stick the blood splatter video asset to the front of the yellow jumpsuit, I don't know if what I want to do is achievable with this type of shot. Here's what I've tried:

 

1 - Motion tracking the zipper at the top of the jumpsuit with the AE tracker - no chance, the tracker point goes away very quickly.

 

2 - 3D camera tracking - I was able to get just a few tracking points on the mask, but they appear and disappear as he moves and turns his head. I knew it wouldn't do the trick, but had to try.

 

3 - Motion tracking with Mocha Pro - I thought that the Power Mesh would have the best chance at this if I want the blood to stick and look more real. I'm not a pro with Mocha, so this stuff is relatively new to me, but from what I understood from tutorials, I should create a mask layer of the hand and the axe as an occlusion object and place it at the top of the list, then add a new layer below with the torso tracking I need for the blood asset. It was a difficult track, because it often said there weren't enough pixels for the tracking. So I had to track larger areas of the object I didn't want. With that aside, when I did the tracking, I turned on the power mesh and tracked it again, but the complexity of the shot didn't produce good enough mesh tracking, it was a total chaos. I still tried to import the blood asset within Mocha in the Insert section to see the result - the blood splatter didn't stay the way it's supposed to during the whole time, plus it was visible over the raised arm and axe (so the occlusion didn't work).

 

I'm uploading a screen recording of the settings I've done and I'll be very thankful for any support.

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2 replies

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Community Expert
Rick GerardCommunity Expert
Community Expert
March 11, 2024

If you want to add the wound to a shot, the shot should be the only thing in the comp. AE is not a video editing app. Use it to create the shot with the effect.

 

Mocha Pro/Mesh Tracking should be able to handle the shot, but you will have to create and probably animate by hand two or more mattes for the arms to place above the Mesh Track of the chest. Mesh tracking, even if done in small pieces, should be able to handle the shot. 

 

Here is how I would set up the comp. 

  1. Pick the first frame where you want the blood spatter to hit the chest of the villain and split the clip (Shift + Ctrl/Cmnd + d)
  2. Go to the frame where the chest is turned away from the camera, and the blood spatter would no longer be visible and split it again
  3.  Move forward a couple of frames to where the blood would be visible again and split the layer again
  4. You now have two layers that are only as long as they need to be (you might be able to have only one) 
  5. Select the layer (or layers) containing frames where the blood will be visible, pre-compose, move all attributes to the new comp, name the comp appropriately (Chest 1 and Chest 2, for example), and open the comp.
  6. Add an adjustment layer to the renamed Chest comp, add Lumetri, use the Color Workspace to see the scopes, and make adjustments to get as much data in the shot as possible. You need a detail-enhanced shot with the maximum range of pixel values so there is sufficient detail to track. Your shot is quite underexposed and flat. The Color Correction will be turned off later.
  7. Pre-compose both layers and name this comp something like Blood Overlay
  8. Add Mocha Pro and start your tracking.
  9. First, create a spline for the arm as it obscures the chest. You'll probably have to hand roto this layer and name it Arm Matte
  10. Add a mesh layer for the blood spatter, drag it below the Arm matte layer, and set layer in and out points
  11. Enable and fine-tune the mesh to give you sufficient data to run the track
  12. Fine-tune and adjust the mesh

 

You should be able to insert your blood spatter as a distorted new layer in the Blood overlay comp as a new layer, set the pre-composed and footage layer as a guide layer so it won't render, then return to the main comp and render the shot.

 

If you are doing extensive color correction, I would render only the blood overlay comp. It will probably take a couple of hours because of the motion blur and the hand roto that will be required. 

 

Boris FX Mocha Training

 should give you a good idea of how to accomplish the workflow. I've used these techniques many times, from adding blood and scars to graphics and even removing or adding tattoos. The only way to make the project manageable, even if you are using Davinci or Fusion, is to trim the shots so you are only working with the frames you need to modify. Trying to work on complex effects shots as part of a longer comp is just asking for trouble.

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Stefan Wills_90Author
Known Participant
March 12, 2024

Hi Rick and thank you for your comment on my thread. I did try to do something similar. If you can see from the screen recording video I did,

 

1 - I did split the layer into 2, but now when I think about it, I need to split it into 3, because after the second time he lifts his hand and swings, the blood should still be somewhat visible (though distorted due to him turning his chest).

 

2 - I also did a separate layer with the xpline tool and manually tracked the whole occlusion, but when I added the mesh track below and inserted the blood asset into mocha from the Insert tab, it wasn't occluded when it should have been. Could you please tell me where I went wrong with this?

 

3 - Could you please confirm if I understand correctly your points 5 and 7 - I should first precompose the split layers I need for the track, then within each comp I should apply an adjustment layer for color correction, then go back to the main comp and pre compose all 3 precomps into 1 (the blood overlay). Is are the child pre comps only with the purpose of color correction? If so, is the first pre comping necessary? What would be the difference if I applied a Lumetri color straight to each of the original split layers and then pre comp them all to add Mocha?

R
Community Expert
Rick GerardCommunity Expert
Community Expert
March 12, 2024

You can apply the color correction after you trim when you are color correcting to improve tracking, but I almost always pre-compose again with a color correction added to keep track of the comp names easily. I always pre-compose, trimming to the layer length when tracking to limit the number of frames for tracking. If you trim a layer and then do any motion tracking without the pre-compose and trim workflow, the other frames are still there, and it's pretty easy to get lost in the interface of Mocha and even other trackers.

 

I use adjustment layers most of the time because there may be other effects on the layer, and the temporary color correction is easier to turn or manage if it is not a separate layer. You can apply the correction directly to the clip if you want. The adjustment layer does not add any render time. It just makes it easier to turn it off before rendering.

Mylenium
Mylenium
Legend
March 10, 2024

DaV inci Studio/ Pro, Nuke and Flame have AI-based normal map generation to place logos and stuff on deformable objects. Otherwise I'd probably not bother with any tracking at all. Unless you want to spend a lot of money on a tool liek Lockdown (https://aescripts.com/lockdown/) it's probably a lot more cost-efficient to just grind through manually mapping stuff with Mesh Warp, reshape, the Puppet Tool. It's the old gag of staying calm and having a cup of coffee or two while doing it. Stabilizing the camera shake anbd reintroducing it afterwards would further expedite the process, as your shout is pretty "flat" and rather short, so doing this manually couldn't take more than a lazy afternoon even with limited experience.

 

Mylenium

S
Stefan Wills_90Author
Known Participant
March 12, 2024

Hi and thank you for the reply to my question. I'll test the puppet pin tool/warp combination to see what I can achieve. I'd still have to have the occlusion with the arm and axe though, right? Could you please give me a hint at what I did wrong regarding this, because I did manually track the occlusion with the spline tool and placed the layer above, but when I played the video, the blood stain didn't get occluded. Also, I see that the Lockdown plugin has a free version. Would it suit me for this purpose?

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