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Summary: Fix some comments to match the recent change to transform_points_screen.

Reviewed By: patricklabatut

Differential Revision: D33243697

fbshipit-source-id: dc8d182667a9413bca2c2e3657f97b2f7a47c795

Summary:
All the renderers in PyTorch3D (pointclouds including pulsar, meshes, raysampling) use align_corners=False style. NDC space goes between the edges of the outer pixels. For a non square image with W>H, the vertical NDC space goes from -1 to 1 and the horizontal from -W/H to W/H.

However it was recently pointed out that functionality which deals with screen space inside the camera classes is inconsistent with this. It unintentionally uses align_corners=True. This fixes that.

This would change behaviour of the following:
- If you create a camera in screen coordinates, i.e. setting in_ndc=False, then anything you do with the camera which touches NDC space may be affected, including trying to use renderers. The transform_points_screen function will not be affected...
- If you call the function “transform_points_screen” on a camera defined in NDC space results will be different. I have illustrated in the diff how to get the old results from the new results but this probably isn’t the right long-term solution..

Reviewed By: gkioxari

Differential Revision: D32536305

fbshipit-source-id: 377325a9137282971dcb7ca11a6cba3fc700c9ce

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

Summary:
API fix for NDC/screen cameras and compatibility with PyTorch3D renderers.

With this new fix:
* Users can define cameras and `transform_points` under any coordinate system conventions. The transformation applies the camera K and RT to the input points, not regarding for PyTorch3D conventions. So this makes cameras completely independent from PyTorch3D renderer.

* Cameras can be defined either in NDC space or screen space. For existing ones, FoV cameras are in NDC space. Perspective/Orthographic can be defined in NDC or screen space.

* The interface with PyTorch3D renderers happens through `transform_points_ndc` which transforms points to the NDC space and assumes that input points are provided according to PyTorch3D conventions.

* Similarly, `transform_points_screen` transforms points to screen space and again assumes that input points are under PyTorch3D conventions.

* For Orthographic/Perspective cameras, if they are defined in screen space, the `get_ndc_camera_transform` allows points to be converted to NDC for use for the renderers.

Reviewed By: nikhilaravi

Differential Revision: D26932657

fbshipit-source-id: 1a964e3e7caa54d10c792cf39c4d527ba2fb2e79

Summary: Collection of spelling things, mostly in docs / tutorials.

Reviewed By: gkioxari

Differential Revision: D26101323

fbshipit-source-id: 652f62bc9d71a4ff872efa21141225e43191353a

Summary: Small fixes to website rendering

Reviewed By: jcjohnson

Differential Revision: D23281746

fbshipit-source-id: c9dc8edd5e52f39d4e0e19f10ecc7e035b39feda

Summary:
Refactor cameras
* CamerasBase was enhanced with `transform_points_screen` that transforms projected points from NDC to screen space
* OpenGLPerspective, OpenGLOrthographic -> FoVPerspective, FoVOrthographic
* SfMPerspective, SfMOrthographic -> Perspective, Orthographic
* PerspectiveCamera can optionally be constructred with screen space parameters
* Note on Cameras and coordinate systems was added

Reviewed By: nikhilaravi

Differential Revision: D23168525

fbshipit-source-id: dd138e2b2cc7e0e0d9f34c45b8251c01266a2063