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NDC doc fix (#28)

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Summary:
- Added clarifications about NDC coordinate system for square and non-square images.

Pull Request resolved: https://github.com/fairinternal/pytorch3d/pull/28

Reviewed By: nikhilaravi

Differential Revision: D31681444

Pulled By: bottler

fbshipit-source-id: f71eabe9b3dd54b9372cef617e08f837f316555b
This commit is contained in:
Ignacio Rocco
2021-10-17 07:41:11 -07:00
committed by Facebook GitHub Bot
parent 14dd2611ee
commit 16ebf54e69
2 changed files with 6 additions and 3 deletions
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docs/notes/cameras.md
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@@ -13,7 +13,7 @@ This is the system the object/scene lives - the world.
* **Camera view coordinate system**
This is the system that has its origin on the image plane and the `Z`-axis perpendicular to the image plane. In PyTorch3D, we assume that `+X` points left, and `+Y` points up and `+Z` points out from the image plane. The transformation from world to view happens after applying a rotation (`R`) and translation (`T`).
* **NDC coordinate system**
This is the normalized coordinate system that confines in a volume the rendered part of the object/scene. Also known as view volume. Under the PyTorch3D convention, `(+1, +1, znear)` is the top left near corner, and `(-1, -1, zfar)` is the bottom right far corner of the volume. For non-square volumes, the side of the volume in `XY` with the smallest length ranges from `[-1, 1]` while the larger side from `[-s, s]`, where `s` is the aspect ratio and `s > 1` (larger divided by smaller side).
This is the normalized coordinate system that confines in a volume the rendered part of the object/scene. Also known as view volume. For square images, under the PyTorch3D convention, `(+1, +1, znear)` is the top left near corner, and `(-1, -1, zfar)` is the bottom right far corner of the volume. For non-square images, the side of the volume in `XY` with the smallest length ranges from `[-1, 1]` while the larger side from `[-s, s]`, where `s` is the aspect ratio and `s > 1` (larger divided by smaller side).
The transformation from view to NDC happens after applying the camera projection matrix (`P`).
* **Screen coordinate system**
This is another representation of the view volume with the `XY` coordinates defined in pixel space instead of a normalized space.