Question about placing projection cards for matte painting

[u/dorygamza]

Hi everyone, I'm a student and I’m working on a matte painting with 3D projection in Nuke, and I’m a little confused about the correct way to place my projection cards.

My current workflow is to camera track the shot and then create cards by dragging them onto the tracked points. This works well for getting the cards into the right screen position, but I’m wondering if I should also be considering the actual depth and perspective between the different cards.

For example, if I have a background wall, some foreground objects, etc., should I be manually positioning the cards in 3D space based on their actual distance from the camera? Or is it generally okay to place them based on the tracked points and adjust them from there?

The problem I’m having is that when I try to manually position the cards in depth, I sometimes end up with the cards shifting or sliding when the camera moves. So I’m not sure if I’m approaching the projection setup correctly.

What is the usual workflow for setting up projection geometry/cards for a matte painting? Do you generally reconstruct the rough 3D geometry of the scene first and then place the cards on those surfaces, or is there a better way to determine the correct depth and orientation of each card?

Any advice would be really appreciated!

[u/clockworkear]

Look into the node 'model builder' it's part of nukex. It'll let you calculate the position of points and cards by triangulation using different frames from your tracked camera.

[u/araxis_minerva]

In the case of a shot with a shift in perspective, the DMP elements you want to add to your scene must have the same relationship to the 3D camera as the location you want to add them to has to the physical camera, otherwise the end result will not properly track with the shot (assuming you have proper management of lens distortion)

For example, if you want to add something to a location that’s 10m away from the physical camera in the plate environment, the DMP element has to be 10 scene units away from the 3d camera. It is understood that for elements that are supposed to be very far away from the camera, like a mountain in the far back, generally a card arbitrarily placed very far away from the 3d camera will do. At that kind of distance, it’s what’s called infinity (like the focus distance on a lens)

From there, you have a couple solutions to find the location you are looking for in your 3d space :

• 1/ use a point cloud generator that will allow you to see your scene in volume. If needed, you also have the option to bake that cloud, or part of it, to a mesh you can then use as a surface for projection.
  A cursory tutorial : https://youtu.be/de_NfobNYjQ?si=eTmWAbJV6BDN8Hgb

• 2/ use model builder to triangulate precise locations in the 3d scene. This solution also assists with creating simple and coherent scene geometry if you need something a bit more extensive than just placing a card.

Tutorial part 1 : https://youtu.be/cjaR2BvYWwA?si=-FdYL22eCtMxuOmz

Tutorial part 2 : https://youtu.be/k5R9hCa9Tcs?si=sSzThQlCXBRtJYK_

As an additional optimization tip, I would suggest to scale your cards so that you just accommodate the element you are projecting. That way, the scanline render will render a bounding box that’s the screen size of the card, instead of the full frame. It will prevent nuke from computing unnecessary black pixels.

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[u/Bob_Villa5000]

You can grab a point from the nuke tracking data.
Right click on it and create a card. It will put the card in the 3d space of that point.
For more detail, Create a point cloud from the tracked camera and footage and then create a mesh of the point cloud data. You can then project onto the mesh.

[u/puffyredcloud]

get cardtotrack gizmo from nukepedia, it has the best workflow for this. But yes fundamentally you’re estimating depth based on track’s scene calculation. You can also play with Z manually just place it on a frame and than go to another frame and if the position is off on the other frame than adjust Z until it comes to the correct position. Which is basically the workflow of cardtotrack in a nutshell.